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BEAVER'S TAIL

[Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger. Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.

BEST TRACK

A subjectively smoothed path, versus a precise and very erratic fix-to-fix path, used to represent tropical cyclone movement. It is based on an assessment of all available data.

BLIZZARD

Snow with winds in excess of 35 mph and visibilities of 1/4 mile or less, for an extended period of time (eg. > 3 hours).

BLOWING DUST

Reduction of visibility by strong winds blowing across dry ground with little or no vegetation. Visibilities of 1/8 mile or less over a widespread area are criteria for a Blowing Dust Advisory.

BLUE WATCH (or Blue Box)

[Slang], a severe thunderstorm watch.

BOUNDARY LAYER

In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly. There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.

BOW ECHO

A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation

BOX (or Watch Box)

[Slang], a severe thunderstorm or tornado watch. See blue box, red box.

BRN

See Bulk Richardson Number.

BROKEN CLOUDS

Clouds which cover between 6/10 and 9/10 of the sky. See Sky Terminology.

BUBBLE HIGH

A mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air.

BULD RICHARDSON NUMBER (or BRN)

A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.

BUST

[Slang], an inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.

BWER [Bounded Weak Echo Region]

(Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. See WER.

CA

Cloud-to-Air lightning.

CAA

Cold Air Advection

CAP (or Capping Inversion)

A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur. See CIN and sounding. The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability

CAPE (Convective Available Potential Energy)

represents the amount of buoyant energy available to accelerate a parcel vertically, or the amount of work a parcel does on the environment. The higher the CAPE value, the more energy available to foster storm growth. CAPE is especially important when air parcels are able to reach the Level of Free Convection (LFC)... i.e. CAPE is Potential energy and will only be used should a parcel be lifted to the LFC. A table below describes the buoyancy relative to the numeric values of CAPE (measured in joules/kilogram)

CAPE VALUECONVECTIVE POTENTIAL
0Stable
0 - 1000Marginally Unstable
1001 - 2500Moderately Unstable
2501 - 3500Very Unstable
3501 +Extremely Unstable

Cb

Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead."

CC

Cloud-to-Cloud lightning.

CEILING

The height of the lowest layer of clouds, when the sky is broken or overcast.

CELL

Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells (see multi-cellular thunderstorm). The term "cell" also is used to describe the radar echo returned by an individual shower or thunderstorm. Such usage, although common, is technically incorrect.

CENTER

The vertical axis or core of a tropical cyclone. It is usually determined by cloud vorticity patterns, wind, and/or pressure distributions.

CENTER / VORTEX FIX

The location of the center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration, satellite, radar, or synoptic data.

CENTRAL NORTH PACIFIC BASIN

The region north of the Equator between 140W and the International Dateline. The Central Pacific Hurricane Center (CPHC) in Honolulu, HI is respobsible for tracking tropical cyclones in this region.

CG

Cloud-to-Ground lightning flash.

CHAFF

Small strips of metal foil, usually dropped in large quantities from aircraft or balloons. Chaff typically produces a radar echo which closely resembles precipitation. Chaff drops once were conducted by the military in order to confuse enemy radar, but now are conducted mainly for radar testing and calibration purposes.

CHANCE

A 30, 40 or 50 percent chance of occurrence of measurable precipitation.

CHINOOK WIND

A foehn wind on the east side of the Rocky Mountains.

CIN or CINH (Convective Inhibition)

A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.) See CAPE and sounding.

CIRRIFORM

High altitude ice clouds with a very thin wispy appearance.

CIRROCUMULUS

Cirrus clouds with vertical development.

CIRROSTRATUS

Cirrus clouds with a flat sheetlike appearance.

CIRRUS

High-level clouds (16,000 feet or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

CLASSIC SUPERCELL

See supercell.

CLEAR

Sky condition of less than 1/10 cloud coverage.

CLEAR SLOT

A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

CLIMATE

The historical record of average daily and seasonal weather events.

CLOSED LOW

A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see cutoff low).

COLD ADVECTION

Transport of cold air into a region by horizontal winds.

CLOUD STREETS

Rows of cumulus or cumulus-type clouds aligned parallel to the low-level flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.

CLOUD TAGS

Ragged, detached cloud fragments; fractus or scud.

COASTAL FLOODING

The inundation of land areas along the coast caused by sea water above normal tidal actions.

COASTAL FORECAST

A forecast of wind, wave and weather conditions between the coastline and 60 miles offshore.

COLD-AIR FUNNEL

A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

COLD FRONT

The boundary between a cold air mass that is advancing and a relatively warmer airmass. Generally characterized by steady precipitation followed by showery precipitation.

COLD POOL

A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

COLLAR CLOUD

A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. This term sometimes is used (incorrectly) as a synonym for wall cloud.

COMMA CLOUD

A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.

COMMA ECHO

A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo.

COMBINED SEAS

The combined height of swell and wind waves.

CONDENSATION

The process of gas changing to liquid.

CONDENSATION FUNNEL

A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.). Compare with debris cloud.

CONFLUENCE

A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow.

CONGESTUS(or Cumulus Congestus)

same as towering cumulus.

CONVECTION

Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

CONVECTIVE OUTLOOK (sometimes called AC)

A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC. The terms approaching, slight risk, moderate risk, and high risk are used to describe severe thunderstorm potential. Local versions sometimes are prepared by local NWS offices.

CONVECTIVE TEMPERATURE

The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding. Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.

CONVERGENCE

A contraction of a vector field; the opposite of divergence. Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result

CONTINENTAL AIR MASS

A dry air mass originating over a large land area.

CORE PUNCH

[Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting.

CORIOLIS FORCE

An apparent force caused by the rotation of the Earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left.

CUMULONIMBUS CLOUD

A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.

CUMULUS CLOUD

A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

CUMULIFORM ANVIL

A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, mushroom.

CUMULUS

Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus.

CUMULUS CONGESTUS (or simply Congestus)

Same as towering cumulus.

CUTOFF LOW

A closed low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression). "Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.

CYCLIC STORM

A thunderstorm that undergoes cycles of intensification and weakening (pulses) while maintaining its individuality. Cyclic supercells are capable of producing multiple tornadoes (i.e., a tornado family) and/or several bursts of severe weather. A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.

CYCLONE

An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. (2)An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

CYCLONIC CIRCULATION

This is circulation flowing in a counter-clockwise direction (Northern Hemisphere) and is associated with low pressure. Anti-cyclonic circulation is the opposite, flowing clockwise and associated with high pressure.

CYCLOGENESIS

Development or intensification of a low-pressure center (cyclone). *Cyclonic Circulation (or Cyclonic Rotation)

dBZ

Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z. The value of Z is a function of the amount of radar beam energy that is backscattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being backscattered by a target. The amount of backscattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates. However, other factors can affect reflectivity, such as width of the radar beam, precipitation type, drop size, or the presence of ground clutter or AP. WSR-88D radars can detect reflectivities as low as -32 dBZ near the radar site, but significant (measurable) precipitation generally is indicated by reflectivities of around 15 dBZ or more. Values of 50 dBZ or more normally are associated with heavy thunderstorms, perhaps with hail, but as with most other quantities, there are no reliable threshold values to confirm the presence of hail or severe weather in a given situation. See VIP for threshold dBZ values associated with each VIP level.

DEBRIS CLOUD

A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. This term is similar to dust whirl, although the latter typically refers to a circulation which contains dust but not necessarily any debris. A dust plume, on the other hand, does not rotate. Note that a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.

DELTA T

A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.

DENSE FOG ADVISORY

Issued when fog reduces visibility to 1/8 mile or less over a widespread area. Most common in California's central valley. See tule fog.

DERECHO

(Pronounced day-RAY-cho), a widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

DEW

Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

DEW POINT (or Dew-point Temperature) [dewpoint]

A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

DIABLO WIND

Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.

DIFFERENTIAL MOTION

Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

DIFLUENCE (or Diffluence)

A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence. Difluence in an upper level wind field is considered a favorable condition for severe thunderstorm development (if other parameters are also favorable). But difluence is not the same as divergence. In a difluent flow, winds normally decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the difluent flow.

DIRECTIONAL SHEAR

The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.

DIURNAL

Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).

DIVERGENCE

The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable) (See also Convergence).

DOPPLER RADAR

A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere.

DOWNBURST

A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder. See dry and wet microburst.

DOWNDRAFT

A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.

DOWNSTREAM

In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

DRIZZLE

Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

DRY ADIABAT

A line of constant potential temperature on a thermodynamic chart. See sounding.

DRY-BULB TEMPERATURE

Technically, the temperature registered by the dry-bulb thermometer of a psychrometer. However, it is identical with the ambient air temperature and may also be used in that sense.

DRY LINE

A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime; see dry punch). These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward. See LP storm.

DRY-LINE BULGE

A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch). Severe weather potential is increased near and ahead of a dry line bulge.

DRY-LINE STORM

Generally, any thunderstorm that develops on or near a dry line. The term often is used synonymously with LP storm, since the latter almost always occurs near the dry line.

DRY MICROBURST

A microburst with little or no precipitation reaching the ground; most common in semi-arid regions. They may or may not produce lightning. Dry microbursts may develop in an otherwise fair-weather pattern; visible signs may include a cumulus cloud or small Cb with a high base and high-level virga, or perhaps only an orphan anvil from a dying rain shower. At the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga. Compare with wet microburst.

DRY PUNCH

[Slang], a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.

DRY SLOT

A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs. Dry slot should not be confused with clear slot, which is a storm-scale phenomenon.

DUST DEVIL

A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions.

DUST PLUME

A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front. If rotation is observed, then the term dust whirl or debris cloud should be used.

DUST WHIRL

A rotating column of air rendered visible by dust. Similar to debris cloud; see also dust devil, gustnado, tornado.

DYNAMICS

Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

EASTERN NORTH PACIFIC BASIN

The region north of the Equator east of 140W. The National Hurricane Center in Miami, FL is responsible for tracking tropical cyclones in this region.

EBB CURRENT

Movement of a tidal current away from shore or down a tidal river or estuary.

ECHO TOPS

This is the highest altitude of a storm/precipitation area, as detected by radar.

ECMWF

European Center for Medium-Range Weather Forecasting. Operational references in forecast discussions typically refer to the ECMWF's medium-range forecast model. See MRF, UKMET.

EDDY

This is a small-scale area of rotation. Usually, in southern California, mention of an eddy refers to the coastal eddy, which is a usually small area of circulation over the coastal waters.

ELEVATED CONVECTION

Convection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft. In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.

EL NINO

A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns.

ENERGY HELICITY INDEX (or EHI)

An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE (in j/kg) as follows

ENHANCED V

A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather. Enhanced V should not be confused with V notch, which is a radar signature.

ENHANCED WORDING

An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches with enhanced wording. Severe thunderstorm watches may include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a derecho event is occurring or forecast to occur. See PDS watch.

ENSO

El Nino-Southern Oscillation.

ENTRANCE REGION

The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also exit region, left exit region.

EQUILIBRIUM LEVEL (or EL)

On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. (See sounding.) The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops. However, strong updrafts will continue to rise past the EL before stopping, resulting in storm tops that are higher than the EL. This process sometimes can be seen visually as an overshooting top or anvil dome. The EL typically is higher than the tropopause, and is a more accurate reference for storm tops.

ETA MODEL

One of the operational numerical forecast models run at NCEP. The Eta is run twice daily, with forecast output out to 48 hours.

EYE

The relatively calm center of the tropical cyclone that is more than one half surrounded by wall cloud.

EYE WALL/WALL CLOUD

An organized band of cumuliform clouds immediately surrounding the center of a tropical cyclone. Eye wall and wall cloud are used synonymously.

EXIT REGION

The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also entrance region, right entrance region.

EXPLOSIVE DEEPENING

A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least six hours.

EXTRATROPICAL

A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.

FAIR

Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds.

F SCALE

See Fujita Scale.

FEEDER BANDS

Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). Same as inflow bands. This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.

FETCH

The area in which ocean waves are generated by the wind. Also refers to the length of the fetch area, measured in the direction of the wind.

FLANKING LINE

A line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm, and generally coincides with the pseudo-cold front.

FLASH FLOOD

A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure.

FLOOD

High flow, overflow or inundation of a normally dry area which causes or threatens damage.

FLOOD STAGE

The level of a river or stream at which considerable inundation of surrounding areas will occur.

FOEHN

A warm dry wind on the lee side of a mountain range. The heating and drying are due to adiabatic compression as the wind descends downslope.

FOG

The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface, limiting visibility.

FORWARD FLANK DOWNDRAFT

The main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is. Compare with rear flank downdraft. See pseudo-warm front and supercell.

FRACTUS

Ragged, detached cloud fragments; same as scud.

FREEZING LEVEL

The altitude in the atmosphere where the temperature drops to 32F.

FREEZING RAIN

Rain which falls as liquid then freezes upon impact, resulting in a coating of ice on exposed objects.

FRONT

A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

FUJITA SCALE (or F Scale)

A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage

FUJIWHARA EFFECT

A binary interaction where tropical cyclones within a certain distance (300-750 nm depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint.

FULL MOON

The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.

FUNNEL CLOUD

A condensation funnel extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.

GALE

Wind speeds from 39 to 54 mph (34 to 47 knots).

GALE WARNING

A warning of 1-minute sustained surface winds in the range 34 kt (39 mph or 63 km/hr) to 47 kt (54 mph or 87 km/hr) inclusive, either predicted or occurring not directly associated with tropical cyclones.

GEOSTATIONARY SATELLITE

A satellite that rotates at the same rate as the earth, remaining over the same spot above the equator.

GOES

Geostationary Operational Environmental Satellite

GREENHOUSE EFFECT

The warming of the atmosphere by the trapping of longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

GROUND CLUTTER

A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.

GROUND FOG

Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in parts of California as tule fog.

GUNGE

[Slang], anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds (stratus), etc.

GUST

A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

GUST FRONT

The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also downburst, gustnado, outflow boundary.

GUSTNADO (or Gustinado)

[Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

HAIL

Precipitation in the form of balls or irregular lumps of ice.

HALOS

Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.

HARD FREEZE

A hard freeze occurs when the temperature drops below 28F, causing seasonal vegetation to be destroyed and the ground surface to become frozen.

HAZE

Fine dry or wet dust or salt particles in the air that reduce visibility.

HEAT INDEX

An index that combines air temperature and humidity to give an apparent temperature (eg. how hot it "feels").

Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.

(-42.379+2.04901523*t+10.14333127*r-.22475541*t*r-(6.83783e-3)*t^2-(5.48 1717e-2)*r^2+(1.22874e-3)*t^2*r+(8.5282e-4)*t*r^2-(1.99e-6)*t^2*r^2)
t=temp deg f and
r=%rel hum

HELICITY

A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.

HIC

Hydrologist In Charge.

HIGH

The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

HIGH RISK (of severe thunderstorms)

Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. (See slight risk, moderate risk, convective outlook.)

HIGH WIND

Sustained winds greater than or equal to 40 mph or gust greater than or equal to 58 mph.

HIGH WIND WARNING

A high wind warning is defined as 1-minute average surface winds of 35 kt (40 mph or 64 km/hr) or greater lasting for 1 hour or longer, or winds gusting to 50 kt (58 mph or 93 km/hr) or greater regardless of duration that are either expected or observed over land.

HODOGRAPH

A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

HOOK (or Hook Echo)

A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development. See BWER and supercell.

HP STORM or HP SUPERCELL

High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events. Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous. See bear's cage.

HUMIDITY

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

HURRICANE

A severe tropical cyclone with wind speeds in excess of 74 mph (64 knots). (2)A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific cyclones north of the Equator west of the International Dateline.

HURRICANE LOCAL STATEMENT

A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property.

HURRICANE SEASON

The portion of the year having a relatively high indidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30.

HURRICANE WARNING

A warning that sustained winds 64 kt (74 mph or 119 km/hr) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

HURRICANE WATCH

An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours.

ICAO

International Civil Aeronautical Organization; the international body governing the operation of commercial aircraft. The ICAO was created in 1944 to promote the safe and orderly development of civil aviation in the world. A specialized agency of the United Nations, it sets international standards and regulations necessary for the safety, security, efficiency and regularity of air transport and serves as the forum for cooperation in all fields of civil aviation among its 187 Contracting Member States. Also [slang] a 4-letter identifier for meterological stations. (relevant website)

IMPULSE

See upper level system.

INDIAN SUMMER

An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

INFLOW BANDS (or Feeder Bands)

Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.

INFLOW JETS

Local jets of air near the ground flowing inward toward the base of a tornado.

INFLOW NOTCH

A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).

INFLOW STINGER

A beaver tail cloud with a stinger-like shape.

INSOLATION

Incoming solar radiation. Solar heating; sunshine.

INSTABILITY

The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather

INVERSION

Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap. see sounding.

ISENTROPIC LIFT

Lifting of air that is traveling along an upward-sloping isentropic surface. Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.

ISENTROPIC SURFACE

A two-dimensional surface containing points of equal potential temperature.

ISOBAR

A line connecting points of equal pressure.

ISODROSOTHERM

A line connecting points of equal dew point temperature.

ISOHYET

A line connecting points of equal precipitation amounts.

ISOPLETH

General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

ISOTACH

A line connecting points of equal wind speed.

ISOTHERM

A line connecting points of equal temperature.

ITCZ

Inter-tropical Convergence Zone. The region where the northeasterly and southeasterly tradewinds converge, forming an often continuous band of clouds or thunderstorms near the equator.

JET MAX (or Speed Max, Jet Streak)

a point or area of relative maximum wind speeds within a jet stream.

JET STREAM

Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as front and low pressure systems.

KNUCKLES

[Slang], lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds. See also cumuliform anvil, anvil rollover.

LAMINAR

Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

LANDSPOUT

[Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

LAPSE RATE

The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring. See sounding.

LARGE-SCALE

See synoptic-scale.

LAST QUARTER

One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

LEFT FRONT QUADRANT (or Left Exit Region)

The area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.

LEFT MOVER

A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm. See also right mover.

LEWP

Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line. The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of a LEWP.

LIFTED INDEX (or LI)

A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability

LOADED GUN (Sounding)

[Slang], a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it. See sounding.

LONGWAVE TROUGH

A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

LOW-LEVEL JET (abbrev. LLJ)

A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer). The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

LP STORM (or LP Supercell)

Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

LSR

Local Storm Report. A product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events.

MAMMATUS CLOUDS

Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well. See HP storm, LP storm, and supercell.

MCC (Mesoscale Convective Complex)

A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs: Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. Duration: Size criteria must be met for at least 6 hours. Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

MCS (Mesoscale Convective System)

A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

MEDIUM RANGE

In forecasting, (generally) three to seven days in advance.

MERIDIONAL FLOW

Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.

MESOCYCLONE

A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

MESOHIGH

A mesoscale high pressure area, usually associated with MCSs or their remnants.

MESOLOW (or Sub-synoptic Low)

A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow. Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.

MESONET

A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.

MESOSCALE

Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems.

MICROBURST

A small, concentrated downburst affecting an area less than 4 kilometers (about 2.5 miles) across. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

MID-LEVEL COOLING

Local cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal. Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.

MODERATE RISK (of severe thunderstorms)

Severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See high risk, slight risk, convective outlook.

MOISTURE ADVECTION

Transport of moisture by horizontal winds.

MOISTURE CONVERGENCE

A measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.

MORNING GLORY

An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer. They are similar to ripples on a water surface; several parallel morning glories often can be seen propagating in the same direction.

MRF

Medium-Range Forecast model; one of the operational forecast models run at NCEP. The MRF is run once daily, with forecast output out to 240 hours (10 days).

MULTI-CELL(ULAR) THUNDERSTORM

A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development. Nearly all thunderstorms (including supercells) are multi-cellular, but the term often is used to describe a storm which does not fit the definition of a supercell.

MULTIPLE-VORTEX (or Multi-vortex) Tornado

a tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.

MUSHROOM

[Slang], a thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.

NCEP

National Centers for Environmental Prediction; the modernized version of NMC.

NEGATIVE-TILT TROUGH

An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

NEXRAD

NEXt-Generation Weather RADar. Technologically-advanced weather radar being deployed to replace WSR-57 and WSR-74 units. NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans. NEXRAD units are known as WSR-88D.

NEW MOON

The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse). New Moon follows waning crescent, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation".

NGM

Nested Grid Model; one of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours.

NMC

National Meteorological Center, with headquarters near Washington D.C.; now known as NCEP.

NOAA

National Oceanographic and Atmospheric Administration.

NOCTURNAL

Related to nighttime, or occurring at night.

NOWCAST

A short-term weather forecast, generally out to six hours or less.

NSSFC

National Severe Storms Forecast Center, in Kansas City MO; now known as SPC.

NSSL

National Severe Storms Laboratory, in Norman OK. (Sometimes pronounced NES-sel.)

NWP

Numerical Weather Prediction.

NWS

National Weather Service.

OCCLUDED MESOCYCLONE

A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.

OROGRAPHIC

Related to, or caused by, physical geography (such as mountains or sloping terrain).

OROGRAPHIC LIFT

Lifting of air caused by its passage up and over mountains or other sloping terrain.

ORPHAN ANVIL

[Slang], an anvil from a dissipated thunderstorm, below which no other clouds remain.

OUTFLOW BOUNDARY

A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).

OVERHANG

Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side). See BWER.

OVERRUNNING

A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. Embedded thunderstorms sometimes develop in such a pattern; severe thunderstorms (mainly with large hail) can occur, but tornadoes are unlikely. Overrunning often is applied to the case of warm air riding up over a retreating layer of colder air, as along the sloping surface of a warm front. Such use of the term technically is incorrect, but in general it refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.

OVERSHOOTING TOP (or Penetrating Top)

A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm. See HP storm, LP storm and supercell.

PDS WATCH

[Slang], a tornado watch with enhanced wording (Particularly Dangerous Situation).

PENDANT ECHO

Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

PENETRATING TOP

Same as overshooting top.

POPCORN CONVECTION

[Slang], Showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

POSITIVE AREA

The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. See sounding. Positive area is a measure of the energy available for convection; see CAPE.

POSITIVE CG

A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)

POSITIVE-TILT TROUGH

An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

POST-STORM REPORT

A report issued by a local National Weather Service office summarizing the impact of a tropical cyclone on it's forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.

POTENTIAL TEMPERATURE

The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

PPINE

Plan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range.

PRELIMINARY REPORT

A report summarizing the life history and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on forecasts and warnings associated with the cyclone. NHC writes a preliminary report on every tropical cyclone in its area of responsibility.

PRESENT MOVEMENT

The best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.

PROBABILITY OF TROPICAL CYCLONE CONDITIONS

The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.

PROFILER

An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

PSEUDO-COLD FRONT

A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. See also pseudo-warm front.

PSEUDO-WARM FRONT

A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.

PULSE STORM

A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See overshooting top, cyclic storm.

PVA (Positive Vorticity Advection)

Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.

RADAP II

RAdar DAta Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, accumulated rainfall, etc.

RADIAL VELOCITY

Component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, can not be detected.)

RAIN FOOT

[Slang], a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.

RAIN-FREE BASE

A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself

RAPID DEEPENING

A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours.

RED WATCH OR RED BOX

[Slang], a tornado watch.

REFLECTIVITY

Radar term referring to the ability of a radar target to return energy; used to derive echo intensity, and to estimate precipitation intensity and rainfall rates. See dBZ, VIP.

RELATIVE HUMIDITY

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

RELOCATED

A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest know position is not necessarily a reasonable representation of the cyclone's movement.

RETROGRESSION (or Retrograde Motion)

Movement of a weather system in a direction opposite to that of the basic flow in which it is embedded, usually referring to a closed low or a longwave trough which moves westward.

RETURN FLOW

South winds on the back (west) side of an eastward-moving surface high pressure system. Return flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).

RIGHT ENTRANCE REGION (or Right Rear Quadrant)

The area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.

RIDGE

An elongated area of relatively high atmospheric pressure; the opposite of trough.

RIGHT MOVER

A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.) See left mover, splitting storm.

RIGHT REAR QUADRANt

see Right Entrance Region.

ROLL CLOUD

A low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds.

ROPE (or Rope Funnel)

A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.

ROPE CLOUD

In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.

ROPE STAGE

The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.

RUC

Rapid Update Cycle, a numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

SCUD (or Fractus)

Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.

SELS

SEvere Local Storms Unit, former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK (formerly in Kansas City, MO).

SEVERE THUNDERSTORM

A thunderstorm which produces tornadoes, hail 0.75 inches or more in diameter, or winds of 50 knots (58 mph) or more. Structural wind damage may imply the occurrence of a severe thunderstorm. See approaching (severe).

SHEAR

Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

SHELF CLOUD

A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

SHORT-FUSE WARNING

A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.

SHORTWAVE (or Shortwave Trough)

A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.

SLIGHT RISK (of severe thunderstorms)

Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. See high risk, moderate risk, convective outlook.

SOUNDING

A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.

SPC

Storm Prediction Center. A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding) and severe winter weather over the contiguous United States.

SPEED SHEAR

The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

SPIN-UP

[Slang], a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.

SPLITTING STORM

A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.

SQUALL LINE

A solid or nearly solid line or band of active thunderstorms.

STACCATO LIGHTNING

A CG lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.

STEERING WINDS (or Steering Currents)

A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

STORM-RELATIVE

Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

STORM-SCALE

Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale.

STORM SURGE

An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

STORM TIDE

The actual level of sea water resulting from the astronomic tide combined with the storm surge.

STORM WARNING

A warning of 1-minute sustained surface winds of 48 kt (55 mph or 88 km/hr) or greater, either predicted or occurring, not directly associated with tropical cyclones.

SUBTROPICAL CYCLONE

A low pressure system that develops over subtropical waters that initially has a non-tropical circulation but in which some elements of tropical cyclone cloud structure are present. Subtropical cyclones can evolve into tropical cyclones. Subtropical cyclones are generally of two types:

  1. An upper level cold low with circulation extending to the surface and maximum sustained winds generally occurring at a radius of about 100 miles or more from the pressure center.
  2. A mesoscale cyclone originating in or near a frontolyzing zone of horizontal wind shear, with radius of maximum sustained winds generally less than 30 miles. The entire circulation sometimes encompasses an area initially no more than 100 miles in diameter. These generally short-lived, marine cyclones may vary in structure from cold to warm core.

SUBTROPICAL DEPRESSION

A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.

SUBTROPICAL STORM

A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 km/hr) or more.

STRAIGHT-LINE WINDS

Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

STRATIFORM

Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).

STRATOCUMULUS

Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

STRATUS

A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.

STRIATIONS

Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of an LP storm.

SUBSIDENCE

Sinking (downward) motion in the atmosphere, usually over a broad area.

SUB-SYNOPTIC LOW

Essentially the same as mesolow.

SUCTION VORTEX (sometimes Suction Spot)

A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

SUPERCELL

A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events

SURFACE-BASED CONVECTION

Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.

SWEAT INDEX

Severe Weather ThrEAT index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows: SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where Td 850 is the dew point temperature at 850 mb, TT is the total-totals index, f 850 is the 850-mb wind speed (in knots), f 500 is the 500-mb wind speed (in knots), and s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer). SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

SWODY1, SWODY2 (sometimes pronounced swoe-dee)

The day-1 and day-2 convective outlooks issued by SELS.

SYNOPTIC SCALE (or Large Scale)

Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale, storm-scale.

SYNOPTIC TRACK

Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.

TAIL CLOUD

A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud).

TAIL-END CHARLIE

[Slang], the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.

THERMODYNAMIC CHART (or Thermodynamic Diagram)

A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability. (See sounding.)

THERMODYNAMICS

In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

THETA-E (or Equivalent Potential Temperature)

The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.

THETA-E RIDGE

An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.

TILT SEQUENCE

Radar term indicating that the radar antenna is scanning through a series of antenna elevations in order to obtain a volume scan.

TILTED STORM OR TILTED UPDRAFT

A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.

TORNADO

A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

TORNADO FAMILY

A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.

TOTAL-TOTALS INDEX

A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices

TOWER

(Short for towering cumulus), a cloud element showing appreciable upward vertical development.

TOWERING CUMULUS

(Same as congestus.) A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)

TRANSVERSE BANDS

Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.

TRANSVERSE ROLLS

Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favorable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.

T ROLLS

[Slang], same as transverse rolls.

TRIPLE POINT

The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

TROPICAL CYCLONE

A warm-core, nonfrontal low pressure system of synoptic scale that develops over tropical or subtropical waters and has a definite organized surface circulation.

TROPICAL CYCLONE PLAN OF THE DAY

A coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.

TROPICAL DEPRESSION

A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.

TROPICAL DISTURBANCE

A discrete tropical weather system of apparently organized convection--generally 100 to 300 nmi in diameter---originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.

TROPICAL STORM

A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 34 kt (39 mph or 63 km/hr) to 63 kt (73 mph or 118 km/hr).

TROPICAL STORM WARNING

A warning for tropical storm conditions including sustained winds within the range of 34 to 63 kt (39 to 73 mph or 63 to 118 km/hr) that are expected in a specified coastal area within 24 hours or less.

TROPICAL STORM WATCH

An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is forecast to attain hurricane strength.

TROPICAL WAVE

A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.

TROPOPAUSE

The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). See sounding.

TROPOSPHERE

The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers

TROUGH

An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

TURKEY TOWER

[Slang], a narrow, individual cloud tower that develops and falls apart rapidly. The sudden development of turkey towers from small cumulus clouds may signify the breaking of a cap.

TVS

Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation

UKMET

A medium-range numerical weather prediction model operated by the United Kingdom METeorological Agency.

UPDRAFT

A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

UPDRAFT BASE

Alternate term for a rain-free base.

UPPER LEVEL SYSTEM

A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.

UPSLOPE FLOW

Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

UPSTREAM

Toward the source of the flow, or located in the area from which the flow is coming.

UVM (or UVV)

Upward Vertical Motion (or Velocity).

VAD

Velocity Azimuth Display. A radar display on which mean radial velocity is plotted as a function of azimuth. See VWP.

VAULT

Same as BWER.

VEERING WINDS

Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds.

VERTICALLY-STACKED SYSTEM

A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.

VIL

Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size.

VIP

Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels

VIRGA

Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst; see dry microburst.

V NOTCH

A radar reflectivity signature seen as a V-shaped notch in the downwind part of a thunderstorm echo. The V-notch often is seen on supercells, and is thought to be a sign of diverging flow around the main storm updraft (and hence a very strong updraft). This term should not be confused with inflow notch or with enhanced V, although the latter is believed to form by a similar process. See supercell.

VOLUME SCAN

A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes. Volume scans are necessary to determine thunderstorm type, and to detect features such as WERs, BWERs, and overhang.

VORTICITY

A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.

VORT MAX

(Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of a fluid.

VWP

VAD Wind Profile. A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets). This product often is referred to erroneously as a VAD.

WALL CLOUD

A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

WANING GIBBOUS MOON

The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

WANING CRESCENT MOON

The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

WARM ADVECTION

Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.

WARNING

A product issued by NWS local offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning). See short-fuse warning.

WATCH

An NWS product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).

WATCH BOX (or Box)

[Slang], a severe thunderstorm or tornado watch.

WATERSPOUT

In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters. The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.

WEDGE (or Wedge Tornado)

[Slang], a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare. Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity

WER [Weak Echo Region]

Radar term for a region of relatively weak reflectivity at low levels on the inflow side of a thunderstorm echo, topped by stronger reflectivity in the form of an echo overhang directly above it. The WER is a sign of a strong updraft on the inflow side of a storm, within which precipitation is held aloft. When the area of low reflectivity extends upward into, and is surrounded by, the higher reflectivity aloft, it becomes a BWER.

WET-BULB DEPRESSION [wetbulb]

The difference in degrees between the dry-bulb temperature and the wet-buld temperature.

WET-BULB TEMPERATURE [wetbulb]

The temperature read from the wet-bulb thermometer. The temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat of vaporization being supplied by the parcel.

WET-BULB THERMOMETER [wetbulb]

In a psychrometer, the thermometer that has the wet, muslin-covered bulb and therefore measures wet-bulb temperature.

WET MICROBURST

A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. See dry microburst.

WIND SHEAR

See shear.

WRAPPING GUST FRONT

A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.

WSR-57, WSR-74

NWS Weather Surveillance Radar units, replaced by WSR-88D units.

WSR-88D

Weather Surveillance Radar

ZONAL FLOW

Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.

ABSOLUTE HUMIDITY

A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

ABSOLUTE INSTABILITY

When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated. Related term: instability

ABSOLUTE TEMPERATURE SCALE

A temperature scale with a freezing point of +273K (Kelvin) and a boiling point of +373K. Related term: Kelvin Temperature Scale

ABSOLUTE ZERO

Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0 Kelvin, -273.15 Celsius and -459.67 Fahrenheit.

ABSORPTION

The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.

ABYSSAL PLAIN

The flat, gently sloping or nearly level region of the sea floor.

AC

Convective outlook issued by the SPC. Abbreviation for Anticipated Convection; the term originates from the header coding [ACUS1] of the transmitted product.

ACCAS (usually pronounced ACK-kis)

AltoCumulus CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

ACCESSORY CLOUD

A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.

ACID RAIN

Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH < 5.6).

ADIABATIC PROCESS

A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansion will result in cooling.

ADVECTION

The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection".

ADVECTION FOG

Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point.

ADVISORY

Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence.

ADVISORY (2) [tropical]

Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe:

  1. tropical cyclones prior to issuance of watches and warnings and
  2. subtropical cyclones.

AFOS

Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data.

AIR

This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are Nitrogen (N2) at 78.09%, Oxygen (O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (CO2) at 0.033%. One of the most important constituents of air and most important gases in meteorology is water vapor (H2O).

AIR MASS

A large body of air having similar horizontal temperature and moisture characteristics.

AIR-MASS THUNDERSTORM

Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.

AIR POLLUTION

The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors.

AIR QUALITY STANDARDS

The maximum level which will be permitted for a given pollutant. Primary standards are to be sufficiently stringent to protect the public health. Secondary standards must protect the public welfare, including property and aesthetics.

ALASKAN WINDS

The downslope air flow that blows through the Alaskan valleys. It is usually given local names, such as Knik, Matanuska, Pruga, Stikine, Taku, Take, Turnagain, or Williwaw.

ALERT

Automated Local Event Reporting in Real Time. Network of automatic raingauges that transmit via VHF radio link when precipitation occurs. Some sites are also equipped with other sensors such as temperature, wind, pressure, river stage or tide level.

ALBERTA CLIPPER

A fast moving, snow-producing weather system that originates in the lee of the Canadian Rockies. It moves quickly across the northern United States, often bring gusty winds and cold Arctic air.

ALBEDO

The percentage of light reflected by an object.

ALEUTIAN LOW

A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum intensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.

ALGORITHM

A computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.

ALTIMETER

An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.

ALTIMETER SETTING

The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined.

ALTITUDE

In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

ALTOCUMULUS

Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. In the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. A defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. Sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. It may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. Virga may also fall from these clouds.

ALTOCUMULUS CASTELLANUS

A middle cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. Its formation indicates instability and turbulence at the altitudes of occurrence.

ALTOSTRATUS

This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Virga often falls from these clouds.

AM

Area Manager.

AMERICAN METEOROLOGICAL SOCIETY

An organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences.

ANABATIC WIND

A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are an example of these winds. The opposite of a katabatic wind.

ANEMOMETER

An instrument that measures wind speed.

ANEROID BAROMETER

An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars.

ANOMALOUS PROPAGATION

This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.

ANTARCTIC

Of or relating to the area around the geographic South Pole, from 90 South to the Antarctic Circle at approximately 66 1/2South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).

ANTARCTIC OCEAN

Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.

ANTICYCLONE

A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.

ANTICYCLONIC CIRCULATION

See cyclonic circulation.

ANVIL

The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (see back-sheared anvil).

ANVIL DOME

A large overshooting top or penetrating top.

ANVIL ROLLOVER

[Slang], a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, mushroom.

ANVIL ZITS

[Slang], frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

AP

Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.

APHELION

The point on the earth's orbit that is farthest from the sun. Although the position is part of a 21,000 year cycle, currently it occurs around July, when the earth is about 3 million miles farther from the sun than at perihelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of perihelion.

APOGEE

The point farthest from the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of perigee.

APPROACHING (severe levels)

A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter. See severe thunderstorm.

ARCTIC

Of or relating to the area around the geographic North Pole, from 90 North to the Arctic Circle at approximately 66 1/2 North latitude.

ARCTIC AIR MASS

An air mass that develops around the Arctic, it is characterized by being cold from surface to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continuous feature. When this air mass moves from its source region, it may become more shallow in height as it spreads southward.

ARCTIC JET

The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circles. It marks the boundary of polar and arctic air masses.

ARCTIC SEA SMOKE

A type of advection fog that forms primarily over water when cold air passes across warmer waters.

ARCUS

A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.

ARGON (A)

A colorless, odorless inert gas that is the third most abundant constituent of dry air, comprising 0.93% of the total.

ARID

A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.

ASOS

Automated Surface Observing System. Observes sky conditions, temperature and dewpoint, wind direction and speed, and barometric pressure.

ASTRONOMICAL TWILIGHT

The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18 below the horizon.

ATMOSPHERE

The gaseous or air portion of the physical environment that encircles a planet. In the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. The divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.

ATMOSPHERIC PRESSURE

The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).

ATWC

Alaska Tsunami Warning Center, located in Palmer, AK.

AUTUMN

The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period between the autumnal equinox and the winter solstice.

AURORA

It is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. It is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. In the Northern Hemisphere, it is known as the aurora borealis or Northern Lights, and in the Southern Hemisphere, this phenomena is called the aurora australis.

AVHRR

Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.

AVIATION WEATHER CENTER

As one of the National Centers for Environmental Prediction, it is the national center for weather information that is used daily by the Federal Aviation Administration, commercial airlines, and private pilots. It is entering a new phase of service, growing to accept global forecasting responsibilities.

AVN

AViatioN model; one of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.

AWIPS

Advanced Weather Information Processing System. New NWS computer system integrating graphics, satellite and radar imagery. The successor to AFOS.

AZORES HIGH

A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. In the Northern Hemispheric winter and early spring, when the Icelandic Low dominates the North Atlantic, it is primarily centered near the Azores Islands. When it is displaced westward, during the summer and fall, the center is located in the western North Atlantic, near Bermuda, and is known as the Bermuda High.

BACK-BUILDING THUNDERSTORM

A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.

BACKING

A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is clockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the northeast to the north to the northwest. It is the opposite of veering.

BACKING WINDS

Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels. Backing winds with height are indicative of cold air advection (CAA). The opposite of Veering Winds.

BACKSCATTER

A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. Also the scattering of radiant energy into space before it reaches the earth's surface.

BACK-SHEARED ANVIL

[Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential.

BALL LIGHTNING

A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air.

BAPSU

Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.

BAR

An obstacle formed at the shallow entrance to the mouth of a river or bay.

BARBER POLE

[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.

BAROCLINIC ZONE

A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.

BAROCLINITY

The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. Also known as baroclinicity. An example is the tight temperature gradient along the East Coast of the United States during the winter that gives rise to intense cyclogenesis.

BAROGRAPH

An instrument that continuously records a barometer's reading of atmospheric pressure.

BAROMETER

An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.

BAROMETRIC PRESSURE

The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).

BAROTROPIC SYSTEM

A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone. Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems

BAROTROPY

The state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. It is considered zero baroclinity.

BARRIER WINDS

Refers to the westerly flow of air along the northern slope of the Brooks Range in northern Alaska that precedes the arrival of colder air from the north.

BATHYTHERMOGRAPH

A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referred to as a B.T.

BEAR'S CAGE

[Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.

BEAUFORT WIND SCALE

A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

BELLOT WINDS

Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somerset Island and the Boothia Peninsula, connecting the Gulf of Boothia and Franklin Strait.

BERMUDA HIGH

A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores Islands.

BERNOULLI'S THEOREM

A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. Theorized by Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist.

BIOSPHERE

The transition zone between the earth and the atmosphere within which most terrestrial life forms are found. It is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere.

BLACK BLIZZARD

A local term for a violent duststorm on the south-central Great Plains that darkens the sky and casts a pall over the land.

BLACK ICE

Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.

BLIZZARD

A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10F, winds exceeding 45 mph, and visibility reduced by snow to near zero.

BLOCKING HIGH

The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. It prevents the movement of migratory cyclones across its latitudes.

BLOWING DUST

Dust that is raised by the wind to heights of six feet or greater. It is reported as "BLDU" in an observation and on the METAR.

BLOWING SAND

Sand that is raised by the wind to heights of six feet or greater. It is reported as "BLSA" in an observation and on the METAR.

BLOWING SNOW

Snow that is raised by the wind to heights of six feet or greater. It is reported as "BLSN" in an observation and on the METAR.

BLOWING SPRAY

Salt spray that is raised by the wind to heights of six feet or greater. It is reported as "BLPY" in an observation and on the METAR.

BLUE NORTHER

Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-black sky with strong wintery winds from the northwest or north and temperatures that may drop 20F to 30F in a few minutes.

BOILING POINT

The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100C or 212F.

BOULDER WIND

A local name referring to an extremely strong downslope wind in the Front Range of the Rocky Mountains near Boulder, Colorado.

BOUNDARY LAYER

The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.

BOW ECHO

A radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. It is bent outward in a "bow" shape.

BOYLE'S LAW

States that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert Boyle (1627-1691), an Irish physicist and chemist and co-founder of the Royal Society.

BRIGHT BAND

A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest.

BROKEN

The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer.

BUBBLE HIGH

A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. Convergence along the leading edge of a bubble high may help form additional thunderstorms.

BUYS BALLOT'S LAW

Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere. This law was named after the Dutch meteorologist, Buys Ballot, who developed the formula in 1857.

BWER

Acronym for Bounded Weak Echo Region. Refers to radar echo signatures with low reflectivity in the center, surrounded by higher reflectivity. It is associated with strong updrafts and is found in the inflow region of a thunderstorm.

CALM

Atmospheric conditions devoid of wind or any other air motion. In oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell.

CALORIE

In meteorology, it is the amount of heat required to raise the temperature of one (1) gram of water one (1) degree Celsius. It is a unit of heat energy.

CAP

Composed of a layer of warmer, dryer air aloft which may suppress or delay the development of thunderstorms. As an air parcel rises, it becomes cooler relative to the ambient, or surrounding, air in the cap and therefore, less buoyant and unable to rise further. Also referred to as a lid.

CAPE

Acronym for Convective Available Potential Energy. The amount of energy available to create convection, with higher values increasing the possibility for severe weather.

CAPE VERDE ISLANDS

A group of volcanic islands in the eastern Atlantic Ocean off the coast of West Africa. A Cape Verde hurricane originates near here.

CARBON DIOXIDE (CO2)

A heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of the total.

CATALINA EDDY

A weak low pressure circulation that may form off the Southern California coast.

CEILING

The lowest cloud layer that is reported as broken or overcast. If the sky is totally obscured, then it is the height of the vertical visibility.

CEILING LIGHT

An instrument consisting of a drum and an optical system that projects a narrow vertical beam of light onto a cloud base.

CEILOMETER

An instrument that is used to measure the angular elevation of a projected light on the base of a cloud. It measures the angle of the cloud base included by the observer (or machine), the ceiling light and the illuminated spot on the cloud.

CELESTIAL EQUATOR

The projection of the plane of the geographical equator upon the celestial sphere.

CELESTIAL SPHERE

The apparent sphere of infinite radius having the earth as its center. All heavenly bodies (planets, stars, etc.) appear on the "inner surface" of this sphere and the sun moves along the ecliptic.

CELSIUS TEMPERATURE SCALE

A temperature scale where water at sea level has a freezing point of 0C (Celsius) and a boiling point of +100C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."

CENTRAL PRESSURE

The atmospheric pressure at the center of a high or low. It is the highest pressure in a high and the lowest pressure in a low, referring to the sea level pressure of the system on a surface chart.

CENTRIFUGAL FORCE

The apparent force in a rotating system that deflects masses radially outward from the axis of rotation. This force increases towards the equator and decreases towards the poles.

CENTRIPETAL FORCE

The force required to keep an object moving in a curved or circular path. It is directed inwards toward the center of the curved path.

CHARLES' LAW

States that when the pressure is held constant, the volume of a gas varies directly with the temperature. Therefore, if the pressure remains constant, the volume of a gas will increase with the increase of temperature. It was developed by Jacques Charles and is also known as the Charles-Guy-Lussac Law.

CHEMOSPHERE

A vaguely defined region of the upper atmosphere in which photochemical reactions take place. It includes the top of the stratosphere, all of the mesosphere, and sometimes the lower part of the thermosphere.

CHEYENNE FOG

An upslope fog formed by the westward flow of air from the Missouri River Valley, producing fog on the eastern slopes of the Rockies.

CHINOOK

A type of foehn wind. Refers to the warm downslope wind in the Rocky Mountains that may occur after an intense cold spell when the temperature could rise by 20F to 40F in a matter of minutes.

CHOCOLATTA NORTH

A West Indian gale that blows from the northwest.

CHROMOSPHERE

A thin layer of relatively transparent gases above the photosphere of the sun. It is observed best during a total eclipse of the sun.

CIRCULATION

The flow or motion of a fluid in or through a given area or volume. In meteorology, it is used to describe the flow of air as it moves around a pressure system in the atmosphere. It describes smaller patterns in semi-permanent pressure systems as well as the relatively permanent global currents of air. In oceanic terms, it is used to describe a water in current flow within a large area, usually a closed circular pattern such as in the North Atlantic.

CIRCULATION CELLS

Large areas of air movement created by the rotation of the earth and the transfer of heat from the equator toward the poles. Circulation is confined to a specific region, such as the tropics, temperate, or polar, that influences the type of weather prevailing there.

CIRRIFORM

Clouds composed of small particles, mostly ice crystals. Because the particles are fairly widely dispersed, this usually results in relative transparency and whiteness, often producing a halo phenomena not observed in other clouds forms. These clouds generally have bases above 20,000 feet in the mid-latitudes, and are classified as high clouds. They include all varieties of cirrus, cirrocumulus, and cirrostratus clouds.

CIRROCUMULUS

A cirriform cloud with vertical development, appearing as a thin sheet of small white puffs which give it a rippled effect. It often creates a "mackerel sky", since the ripples may look like fish scales. Sometimes it is confused with altocumulus, however, it has smaller individual masses and does not cast a shadow on other elements. It is also the least common cloud type, often forming from cirrus or cirrostratus, with which it is associated in the sky.

CIRROSTRATUS

A cirriform cloud that develops from cirrus spreading out into a thin layer, creating a flat sheetlike appearance. It can give the sky a slightly milky or veiled look. When viewed from the surface of the earth, these ice crystals can create a halo effect around the sun or moon. This cloud is a good precursor of precipitation, indicating it may occur within 12 to 24 hours.

CIRRUS

One of the three basic cloud forms (the others are cumulus and stratus). It is also one of the three high cloud types. Cirrus are thin, wispy clouds composed of ice crystals and often appear as veil patches or strands. In the mid-latitudes, cloud bases are usually found between 20,000 to 30,000 feet, and it is the highest cloud that forms in the sky, except for the tops, or anvils, of cumulonimbus, which occasionally build to excessive heights.

CIVIL TWILIGHT

The time between the moment of sunset, when the sun's apparent upper edge is just at the horizon, until the center of the sun is 6 directly below the horizon.

CLEAR

The state of the sky when no clouds or obscurations are observed or detected from the point of observation.

CLEAR AIR TURBULENCE

Name given to turbulence that may occur in perfectly clear air without any visual in warning in the form of clouds. It is often found in the vicinity of the jet stream where large shears in the horizontal and vertical are found, although this turbulence is not limited just to jet stream locale. Other areas where it may occur include near mountains, in closed lows aloft, and in regions of wind shear. May be referred to as CAT.

CLEAR ICE

A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled in water droplets. The droplets spread out over an object, such as an aircraft wing's leading edge, prior to complete freezing and forms a sheet of clear ice.

CLIMATE

The historical record and description of average daily and in seasonal weather events that help describe a region. Statistics are generally drawn over several decades. The word is derived from the Greek klima, meaning inclination, and reflects the importance early scholars attributed to the sun's influence.

CLIMATE ANALYSIS CENTER (CAC)

The U.S. National Weather Service division that applies new technology and approaches to the analysis, diagnosis, and projection of short term climate fluctuations on a regional and global basis.

CLIMATE PREDICTION CENTER (CPC)

A branch of the National Centers for Environmental Prediction,the Center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them.

CLIMATOLOGY

The study of climate. It includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems.

CLINOMETER

An instrument used to measure angles of inclination. Used in conjunction with a ceiling light, it determines cloud height at night, based on the angle of a projected light on the clouds, the observer, and the ceiling light.

CLOSED LOW

A region of low pressure distinguished by a center of counterclockwise circulation (in the Northern Hemisphere), and is surrounded by one or more isobars or height contours. Closed lows aloft (i.e., above the surface) may become disconnected from the primary westerly flow and thus progress eastward more slowly. It is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows.

CLOUD

A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the free air. A cloud forms in the atmosphere as a result of condensation of water vapor. Condensation nuclei, such as in smoke or dust particles, form a surface upon which water vapor can condense.

CLOUD BANK

A well-defined cloud mass that can be observed at a distance. It covers the horizon, but is not directly overhead.

CLOUDBURST

A sudden, heavy rainfall of a showery nature.

COALESCENCE

The merging of two water drops into a single larger drop.

COLD

A condition marked by low or decidedly subnormal temperature. The lack of heat.

COLD ADVECTION

The horizontal movement of colder air into a location. Contrast with warm advection.

COLD AIR FUNNEL

Funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very in cold. Cold air funnels may touch down briefly, but in general are less violent than most other types of tornadoes.

COLD CORE THUNDERSTORMS

Thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlies warmer surface air.

COLD FRONT

The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front.

COLD HIGH

A high pressure system that has its coldest temperatures at or near the center of circulation, and horizontally, is thermally barotropic. It is shallow in nature, as circulation decreases with height. Associated with cold Arctic air, it is usually stationary. Also known as a cold core high. Contrast with a warm high.

COLD LOW

A low pressure system that has its coldest temperatures at or near the center of circulation, and is thermally barotropic with respect to a horizontal plane. Also known as a cold core low. A cut off low is an example, where an isolated pool of colder air is located south of the main westerlies.

COLD WAVE

A rapid fall in temperature within twenty-four hours to temperatures requiring substantially increased protection to agriculture, industry, commerce, and social activities. National Weather Service criteria includes the rate of temperature fall and the minimum to which it falls, depending on the region of the country and time of the in year. The Weather Channel uses the following criteria for a cold wave: a cold spell of two days or more with below normal temperatures in at least fifteen states, with at least five of them more than fifteen degrees below normal.

COLLADA

A strong, steady wind blowing from the north or northwest in the upper part of the Gulf of California and from the northeast in the lower part.

COLORADO LOW

A low pressure disturbance that forms in the lee of the Rocky Mountains, usually in southeastern Colorado.

COMMA CLOUD

A feature seen on satellite images with a distinctive comma-shape. This is indicative of a synoptic cloud pattern associated with large, well-developed low pressure systems.

CONDENSATION

The process by which water vapor undergoes a change in state from a gas to a liquid. It is the opposite physical process of evaporation.

CONDENSATION FUNNEL

A funnel-shaped cloud consisting of condensed water drops that has possible rotation.

CONDENSATION NUCLEI

A particle upon which condensation of water vapor occurs. It may be either in a solid or liquid state.

CONDITIONAL INSTABILITY

Stable unsaturated air that will result in instability in the event or on the condition that the air becomes saturated. If the air is saturated, it is considered unstable; if air is unsaturated, it is considered stable.

CONDUCTION

The transfer of heat through a substance by molecular action or from one substance by being in contact with another.

CONFLUENCE

A rate at which wind flow comes together along an axis oriented normal to the flow in question. The opposite of diffluence.

CONSTANT PRESSURE CHART

A chart of a constant pressure surface in which atmospheric pressure is uniform everywhere at any given moment. Elements may include analyses of height above sea level, wind, temperature, and humidity.

CONSTANT PRESSURE SURFACE

A surface along which the atmospheric pressure is equal everywhere.

CONTINENT

A large land mass rising abruptly from the deep ocean floor, including marginal regions that are shallowly submerged. Continents constitute about one-third of the earth's surface.

CONTINENTAL AIR MASS

An air mass with continental characteristics. It is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region. For example, cP is an air mass that is continental polar in nature.

CONTINENTAL SHELF

The zone around the continents extending from the low-water mark seaward, typically ending in steep slope to the depths of the ocean floor.

CONTRAIL

Acronym for CONdensation TRAIL. A cloud-like streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere.

CONVECTION

Motions in a fluid that transport and mix the properties of the fluid. These properties could be heat and/or moisture. When used to imply only upward vertical motion, it is then the opposite of subsidence.

CONVECTIVE CONDENSATION LEVEL (CCL)

The height at which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated.

CONVERGENCE

Wind movement that results in a horizontal net inflow of air into a particular region. Convergent winds at lower levels are associated with upward motion. Contrast with divergence.

COOLING DEGREE DAY

A cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees Fahrenheit. It is used to estimate the energy requirements and is an indication of fuel consumption for air conditioning or refrigeration.

CORIOLIS EFFECT

A force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. It is dependent on the latitude and the speed of the moving object. In the Northern Hemisphere, air is deflected to the right of its path, while in the Southern Hemisphere, air is deflected to the left of its path. It is greatest at the poles, North and South, and almost nonexistent at the equator.

COROMELL

The prevailing evening land breeze which takes place from November to May in the vicinity of La Paz, at the southern tip of Baja California, Mexico.

CORONA

A pastel halo around the moon or sun created by the diffraction of water droplets. The droplets in the cloud, such as cirrostratus, and the cloud layer itself must be almost perfectly uniform in order for this phenomena to occur. The color display sometimes appears to be iridescent.

CORPOSANT

A luminous, sporadic, and often audible, electric discharge. It occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. It often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples.

CREPUSCULAR RAYS

Contrasting, alternating bright and dark rays in the sky. Sunlight is scattered by molecules and particles rendering these bright rays visible. Contrast is enhanced by haze, dust, or mist. These rays are more likely to be seen in the late afternoon, as clouds come between the sun and the observer. A similar effect occurs when the sun shines though a break in a layer of clouds.

CRYSTALLIZATION

The process of a substance going directly from a vapor form (water vapor) to a solid (ice) at the same temperature, without going through the liquid phase (water). The opposite of sublimation.

CUMULIFORM

Clouds composed of water droplets that exhibit vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. With increasing vertical height, they are often associated with convection. Bases of these clouds are generally no more than 3,000 feet above the ground, but they can develop past the troposphere in both temperate and tropical latitudes. They are classified as low clouds and include all varieties of cumulus and cumulonimbus. The opposite in type are the horizontal development of stratiform clouds.

CUMULONIMBUS

A vertically developed cumulus cloud, often capped by an anvil-shaped cirriform cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail, tornadoes or strong, gusty winds.

CUMULONIMBUS MAMMATUS

A portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms. They may slowly vary in size, since they are an area of negative buoyancy convection, and is associated with severe turbulence in the lower sections of the cloud.

CUMULUS

One of the three basic cloud forms (the others are cirrus and stratus). It is also one of the two low cloud types. A cloud that develops in a vertical direction from the base (bottom) up. They have flat bases and dome- or cauliflower-shaped upper surfaces. The base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. Small, separate cumulus are associated with fair weather (cumulus humilis). With additional heating from the earth's surface, they can grow vertically throughout the day. The top of such a cloud can easily reach 20,000 or more into the troposphere. Under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. Further development may create a cumulonimbus.

CUMULUS CONGESTUS

A strongly sprouting cumulus cloud with generally sharp outlines and often with great vertical development. It may occur as tower-like clouds with cauliflower tops. These clouds may produce abundant showers and may develop further into cumulonimbus.

CUMULUS FRACTUS

Cumulus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in stratus clouds (called stratus fractus), but not in cirrus clouds.

CUMULUS HUMILIS

Cumulus clouds with little or no vertical development characterized by a generally flat appearance. Their growth is usually limited by a temperature inversion, which is marked by the unusually uniform height of the clouds. Also called fair-weather cumulus.

CUMULUS MEDIOCRIS

Cumulus clouds characterized by moderate vertical development with upper protuberances not very marked in appearance. This cloud does not produce precipitation, but could develop into towering cumulus or cumulonimbus which do.

CURRENT

A horizontal movement of water, such as the Gulf Stream off the east coast of North America, or air, such as the jet stream.

CUT-OFF HIGH

A warm high which has become displaced and is on the polarward side of the jet stream. It occurs mostly during the spring and is most frequent over northeastern Siberia, Alaska, and Greenland. It is an example of a blocking high.

CUT-OFF LOW

A closed cold core low completely removed from the primary westerly flow. Cutoff lows may remain detached from the westerlies for days while exhibiting very little forward (eastward) progress. In some instances, a cutoff low may move to the west, or retrograde, opposite to the prevailing flow. It is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows.

CYCLOGENESIS

The process that creates a new low pressure system or cyclone, or intensifies a pre-existing one. It is also the first appearance of a trough.

CYCLONE

An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also called a low pressure system and the term used for a tropical cyclone in the Indian Ocean. Other phenomena with cyclonic flow may be referred to by this term, such as dust devils, tornadoes, and tropical and extratropical systems. The opposite of an anticyclone or a high pressure system.

CYCLONIC FLOW

Winds that blow in and around a cyclone, that is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

DAILY MEAN

The average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures.

DALTON'S LAW

States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases. Formulated by John Dalton, an English physicist.

DATA BUOYS

Buoys placed throughout the Gulf of Mexico and along the Atlantic and Pacific coasts of the United States that relay information on air and water temperature, wind speed, air pressure, and wave conditions via radio signals.

DAWN

The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twilight. The visual display is created by the scattering of light reaching the upper atmosphere prior to the sun's rise to the observer's horizon.

DAY

Considered a basic unit of time as defined by the earth's motion. It represents the time needed for one complete revolution of the earth about its own axis. Also know as a sidereal day, it is approximately equal to 23 hours, 56 minutes, and 4.09 seconds.

DEBRIS CLOUD

Considered a rotating cloud of debris or dust that is on the ground or near the ground. The debris cloud appearing beneath a thunderstorm will most likely confirm the presence of a tornado.

DEEPENING

Used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. The opposite of filling.

DEGREE

A measure of temperature difference representing a single division on a temperature scale.

DEGREE DAY

A measure of the departure of the mean daily temperature from a given standard. That is one degree day for each degree (Fahrenheit or Celsius) of departure above or below the standard during one day.

DENSE FOG ADVISORY

Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions.

DENSITY

The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0C or 39.2F.

DENSITY ALTITUDE

The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is computed from the station pressure at takeoff and the virtual temperature at the particular altitude under consideration.

DEPRESSION

In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features.

DERECHO

A line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. They are characterized by damaging straight-line winds over hundreds of miles. Spanish for straight.

DEW

Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.

DEW POINT

The temperature to which air must be cooled at a constant pressure to become saturated.

DIABLO WINDS

Dry winds in the Diablo mountain range in central California that can exceed 60 miles per hour. Similar to the Santa Ana winds, they develop as the wind flows from high pressure over Nevada to lower pressure along the central California coast.

DIFFLUENCE

A rate at which wind flow spreads apart along an axis oriented normal to the flow in question. The opposite of confluence.

DIFFRACTION

The result of light waves interfering with other after passing through a narrow aperture, causing them to bend or spread.

DIRECTIONAL SHEAR

The shear created by a rapid change in wind direction with height.

DISCONTINUITY

Comparatively large contrast in meteorological elements over a relatively small distance or period of time. In oceanography, it is the abrupt change or jump of a variable at a line or surface.

DISTURBANCE

This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features.

DIURNAL

Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. Meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind.

DIVERGENCE

Wind movement that results in a horizontal net outflow of air from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft. Contrast with convergence.

DOG DAYS

The name given to the very hot summer weather that may persists for four to six weeks between mid-July through early September in the United States. In western Europe, this period may exist from the first week in July to mid-August and is often the period of the greatest frequency of thunder. Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months.

DOLDRUMS

Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. Also a nautical term for the equatorial trough.

DOPPLER RADAR

Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.

DOWNBURST

A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air creates damaging winds at or near the surface. Sometimes the damage resembles tornadic damage.

DOWNDRAFT

A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.

DOWNPOUR

A heavy rain.

DOWNSLOPE EFFECT

The warming of an air flow as it descends a hill or mountain slope.

DRAINAGE WIND

A katabatic wind, it is caused by the cooling of air along the slopes of a mountain.

DRIFTING SNOW

Snow particles blown from the ground by the wind to a height of less than six feet.

DRIFTS

Normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind.

DRIZZLE

Slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches or 0.5 millimeters. It falls from stratus clouds and is often associated with low visibility and fog. It is reported as "DZ" in an observation and on the METAR.

DROPSONDE

A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure the atmosphere below.

DROUGHT

Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.

DRY ADIABAT

The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As a parcel rises adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel. When an air parcel is unsaturated and rises, then the temperature decreases at a rate of 1C per 100 meters (5.5F per 1,000 feet).

DRY BULB THERMOMETER

A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

DRY LINE

The boundary between the dry desert air mass of the Southwest U.S. and the moist air mass from the Gulf of Mexico. It usually lies north-south across the central and southern High Plains states during spring and summer. The passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. Its presence influences severe weather development in the Great Plains.

DRY SLOT

An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.

DUSK

The period of waning light from the time of sunset to dark.

DUST

Small particles of earth or other matter suspended in the air. It is reported as "DU" in an observation and for wide spread dust on the METAR.

DUST BOWL

The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado, and New Mexico that was most greatly affected during the Great Drought of the 1930's.

DUST DEVIL

A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating.

DUSTSTORM

A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to between 5/8ths and 5/16ths statute mile. It is reported as "DS" in an observation and on the METAR.

D-VALUE

The deviation of actual altitude along a constant pressure surface from the standard atmosphere altitude of that surface.

DYNAMICS

A branch of mechanics that deals with forces and their relations to patterns of motion. In metorology, this relates especially to wind and water patterns.

EARTHLIGHT (EARTHSHINE)

The faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere.

EARTHQUAKE

A sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity.

EASTERLIES

Usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds.

EASTERLY WAVE

An inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. The low level convergence and associated convective weather occur on the eastern side of the wave axis. Normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. It is often associated with possible tropical cyclone development and is also known as a tropical wave.

ECHO

The energy return of a radar signal after it has hit the target.

ECLIPSE

The obscuring of one celestial body by another.

ECLIPTIC

The sun's apparent path across the sky that tracks a circle through the celestial sphere.

ECOLOGY

The study of the relationships between living organisms and their environment.

EDDY

A small disturbance of wind in a large wind flow, which can produce turbulent conditions. They can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. In oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current.

ELECTROMAGNETIC RADIATION

Also called radiation, it is waves of energy propagated though space or through a material media.

ELECTROMAGNETIC SPECTRUM

The band of electromagnetic radiation with components that are separated into their relative wave lengths. The portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. The various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays.

ELEVATION

The measure of height with respect to a point on the earth's surface above mean sea level. Sometimes referred to as station elevation.

EL NIO

The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.

ENVIRONMENT

The sum total of all the external conditions that effect an organism, community, material, or energy.

EQUATOR

The geographic circle at 0 degrees latitude on the earth's surface. It is equal distance from the North and South Poles and divides the Northern Hemisphere from the Southern.

EQUATORIAL TROUGH

The quasi-continuous area of low pressure between the subtropical high pressure areas in both the Northern and Southern Hemisphere.

EQUINOX

The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.

EROSION

The movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind.

EVAPORATION

The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

EVAPOTRANSPIRATION

The total amount of water that is transferred from the earth's surface to the atmosphere. It is made up of the evaporation of liquid or solid water plus the transpiration from plants.

EXOSPHERE

This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface.

EXTRATROPICAL CYCLONE

Any cyclone not of tropical origin. Generally considered to be a migratory frontal cyclone found in the middle and high latitudes.

EYE

The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

EYE WALL

An organized band of convection surrounding the eye, or center, of a tropical cyclone. It contains cumulonimbus clouds, intense rainfall and very strong winds.

FAHRENHEIT TEMPERATURE SCALE

A temperature scale where water at sea level has a freezing point of +32F and a boiling point of +212F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.

FAIR

This is a subjective description. Considered as pleasant weather conditions with regard to the time of year and the physical location.

FATHOM

The common unit of depth in the ocean for countries using the English system of measurement. It is six feet or 1.83 meters. It can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile.

FEEDER BANDS

In tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. Also known as outer convective bands, a typical hurricane may have three or more of these bands. They occur in advance of the main rain shield and are usually 40 to 80 miles apart. In thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm.

FETCH

An area of the water surface over which waves are generated by a wind having a constant direction and speed. Also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. One of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture.

FEW

The amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer.

FILLING

Used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. Although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. The opposite of deepening.

FIRST QUARTER

One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

FIREWHIRL

A tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption.

FIRST GUST

Another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm.

FLANKING LINE

A line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell.

FLASH FLOOD

A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

FLOOD

High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

FLOOD PLAIN

Level land that may be submerged by flood waters.

FLOOD STAGE

The level of a river or stream where overflow onto surrounding areas can occur.

FOEHN

A warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. It is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. Classified as a katabatic wind.

FOG

A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on the METAR.

FOG BANK

A fairly well-defined mass of fog observed in the distance. Most commonly seen at sea, over a lake, or along coastal areas.

FOGBOW

A whitish semicircular arc seen opposite the sun in fog. The outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. An additional bow with reversed colors sometimes appears inside the first.

FORECAST

A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.

FRACTUS

The elements of cumulus and stratus clouds that appear in irregular fragments, as if they had been shred or torn. Never appears in cirrus clouds. Also known as scud.

FREEZING DRIZZLE

Drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. It is reported as "FZDZ" in an observation and on the METAR.

FREEZING FOG

Used to describe the phenomena when fog is present and the air temperature is below 0C. It is reported as "FZFG" in an observation and on the METAR.

FREEZING POINT/FREEZE

The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0C or 32F. It is the opposite of fusion. In oceanography, the freezing point of water is depressed with increasing salinity.

FREEZING PRECIPITATION

Precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces.

FREEZING RAIN

Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. It is reported as "FZRA" in an observation and on the METAR.

FRESH WATER

Water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity.

FRICTION

In meteorology, it is the turbulent resistance of the earth on the atmosphere. Considered as the resistance of fluids (air and water) to the relative motion of a solid body. The amount is dependent on the size and shape of the body.

FRICTION LAYER

The thin layer of atmosphere adjacent to the earth's surface. Surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. Above this level, air tends to flow parallel to the isobars. Wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the underlying surface features.

FRONT

The transition zone or interface between two air masses of different densities, which usually means different temperatures. For example, the area of convergence between warm, moist air and cool, dry air.

FRONTAL PASSAGE

It is the passage of a front over a specific point on the surface. It is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. Accompanying a passage may be precipitation and clouds. May be referred to as "fropa."

FRONTOGENESIS

The birth or creation of a front. This occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. It could happen when either air mass, or both, move over a surface which strengthens their original properties. However, it occurs most often along the eastern coasts of North America and Asia, when the air mass moving out over the ocean has a weak or no distinct boundary. The opposite of frontolysis.

FRONTOLYSIS

The destruction or dying of a front where the transition zone is losing its contrasting properties. The opposite of frontogenesis.

FROST

The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.

FROZEN PRECIPITATION

Precipitation that reaches the ground in a frozen state. Examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail.

FUJITA-PEARSON SCALE

A scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. It is normally used to identify the most intense damage exhibited by a tornado. Developed by T. Theodore Fujita and Allen Pearson.

FUNNEL CLOUD

A violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it. It is reported as "FC" in an observation and on the METAR.

FUSION

The change of state from a solid to a liquid at the same temperature. The heat of fusion is the number of gram calories of heat necessary to change one gram of a substance from the solid to the liquid state. It is the opposite of freezing.

GALE

On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).

GALE WARNING

A warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).

GEOPHYSICS

The study of the physics or nature of the Earth and its environment. It deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. Areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. By extension, it often includes astronomy and the related astro-sciences.

GEOSPHERE

Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. At their conjunction is the biosphere.

GEOSTATIONARY SATELLITE

An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Environmental Satellite. Related terms: polar-orbiting satellite and Dave's Dictionary

GEOSTROPHIC WIND

A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

GLACIER WINDS

Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum intensity in the early afternoon.

GLAZE

A smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. A storm that produces the accretion of glaze is called an ice storm.

GRADIENT WIND

A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence.

GRAUPEL

A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.

GRAVITATION

The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere modifies this attraction to produce the field of gravity.

GRAVITY

The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface.

GREEN FLASH

A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.

GREENHOUSE EFFECT

The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.

GREENWICH MEAN TIME (GMT)

The name of the twenty-four hour time scale which is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.

GROUND CLUTTER

A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation.

GROUND FOG

Fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Primarily takes place at night or early morning.

GROWING SEASON

Considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. Usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. The frost-free growing season is between the first and last occurrence of 32F temperatures in spring and autumn.

GULF STREAM

The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States, beginning near Cape Hatteras. The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.

GULLY WASHER

A heavy rain shower that occurs suddenly, possibly creating a flash flood.

GUST

A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

GUST FRONT

The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary.

GUSTNADO

A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud.

HABOOB

Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought.

HAIL

Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones. It is reported as "GR" in an observation and on the METAR. Small hail and/or snow pellets is reported as "GS" in an observation and on the METAR.

HALO

The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight. The most commonly observed is a halo that forms at a 22 radius, although another one at 46 radius may also be seen.

HAZE

A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance. It is reported as "HZ" in an observation and on the METAR.

HEAT

A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.

HEAT BALANCE

The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).

HEAT EXHAUSTION

The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT INDEX

The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.

HEATING DEGREE DAY

One heating degree day is given for each degree that the daily mean temperature is below 65F. It is used as an indication of fuel consumption.

HEAT LIGHTNING

Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.

HEAT STROKE

Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105F, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

HEAT WAVE

A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states must have 90F plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.

HELICITY

A property of a moving fluid, such as air, representing the potential for helical flow (flow that follows a corkscrew pattern). Computed from the vertical wind profile of the lower atmosphere and measured relative to the motion as a storm, it is used to forecast the formation of mesocyclones.

HIGH CLOUDS

A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus,cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

HIGH LATITUDES

The latitude belt roughly between 60 and 90 North and South.

HIGH PRESSURE SYSTEM

An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.

HOARFROST

Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is brought to saturation by cooling.

HOOK ECHO

A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. However, hook echoes and tornadoes do not always accompany each other.

HORIZON

One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth. The geographic horizon, also called the apparent horizon, is the distant line along which earth and sky appear to meet. This is the usual concept of horizon and is used in weather observing. The local horizon is the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions, such as mountains.

HORSE LATITUDES

Located between 30 North and 30 South in the vicinity of the equator, this area typically has calm or light and variable winds.

HUDSON BAY LOW

An area of low pressure over or near the Hudson Bay area of Canada that often introduces cold air to the north central and northeast United States.

HUMBOLDT CURRENT

Also known as the Peru Current, this ocean current flows northward along the western side of South America, offshore Chile and Peru. There is considerable upwelling of the colder subsurface waters due to the prevailing southerly winds. Dominant weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the meteorological-oceanographic pattern known as El Nio.

HUMIDITY

The amount of water vapor in the air. It is often confused with relative humidity or dew point.

HURRICANE

The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.

HURRICANE WARNING

A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.

HURRICANE WATCH

A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are a potential threat to a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

HYDROMETEOR

Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

HYDROLOGIC CYCLE

Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

HYDROLOGY

The study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface.

HYDROSPHERE

Considered as the water portion of the earth's surface. Part of the geosphere.

HYGROGRAPH

An instrument that records the hygrometer's measure of water vapor.

HYGROMETER

An instrument that measures the water vapor content of the atmosphere.

HYPOTHERMIA

This situation occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.

ICE

The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example.

ICE CRYSTALS

Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust." It is reported as "IC" in an observation and on the METAR.

ICE FOG

Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm conditions in the polar latitudes and may produce a halo around the sun or moon.

ICE JAM

An accumulation of broken river ice caught in a narrow channel, frequently producing local flooding. Primarily occurs during a thaw in the late winter or early spring.

ICELANDIC LOW

A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.

ICE PELLETS

Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. They have a diameter of 0.2 inches (5 mm) or less. They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets. It is reported as "PE" in an observation and on the METAR.

ICE STORM

A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems.

ICICLE

Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows.

ICING

The forming or depositing of ice on an object.

INCHES OF MERCURY (Hg)

The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. First devised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

INDIAN SUMMER

A period of abnormally warm weather in mid to late autumn with clear skies and cool nights. A first frost normally precedes this warm spell.

INFRARED

The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vapor, ozone, and carbon dioxide are capable of absorbing or transmitting infrared radiation. May be referred to as IR.

INSOLATION

Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar radiATION.

INSTABILITY

The state of equilibrium in which a parcel of air when displaced has a tendency to move further away from its original position. It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward, often forming cumulus clouds and possibly thunderstorms.

INSTRUMENT FLIGHT RULES (IFR)

Refers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles.

INSTRUMENT SHELTER

A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation.

INTERMOUNTAIN HIGH

An area of high pressure that occurs during the winter between the Rocky Mountains and the Sierra-Cascade ranges. It blocks the eastward movement of Pacific cyclones. Also called Plateau High or Great Basin High.

INTERNATIONAL DATE LINE

The line of longitude located at 180 East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.

INTERTROPICAL CONVERGENCE ZONE (ITCZ)

An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10 degrees North and South of the equator. It is a broad area of low pressure where both the Coriolis force and the low-level pressure gradient are weak, occasionally allowing tropical disturbances to form. It fluctuates in location, following the sun's rays, so that during the Northern Hemisphere summer, the ITCZ moves northward over the southern North Atlantic and southern Asia.

INVERSION

A departure from the usual increase or decrease of an atmospheric property with altitude. It usually refers to an increase in temperature with increasing altitude, which is a departure from the usual decrease of temperature with height.

IONOSPHERE

A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is included as part of the thermosphere.

ISALLOBAR

The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendency.

ISOBAR

The line drawn on a weather map connecting points of equal barometric pressure.

ISODROSOTHERM

The line drawn on a weather map connecting points of equal dew point.

ISOHEL

A line drawn through geographic points having equal duration of sunshine or another form of solar radiation during a specified time period.

ISOHYET

The line drawn through geographic points recording equal amounts of rainfall during a given time or for a given of storm.

ISOPLETH

A line connecting equal points of value. Also called an isoline.

ISOTACH

A line connecting equal wind speeds.

ISOTHERM

The line of equal or constant air temperature. If something is isothermal, it is of equal or constant temperature with respect to either time or space.

JET STREAK

A region of accelerated wind speed along the axis of a jet stream.

JET STREAM

An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. It is marked by a concentration of isotherms and strong vertical shear.

KATABATIC WIND

A wind that is created by air flowing downhill. When this air is warm, it may be called a foehn wind, and regionally it may be known as a Chinook or Santa Ana. When this air is cold or cool, it is called a drainage wind, and regionally it may be known as a mountain breeze or glacier wind. The opposite of an anabatic wind.

KATAFRONT

A front where the warm air descends the frontal surface, except in the low layers of the atmosphere.

KELVIN TEMPERATURE SCALE

A temperature scale with the freezing point of +273K (Kelvin) and the boiling point of +373 K. It is used primarily for scientific purposes. Also known as the Absolute Temperature Scale. Proposed in 1848 by William T. Kelvin, 1st Baron of Largs (1824-1907), Irish-born Scottish physicist and mathematician.

K INDEX

The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer.

KNOT

A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.

LAKE EFFECT SNOW

Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.

LAND BREEZE

A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the temperature difference when the sea surface is warmer than the adjacent land. Predominate during the night, it reaches its maximum about dawn. It blows in the opposite direction of a sea breeze.

LANDFALL

The point at which a tropical cyclone's eye first crosses a land mass.

LANDSPOUT

A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the land equivalent of a waterspout.

LAPSE RATE

The change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height and is a sign of instability.

LATENT HEAT

The energy released or absorbed during a change of state.

LATITUDE

The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 North and South latitude.

LEE/LEESIDE/LEEWARD

The side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore, protected from the direct force of the wind. The opposite of windward.

LENTICULAR CLOUD

A cloud species which has elements resembling smooth lenses or almonds and more or less isolated. These clouds are caused by a wave wind pattern created by the mountains. They are also indicative of down-stream turbulence on the leeward side of a barrier.

LEVEL OF FREE CONVECTION (LFC)

The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

LIFTED INDEX (LI)

A measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level and comparing it to the actual temperature at that altitude. Positive values indicate more stable air and negative values indicate instability.

LIFTING CONDENSATION LEVEL (LCL)

The height at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.

LIGHTNING

A sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air.

LIGHT WAVES

That part of the electromagnetic spectrum that contains visible light. The colors, from longest wave length to shortest, are red, orange, yellow, green, blue, indigo, and violet (ROY G. BIV).

LINE ECHO WAVE PATTERN (LEWP)

A wave-shaped bulge in a line of thunderstorms. It may often be seen as a "S"-shaped radar echo signature and is often associated with severe weather.

LITHOMETEOR

Atmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze.

LITHOSPHERE

The solid, outer portion of the earth's crust coupled to the rigid upper mantle. Part of the geosphere.

LONGITUDE

The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.

LONG WAVE TROUGH

A wave in the prevailing westerly flow aloft which is characterized by a large length and amplitude. A long wave moves slowly and is persistent. Its position and intensity govern weather patterns over a period of days or weeks.

LOW CLOUDS

A term used to signify clouds with bases below 6,000 feet and are of a stratiform or a cumuliform variety. Stratiform clouds include stratus and stratocumulus. Cumuliform clouds include cumulus and cumulonimbus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

LOW LATITUDES

The latitude belt between 30 and 0 degrees North and South of the equator. Also referred to as the tropical or torrid region.

LOW LEVEL JET (LLJ)

Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. It is often amplified at night. The southerly wind over the US Plains states during spring and summer is a notable example.

LOW PRESSURE SYSTEM

An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.

LUNAR ECLIPSE

An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color.

MACKEREL SKY

The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales.

MACROBURST

A large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds.

MACROSCALE

The meteorological scale covering an area ranging from the size of a continent to the entire globe.

MAGNETIC POLES

Either of the two points on the earth's surface where the magnetic meridians converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other.

MAMMATOCUMULUS

An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms.

MARE'S TAIL

The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.

MARGINAL VISUAL FLIGHT RULES (MVFR)

Refers to the general weather conditions pilots can expect at the surface. MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.

MARITIME AIR MASS

An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.

MAXIMUM

The greatest value attained by a function, for example, temperature, pressure, or wind speed. The opposite of minimum.

MEAN SEA LEVEL

The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL.

MEAN TEMPERATURE

The average of temperature readings taken over a specified amount of time. Often the average of the maximum and minimum temperatures.

MEASURED CEILING

A ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway.

MELTING LEVEL

The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere.

MELTING POINT

The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.

MERCURIAL BAROMETER

An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

MERIDIONAL FLOW

Atmospheric circulation in which the north and south, or meridional, component of motion is unusually pronounced. This weakens the zonal flow.

MESOCYCLONE

A area of rotation of storm size that may often be found on the southwest part of a supercell. Its circulation can be larger than the tornado that may develop within it, but not necessarily. Originally a radar term for a rotation signature that met certain criteria, it is best seen on Doppler radar.

MESOHIGH

A small, concentrated area of high pressure that may be created by the cold outflow and rain-cooled air from thunderstorms. It often forms a pseudo cold front or squall line on its leading edges.

MESOLOW

A small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles.

MESOSCALE

The scale of meteorological phenomena that range in size from several kilometers to around 100 kilometers. This includes MCCs, MCSs, and squall lines. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale.

MESOSCALE CONVECTIVE COMPLEX (MCC)

A large mesoscale convective system (MCS) which is about the size of the state of Ohio or Iowa and lasts at least 6 hours. Generally forming during the afternoon and evening, the complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat. Severe weather may occur at anytime.

MESOSCALE CONVECTIVE SYSTEM (MCS)

A large organized convective weather system comprised of a number of individual thunderstorms. It normally persists for several hours and may be rounded or linear in shape. This term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (MCC).

MESOSPHERE

The layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures drop rapidly with increasing height. It extends between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

METAR

Acronym for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.

METEOROLOGY/METEOROLOGIST

The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A scientist who studies the atmosphere and atmospheric phenomena.

MICROBAROGRAPH

A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.

MICROBURST

A severe localized wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes.

MICROSCALE

The smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. Larger phenomena are classified as mesoscale. It also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.

MIDDLE CLOUDS

A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

MIDDLE LATITUDES

The latitude belt roughly between 35 and 65 degrees North and South. May be referred to as the temperate region.

MILLIBAR (MB)

The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

MINIMUM

The least value attained by a function, for example, temperature, pressure, or wind speed. The opposite of maximum.

MIST

A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.

MIXED LAYER

It is the upper portion of the boundary layer in which air is thoroughly mixed by convection. In oceanography, it is the layer of the water that is mixed through wave action or thermohaline convection.

MIXED PRECIPITATION

Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.

MOIST ADIABAT

The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55C per 100 meters (2-3F per 1,000 feet).

MOISTURE

Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

MONSOON

The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States.

MOON PHASE

The phases of the Moon are related to (actually, caused by) the relative positions of the Moon and Sun in the sky. For example, New Moon occurs when the Sun and Moon are quite close together in the sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky - which is why a Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth. First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the two "half Moon" phases are called First Quarter and Last Quarter because they occur when the Moon is, respectively, one- and three-quarters of the way around the sky (i.e., along its orbit) from New Moon. The relationship of the Moon's phase to its angular distance in the sky from the Sun allows us to establish very exact definitions of when the primary phases occur, independent of how they appear. Technically, the phases New Moon, First Quarter, Full Moon, and Last Quarter are defined to occur when the excess of the apparent ecliptic (celestial) longitude of the Moon over that of the Sun is 0, 90, 180, and 270 degrees, respectively. These definitions are used when the dates and times of the phases are computed for almanacs, calendars, etc. Because the difference between the ecliptic longitudes of the Moon and Sun is a monotonically and rapidly increasing quantity, the dates and times of the phases of the Moon computed this way are instantaneous and well defined.

MOUNTAIN BREEZE

A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite direction of a valley breeze.

MOUNTAIN WAVE

A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. May be called a standing wave or a lee wave.

MUD SLIDE

Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.

MUGGY

A subjective term for warm and excessively humid weather.

MULTICELL STORM

A thunderstorm made up of two or more single-cell storms.

MULTIPLE VORTEX TORNADO

A tornado which has two or more condensation funnels or debris clouds, often rotating around a common center.

NADIR

The point on any given observer's celestial sphere diametrically opposite of one's zenith.

NATIONAL CENTER FOR ATMOSPHERIC RESEARCH (NCAR)

A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities.

NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION (NCEP)

As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Aviation Weather Center, the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. Formerly known as NMC.

NATIONAL CLIMATIC DATA CENTER (NCDC)

The agency that archives climatic data from the National Oceanic and Atmospheric Administration as well as other climatological organizations.

NATIONAL HURRICANE CENTER (NHC)

A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific.

NATIONAL METEOROLOGICAL CENTER (NMC)

Now incorporated into the National Centers for Environmental Prediction, it was the division of the National Weather Service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations.

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA)

A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources.

NATIONAL SEVERE STORMS FORECAST CENTER (NSSFC)

As of October 1995, the responsibilities of this Center were divided into two branches, the Storm Prediction Center and the Aviation Weather Center.

NATIONAL SEVERE STORMS LABORATORY (NSSL)

A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms.

NATIONAL WEATHER ASSOCIATION (NWA)

An organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer.

NATIONAL WEATHER SERVICE (NWS)

A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings.

NAUTICAL MILE

A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles.

NAUTICAL TWILIGHT

The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars.

NEAP TIDE

A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or three-quarters full.

NEGATIVE VORTICITY ADVECTION

The advection of lower values of vorticity into an area.

NEPHELOCOCCYGIA

A term applied when people find familiar objects within the shape of a cloud.

NEWHALL WINDS

The local name for winds blowing downward from desert uplands through the Newhall Pass southward into the San Fernando Valley, north of Los Angeles.

NEWTON

The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.

NEXRAD

Acronym for NEXt Generation Weather RADar. A network of advanced Doppler radars implemented in the United States between 1992 and 1996, it detects the location and intensity of precipitation out to a range of 143 miles from the radar site. NEXRAD Doppler radar is highly sensitive and can detect precipitation from very light rain and snow up to the strongest thunderstorms with accuracy and detail. However, sometimes the radar's extreme sensitivity will cause ground clutter and other non-precipitation echoes to be displayed in the vicinity of the radar site.

NIGHT

The period of the day between dusk and dawn.

NIMBOSTRATUS

This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

NITROGEN (N2)

A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

NOCTILUCENT CLOUDS

Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

NOCTURNAL THUNDERSTORMS

Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.

NOR'EASTER

A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

NORMAL

The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

NORTH PACIFIC HIGH

A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate.

NOWCAST

A short-term weather forecast for expected conditions in the next few hours.

NUMERICAL FORECASTING

The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. These models are run on high-speed computers at the National Centers for Environmental Prediction.

OBSCURATION

Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. According to the National Weather Service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic ash. It is reported as "X" in an observation and on the METAR.

OBSERVATION

In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.

OCCLUDED FRONT

Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet.

OCEAN

The intercommunicating body of salt water occupying the depressions of the earth's surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.

OCEANOGRAPHY

The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.

OMEGA BLOCK

A warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts.

OPAQUE

A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.

OROGRAPHIC LIFTING

Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, and the dissipation of clouds. May be called an orographic uplift.

OUTFLOW

Also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and temperature drop.

OVERCAST

The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

OVERRUNNING

This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event includes cloudiness, cool temperatures, and steady precipitation.

OXYGEN (O2)

A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%.

OZONE (O3)

A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.

OZONE LAYER

An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.

PALMER DROUGHT INDEX

A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agriculture) Joint Agricultural Weather Facility. The index depicts prolonged times, as in months or years, of abnormal dryness or wetness. It responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration.

PALOUSER

A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley in northern Idaho and eastern Washington. May be called a Cow-Killer.

PARCEL

A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

PARHELION

The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns.

PARTIAL OBSCURATION

Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. It often reduces horizontal visibility but not the vertical. It is reported as "X" in an observation and on the METAR.

PARTLY CLOUDY

The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. The National Weather Service does not have an amount of sky cover for this condition.

PASCAL

The unit of pressure produced when one newton acts on about one square meter.

PASCAL'S LAW

When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. Formulated by Blaise Pascal (1623-1662), a French mathematician, theologian, and physicist.

PEAK GUST

The highest instantaneous wind speed observed or recorded.

PERIGEE

The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of apogee.

PERIHELION

The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000 year cycle, currently it occurs around January, when the earth is about 3 million miles closer to the sun than at aphelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of aphelion.

PHOTOMETER

Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not directly cause adverse weather. They include halos, coronas. rainbows, and fogbows.

PHOTOSPHERE

The intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000F, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.

PILOT BALLOON

A small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. Also known as a pibal.

PILOT REPORT

A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.

PLAN POSITION INDICATOR

Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.

PLOW/PLOUGH WIND

The spreading downdraft and strong straight-line winds preceding a thunderstorm. So named in the American Midwest because of its ability to flatten tall grasses as it passes.

POLES/POLAR

The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 60 and 90 latitude, both North and South.

POLAR AIR MASS

An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime polar air (mP), produced over warmer waters, is less stable with high moisture.

POLAR FRONT

A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An integral part of an early meteorological theory known as the Polar Front Theory.

POLAR JET

Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.

POLAR-ORBITING SATELLITE

A satellite whose orbit passes over both of the earth's between poles.

POLLUTANT

Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.

POSITIVE VORTICITY ADVECTION

The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.

POUNDS PER SQUARE INCH (PSI)

A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one square inch.

PRECIPITATION

Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

PRE-FRONTAL SQUALL LINE

A line of thunderstorms that precedes an advancing cold front.

PRE-FRONTAL TROUGH

An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.

PRESSURE

The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.

PRESSURE ALTIMETER

An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.

PRESSURE ALTITUDE

The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.

PRESSURE CHANGE

The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.

PRESSURE CHARACTERISTIC

The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.

PRESSURE GRADIENT

The amount of pressure change that occurs over a fixed distance at a fixed altitude.

PRESSURE JUMP

A sudden increase in the observed atmospheric pressure or station pressure.

PRESSURE TENDENCY

The pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation.

PREVAILING WIND

A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

PREVAILING VISIBILITY

It is considered representative of visibility conditions at the observation station. It is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous.

PROFILER

A type of Doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere.

PROGNOSTIC CHART

A chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. Also known as a prog.

PSYCHROMETER

An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

PULSE

A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic radiation emitted by the radar.

QUANTITATIVE PRECIPITATION FORECAST (QPF)

A forecast of rainfall, snowfall or liquid equivalent of snowfall.

QUASI-STATIONARY FRONT

A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.

RADAR

Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back.

RADARSONDE OBSERVATION

An upper air observation used to determine winds and other meteorological data, by tracking the range, elevation, and azimuth of a radar target carried aloft. A type of rawinsonde.

RADIAL VELOCITY

A type of velocity that expresses motion toward or away from a given location. In Doppler radar, it is the component of motion that is parallel to the radar beam.

RADIATION

The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

RADIATIONAL COOLING

The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation.

RADIATION FOG

Fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning.

RADIOSONDE

An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends. May be referred to as a RAOB, an acronym for RAdiosonde OBservation.

RAIN

Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on the METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.

RAINBOW

A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.

RAINFALL

The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge.

RAIN FOREST

A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.

RAIN GAUGE

An instrument used to measure the amount of rain that has fallen. Measurement is done in hundredths of inches (0.01").

RAIN SHADOW

Also referred to as a precipitation shadow, it is the region on the lee side of a mountain or similar barrier where the precipitation is less than on the windward side. For example, the relatively dry Washoe Valley of western Nevada is in the rain shadow of the Sierra Nevada.

RANGE RESOLUTION

The ability of radar to distinguish between targets on the same azimuth but at different ranges.

RAWINSONDE

An upper air observation that evaluates the winds, temperature, relative humidity, and pressure aloft by means of a balloon-attached radiosonde that is tracked by a radar or radio direction-finder. It is a radiosonde observation combined with a winds-aloft observation, called a rawin.

RECONNAISSANCE (RECCO) CODE

An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.

REFLECTIVITY

A measure of the process by which a surface can turn back a portion of incident radiation into the medium through which the radiation approached. It also refers to the degree by which precipitation is able to reflect a radar beam.

REFRACTION

The bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.

RELATIVE HUMIDITY

A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage.

RELATIVE VORTICITY

The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis.

RESOLUTION

In relation to radar, it is the ability to read two distinct targets separately. The clearer the resolution, the nearer the two objects can be to each other and still be distinguishable.

RETROGRESSION

In meteorology, it is the movement of a weather system in a direction opposite to the direction of the basic flow in which it is embedded. Often used in reference to a long wave trough or other macroscale feature. For example, a long wave trough that may move slightly westward when the "normal" movement and flow is eastward.

REX BLOCK

A blocking pattern where there is an upper level high located directly north of a closed low.

RIDGE

An elongated area of high atmospheric pressure that is associated with an area of maximum anticyclonic circulation. The opposite of a trough.

RIME

The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing.

RIP CURRENT

It is formed by a strong surface water movement, or current, of a short duration that flows seaward from the shore. The return flow is piled up onshore by the incoming waves and wind. It is localized, of narrow width, and its position relative to the beach can change as the wave condition changes. Therefore, the higher the waves, the stronger the current.

ROCKETSONDE

A type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data during its parachute descent from a higher position in the atmosphere than a balloon could reach.

ROLL CLOUD

A relatively rare, low-level, horizontal, tube-shaped cloud. Although they are associated with a thunderstorm, they are completely detached from the base of the cumulonimbus cloud.

ROSSBY WAVES

The movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling the earth. Named for Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee, who first theorized about the existence of the jet stream in 1939.

ROTATION

The spinning of a body, such as the earth, about its axis.

ROTOR CLOUD

An altocumulus cloud formation that can be found in the lee of a mountain or similar barrier. The air rotates around a horizontal axis, creating turbulence. Altocumulus lenticularis is an example.

RUNWAY VISUAL RANGE (RVR)

It is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.

SAFFIR-SIMPSON DAMAGE-POTENTIAL SCALE

Developed in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then Director of the National Hurricane Center, it is a measure of hurricane intensity on a scale of 1 to 5. The scale categorizes potential damage based on barometric pressure, wind speeds, and surge.

ST. ELMO'S FIRE

A luminous, and often audible, electric discharge that is sporadic in nature. It occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. It often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. Also known as corposant or corona discharge.

SALINITY

A measure of the quantity of dissolved salts in sea water. The total amount of dissolved solids in sea water in parts per thousand by weight.

SALT WATER

The water of the ocean, distinguished from fresh water by its appreciable salinity.

SAND

Loose particles of hard, broken rock or minerals. In observing, sand is reported when particles of sand are raised to sufficient height that reduces visibility. It is reported as "SA" in an observation and on the METAR.

SANDSTORM

A strong wind carrying sand particles through the air. They are low level occurences, usually only ten feet in height to not more than fifty feet above the surface. Due to the frequent winds created by surface heating, they are most predominate during the day and die out in the night. Visibility is reduced to between 5/8ths and 6/16ths statute mile, and if less than 5/16ths, then the storm is considered a heavy sandstorm. It is reported as "SS" in an observation and on the METAR.

SANTA ANA WINDS

The hot, dry winds, generally from the east, that funnel through the Santa Ana river valley south of the San Gabriel and San Bernadino Mountains in southern California, including the Los Angeles basin. Classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind.

SARGASSO SEA

An area of the North Atlantic Ocean between Bermuda and the Azores. It is in the middle of the North Atlantic oceanic gyre, with converging surface waters. Consequently, it has less biological features than any other region of the ocean because the lack of mixing with more nutrient-rich waters.

SATELLITE

Any object that orbits a celestial body, such as a moon. However, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. Some of the information that is gathered by weather satellites, such as GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world.

SATELLITE IMAGES

Images taken by a weather satellite that reveal information, such as the flow of water vapor, the movement of frontal system, and the development of a tropical system. Looping individual images aids meteorologists in forecasting. One way a picture can be taken is as a visible shot, that is best during times of visible light (daylight). Another way is as an IR (infrared) shot, that reveals cloud temperatures and can be used day or night.

SATURATE

To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.

SATURATION POINT

The point when the water vapor in the atmosphere is at its maximum level for the existing temperature.

SCATTERED

The amount of sky cover for a cloud layer between 3/8ths and 4/8ths, based on the summation layer amount for that layer.

SCATTERING

The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. When working with radars, this often refers to the more or less random changes in direction of radio energy.

SCUD

Low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. They are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. When observed from a distance, they are sometimes mistaken for tornadoes.

SEA BREEZE

A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

SEA BREEZE FRONT

A coastal phenomena, it is restricted to large bodies of water and their immediate coast lines. This is usually the landward extent of the sea breeze. Due to the imbalance of heating between land and water, a region of maximum upward motion or convergence occurs by mid-afternoon in the summer some 10 to 15 miles inland. Air mass thunderstorms or a line of towering cumulus clouds with showers can form along the front. At the beach, there are blue skies and a light breeze. This often occurs along the coast of the Gulf of Mexico and Florida's east coast.

SEA FOG

A type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water.

SEA ICE

Ice that is formed by the freezing of sea water. It forms first as small crystals, thickens into sludge, and coagulates into sheet ice, pancake ice, or ice floes of various shapes and sizes.

SEA LEVEL

The height or level of the sea surface at any time. It is used as a reference for elevations above and below.

SEA LEVEL PRESSURE

The atmospheric pressure at mean sea level, usually determined from the observed station pressure.

SEA MILE

A unit of length distinguished from a nautical mile. One sea mile is equivalent to 1,000 fathoms (6,000 feet).

SEASON

A division of the year according to some regularly recurring phenomena, usually astronomical or climatic. For example, in the Northern Hemisphere, winter is said to begin on the winter solstice and end on the vernal equinox when spring begins, covering the months of December, January, and February. In the tropics, there is the dry and the rainy season, depending on the amount of precipitation.

SEA SPRAY

Sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave.

SEA SURFACE TEMPERATURE (SST)

The temperature of the water's surface. It is measured using buoy and ship data, infrared satellite imagery, and coastal observations.

SEMI-PERMANENT PRESSURE SYSTEMS

A relatively stable, stationary pressure-and-wind system where the pressure is predominately high or low with the changing season. They are not of a transitory nature, like migratory lows that develop from temperature and density differences.

SEVERE WEATHER

Generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes.

SEVERE THUNDERSTORM

A thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornadoes. Severe thunderstorms may also produce torrential rain and frequent lightning.

SHEAR

It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.

SHEAR LINE

A line of maximum horizontal wind shear. A narrow zone across which there is an abrupt change in the horizontal wind component parallel to it.

SHORT WAVE

A progressive wave of smaller amplitude, wave length, and duration than a long wave. It moves in the same direction as the basic current in which it is embedded and may induce upward vertical motion ahead of it. They are more numerous than long waves and often disappear with height in the atmosphere.

SHOWALTER STABILITY INDEX

A measure of the local static stability of the atmosphere. It is determined by lifting an air parcel to 500 millibars and then comparing its temperature to that of the environment. If the parcel is colder than its new environment, then the atmosphere is more stable. If the parcel is warmer than its new environment, then the atmosphere is unstable and the potential for thunderstorm development and severe weather increases.

SHOWER

Precipitation from a convective cloud that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE). It is reported as "SH" in an observation and on the METAR.

SIBERIAN EXPRESS

A fierce, cold flow of air that originates in Siberia, then moves into Alaska and northern Canada before moving southward into the United States.

SIBERIAN HIGH

The semi-permanent high pressure area that forms over Siberia during the winter. The average central pressure exceeds 1030 millibars from late November to early March. It is characterized by clear, dry weather. Over southern Asia, the predominate surface wind is northeasterly, just the opposite of the predominate summer winds which bring the monsoon.

SIDEREAL TIME

The measure of time as defined by the diurnal motion of the vernal equinox. A sidereal day is equivalent to one complete rotation of the earth relative to the equinox, which is 23 hours, 56 minutes, and 4.091 seconds. A sidereal year is the interval required for the earth to make one absolute revolution around the sun, which is 365 days, 6 hours, 9 minutes, and 9.5 seconds. Compare with the solar day.

SKEW T-LOG P DIAGRAM

A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is used to evaluate and forecast air parcel properties. Some values that can be determined are the Convective Condensation Level (CCL), the Lifting Condensation Level (LCL), and the Level of Free Convection (LFC).

SKY

The vault-like apparent surface against which all aerial objects are seen from the earth.

SKY COVER

The amount of the celestial dome that is hidden by clouds and/or obscurations.

SLEET

Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface. It is reported as "PE" in an observation and on the METAR.

SLUSH

Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.

SMALL CRAFT ADVISORY

An advisory issued for marine interests, especially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).

SMOKE

Small particles produced by combustion that are suspended in the air. A transition to haze may occur when the smoke particles have traveled great distance (25 to 100 miles or more), and when the larger particles have settled out. The remaining particles become widely scattered through the atmosphere. It is reported as "FU" in an observation and on the METAR.

SNOW

Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes. It is reported as "SN" in an observation and on the METAR.

SNOW ADVISORY

A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns of less severe weather conditions than a winter storm

SNOW BANNER

A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.

SNOW BLINDNESS

Temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. The medical term is niphablepsia.

SNOWBURN

A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.

SNOW COVER

The areal extent of ground covered by the snow. It is usually expressed as a percent of the total area of a given region.

SNOW CREEP

A continuous, extremely slow, downhill movement of a layer of snow.

SNOW CRUST

The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.

SNOW DEPTH

The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms.

SNOW DEVIL

A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a waterspout). Formed mechanically by the convergence of local air currents. May be called a snowspout.

SNOW EATER

Any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow.

SNOWFALL

The rate at which snow falls, usually expressed in inches of snow depth over a six hour period.

SNOWFLAKES

An ice crystal or an aggregate of ice crystals which fall from clouds.

SNOW FLURRY/FLURRIES

Light showers of snow, generally very brief without any measurable accumulation. May be reported as "SHSN--" in an observation and on the METAR.

SNOW GARLAND

Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by the surface tension of thin films of water bonding individual snow crystals.

SNOW GRAINS

Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle. It is reported as "SG" in an observation and on the METAR.

SNOW LEVEL

The elevation in mountainous terrain where the precipitation changes from rain to snow, depending on the temperature structure of the associated air mass.

SNOW LINE

The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.

SNOWPACK

The amount of annual accumulation of snow at higher elevations.

SNOW PELLETS

Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail. It is reported as "GS" in an observation and on the METAR.

SNOW ROLLER

The product of moist, cohesive snow that when initiated by wind rolls across the landscape, gathering snow until it can no longer move. It is shaped like a rolled sleeping bag, some reaching four feet across and seven feet in diameter.

SNOW SHOWER

Frozen precipitation in the form of snow, characterized by its sudden beginning and ending. It is reported as "SHSN" in an observation and on the METAR.

SNOW SQUALL

A heavy snow shower accompanied by sudden strong winds, or a squall.

SOLAR DAY

The complete rotation of the earth in relation to the sun. Although it varies, an average has determined a mean solar day of 24 hours. It is universally used for civil purposes.

SOLAR ECLIPSE

An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.

SOLSTICE

The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21 and the summer solstice on or about June 21.

SOUNDING

A plot of the atmosphere, using data rom upper air or radiosonde observations. Usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location.

SOUTHERN OSCILLATION

A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nio events. It is represents the distribution of temperature and pressure over an oceanic area.

SPECIFIC HUMIDITY

The ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. It is expressed in grams per gram or in grams per kilograms. The specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel.

SPRING

The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the North Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.

SPRING TIDE

A tide of increased range, which occurs about every two weeks when the moon is new or full.

SQUALL

A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust. It is reported as "SQ"s in an observation and on the METAR.

SQUALL LINE

A narrow band or line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a mesohigh.

STABLE/STABILITY

Occurs when a rising air parcel becomes denser than the surrounding air. It will then return to its original position. When the density of the air parcel remains the same as the surrounding air after being lifted, it is also considered stable, since it does not have the tendency to rise or sink further. Contrast with unstable air and instability.

STAGNATION AREA

An area that has a combination of stable stratification, weak horizontal wind speed, and little, if any, significant precipitation. It is usually associated with an area of high pressure.

STANDARD ATMOSPHERE

A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO). It assumes a mean sea level temperature of 15C a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury, and a temperature lapse rate of 0.65C per 100 meters up to 11 kilometers in the atmosphere.

STANDARD SURFACE PRESSURE

The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter.

STANDING CLOUD

Any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain.

STANDING WAVE

An atmospheric wave that is stationary with respect to the medium in which it is embedded.

STATIONARY FRONT

A front which is nearly stationary or moves very little since the last synoptic position. May be known as a quasi-stationary front.

STATION ELEVATION

The vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station.

STATION PRESSURE

The atmospheric pressure with respect to the station elevation.

STEAM FOG

A type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. Also called sea smoke when it occurs over the ocean.

STORM

An individual low pressure disturbance, complete with winds, clouds, and precipitation. The name is associated with destructive or unpleasant weather. Storm-scale refers to disturbances the size of individual thunderstorms.

STORM PREDICTION CENTER (SPC)

A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the United States. Formerly known as the Severe Local Storms (SELS) unit of the National Severe Storms Forecast Center.

STORM TRACKS

The path or tracks generally followed by a cyclonic disturbance.

STORM WINDS

On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).

STRAIGHT-LINE WINDS

Any surface wind that is not associated with rotation. An example is the first gust from a thunderstorm, as opposed to tornadic winds.

STRATIFORM

Clouds composed of water droplets that exhibit no or have very little vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus. The opposite in type are the vertical development of cumuliform clouds.

STRATOCUMULUS

A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them.

STRATOPAUSE

The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temperature with increasing altitude.

STRATOSPHERE

The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.

STRATUS

One of the three basic cloud forms (the others are cirrus and cumulus. It is also one of the two low cloud types. It is a sheetlike cloud that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth's surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces heavy precipitation. Clouds producing heavy precipitation may exist above a layer of stratus.

STRATUS FRACTUS

Stratus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds.

SUBLIMATION

The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). The opposite of crystallization.

SUBPOLAR

The region bordering the polar region, between 50 and 70 North and South latitude. This is generally an area of semi-permanent low pressure that exists and where the Aleutian and Icelandic Lows may be found. However, a dome of high pressure may form over the cold continental surfaces during the winter, for example, the North American High and the Siberian High.

SUBREFRACTION

Less than normal bending of light or a radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.

SUBSIDENCE

A sinking or downward motion of air, often seen in anticyclones. It is most prevalent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.

SUBTROPICAL

The region between the tropical and temperate regions, an area between 35 and 40 North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.

SUBTROPICAL AIR

An air mass that forms over the subtropical region. The air is typically warm with a high moisture content due to the low evaporative process.

SUBTROPICAL JET

Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25 and 35 North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.

SUMMATION LAYER AMOUNT

The amount of sky cover for each layer is given in eighths of sky cover attributable to clouds or obscurations. The summation amount for any given layer is equal to the sum of the sky cover for the layer being evaluated plus the sky cover for all lower layers, including partial obscuration. A summation amount for a layer can not exceed 8/8ths.

SUMMER

Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the Southern Hemisphere.

SUN DOG

Either of two colored luminous spots that appear at roughly 22 on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. The scientific name for sun dogs is parhelion and they are also known as mock suns.

SUN PILLAR

Horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's surface, reflect sunlight into vertical sun pillars for a spectacular display.

SUNRISE

The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level.

SUNSET

The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level.

SURGE

The increase in sea water height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.

SUPERCELL

A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.

SUPERCOOLING

The reduction of the temperature of any liquid below the melting point of that substance's solid phase. Cooling a substance beyond its nominal freezing point. Supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.

SUPERREFRACTION

Greater than normal bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature.

SURFACE BOUNDARY LAYER

The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.

SWELL

Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch.

SYNOPTIC CHART

Any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time.

SYNOPTIC SCALE

The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more.

SYZYGY

The points in the moon's orbit about the earth at which the moon is new or full.

TELECONNECTIONS

Information used by forecasters to determine what the weather might be elsewhere when compared with past weather conditions at the same degree of longitude.

TEMPERATE CLIMATE

Climates with distinct winter and summer seasons, typical of regions found between the Tropics of Cancer and Capricorn and the Arctic and Antarctic Circles. Considered the climate of the middle latitudes.

TEMPERATURE

The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.

TERMINAL DOPPLER WEATHER RADAR (TDWR)

Doppler radar installed at major airports throughout the United States to detect microbursts.

TERRESTRIAL RADIATION

Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space.

TEXAS NORTHER

Local name in the south-central Great Plains for strong winter winds blowing north or northwest following a sharp cold front with dropping temperatures. Marked by a dark, blue-black sky.

THAW

A warm spell of weather when ice and snow melt. To free something from the binding action of ice by warming it to a temperature above the melting point of ice.

THEODOLITE

An optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles.

THERMAL LOW

Also known as heat low, it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. It tends to remain stationary over its source area, with weak cyclonic circulation. There are no fronts associated with it. An example is the low that develops over southwestern United States and northwestern Mexico during the summer months.

THERMOCLINE

A vertical negative temperature gradient in some layer of a body of water which is appreciably greater than the gradients above and below it. In the ocean, this may be seasonal, due to the heating of the surface water in the summer, or permanent.

THERMODYNAMICS

Study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body.

THERMOGRAPH

Essentially, a self-recording thermometer. A thermometer that continuously records the temperature on a chart.

THERMOHALINE

In oceanography, it pertains to when both temperature and salinity act together. An example is thermohaline circulation which is vertical circulation induced by surface cooling, which causes convective overturning and consequent mixing.

THERMOMETER

An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.

THERMOSPHERE

A thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. It is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere.

THICKNESS

The thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. The layer most often used in meteorology is between 1000 and 500 millibars. There can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. For example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. Likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer.

THUNDER

The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 C in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

THUNDER SNOW

A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. Intense snowfall rates often occur during these situations.

THUNDERSTORM

Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.

TIDE

The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth's waters.

TILT

The inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. For example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere.

TIROS

A series of Television InfraRed Observation Satellites that demonstrated the feasibility and capability of observing the cloud cover and weather patterns of earth from space. An experimental program, it was the first spaceborne system that allowed meteorologists to acquire information that was immediately put to use in an operational setting. The first U.S. weather satellite, TIROS I, was launched on April 1, 1960, and TIROS X, the last of the series, was launched on July 2, 1965.

TORNADO

A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.

TORNADO ALLEY

A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In terms of sheer numbers, this section of the United States receives more tornadoes than any other.

TOWERING CUMULUS

Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. Its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.

TRACE

Generally, an unmeasurable or insignificant quantity. A precipitation amount of less than 0.005 inch.

TRADE WINDS

Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. Primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.

TRAJECTORY

The curve that a body, such as a celestial object, describes in space. This applies to air parcel movement also.

TRANSLUCENT

Not transparent, but clear enough to allow light to pass through.

TRANSMISSOMETER

An electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. By showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined.

TRANSPARENT

A condition where a material is clear enough not to block the passage of radiant energy, especially light.

TRANSPIRATION

The process by which water in plants is transferred as water vapor to the atmosphere.

TRIPLE POINT

The point at which any three atmospheric boundaries meet. It is most often used to refer to the point of occlusion of an extratropical cyclone where the cold, warm, and occluded fronts meet. Cyclogenesis may occur at a triple point. It is also the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium.

TROPICS/TROPICAL

The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitiation during part of the year.

TROPICAL AIR MASS

An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.

TROPICAL CYCLONE

A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.

TROPICAL DEPRESSION

A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.

TROPICAL DISTURBANCE

An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.

TROPICAL PREDICTION CENTER (TPC)

A division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities. The National Hurricane Center is a branch.

TROPICAL STORM

A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.

TROPICAL WAVE

Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

TROPIC OF CANCER

The most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees North latitude.

TROPIC OF CAPRICORN

The most southern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees South latitude.

TROPOPAUSE

The boundary zone or transition layer between the troposphere and the stratosphere. This is characterized by little or no increase or decrease in temperature or change in lapse rate with increasing altitude.

TROPOSPHERE

The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude.

TROUGH

An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. The opposite of a ridge.

TSUNAMI

An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower water. Also known as a seismic sea wave, and incorrectly, as a tidal wave.

TULE FOG

Ground fog in the central valley of California and the leading cause of weather-related casualties in that state. It forms at night and in the early morning when the ground cools, lowering the air temperature near the ground to or below its initial dew point.

TURBULENCE

The irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.

TWILIGHT

Often called dusk, it is the evening period of waning light from the time of sunset to dark. The time of increasing light in the morning is called dawn. Twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. Civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. Nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.

TWISTER

A slang term used in the United States for a tornado.

TYPHOON

The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.

ULTRAVIOLET

Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.

UNDERCAST

In aviation, it is an opaque cloud layer viewed from an observation point above the layer. From the ground, it would be considered an overcast.

UNITED STATES WEATHER BUREAU

The official name of the National Weather Service prior to 1970.

UNIVERSAL TIME COORDINATE

One of several names for the twenty-four hour time which is used throughout the scientific and military communities.

UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH (UCAR)

A non-profit university membership consortium which carries out programs to benefit atmospheric, oceanic, and related sciences around the globe. Among other activities, UCAR operates the National Center for Atmospheric Research (NCAR) with National Science Foundation sponsorship.

UNSTABLE/ INSTABILITY

Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own.

UPDRAFT

A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development.

UPPER AIR/UPPER LEVEL

The portion of the atmosphere which is above the lower troposphere. It is generally applied to the levels above 850 millibars. Therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface.

UPSLOPE EFFECT

The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable, stratiform clouds and precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.

UPSLOPE FOG

Fog that forms when warm, moist surface air is forced up a slope by the wind. It is adiabatically cooled to below its initial dew point, which means the air cools by expansion as it rises. It forms best where there is a gradual slope, and it can become quite deep, requiring considerable time to dissipate.

UPWELLING

The process by which water rises from a lower to a higher depth, usually as a result of divergence and offshore currents. It influences climate by bringing colder, more nutrient-rich water to the surface. A vital factor of the El Nio event.

UPWIND

The direction from which the wind is blowing. Also the windward side of an object. The opposite of the downwind or leeward side.

VALLEY BREEZE

An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.

VAPOR PRESSURE

The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.

VAPOR TRAIL

A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.

VARIABLE CEILING

Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.

VEERING

A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen horizontally or vertically (with height). For example, the wind shifts from the north to the northeast to the east. It is the opposite of backing.

VERNAL EQUINOX

Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.

VERTICAL TEMPERATURE PROFILE

A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VERTICAL VISIBILITY

The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.

VERTICAL WIND PROFILE

A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

VIRGA

Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.

VISIBILITY

A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

VISIBLE LIGHT

The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays.

VISUAL FLIGHT RULES (VFR)

Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.

VORTEX

Any circular or rotary flow in the atmosphere that possesses vorticity.

VORTICITY

The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.

VORTICITY MAXIMUM

A center of vorticity, or the maximum of the vorticity field of a fluid.

WALKER CIRCULATION

A deep east-west overturning in the atmosphere normally confined to within about 20 degrees latitude of the equator extending from low-levels to near the tropopause. One side of the Walker Circulation is associated with rising motion, clouds, and rain while the opposite side with sinking motion, generally fair weather and little or no rain. This circulation is large in spatial extend normally extending anywhere from way around the world or completely around the world (thus either 1 or 2 upward and 1 or 2 downward branches to the circulation). The rising and sinking branches are climatologically anchored to specific geographical locations, but these locations are seasonally dependent. In winter the the rising branch is centered near the "maritime continent" of greater Indonesia while in summer it is centered near southern India. The sinking branches are generally found in the eastern equatorial Pacific in winter and summer. Sub-branches of rising motion are generally tied to equatorial areas of South America and Africa in summer.

WALL CLOUD

An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. Forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. It often develops before strong or violent tornadoes.

WARM

To have or give out heat to a moderate or adequate degree. A subjective term for temperatures between cold and hot. In meteorology, an air parcel that is warm is only so in relation to another parcel.

WARM ADVECTION

The horizontal movement of warmer air into a location.

WARM FRONT

The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produced fog in the warm air.

WARM HIGH

A high pressure system that has its warmest temperatures at or near the center of circulation. Contrast with a cold high.

WARM LOW

A low pressure system that has its warmest temperatures at or near the center of circulation. Also referred to as a warm core low.

WARNING

A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

WASATCH WINDS

Strong winds blowing easterly out of the Wasatch Mountains in Utah, sometimes reaching speeds greater than 75 miles per hour.

WATCH

A forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows.

WATER

Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life-sustaining part of life on earth.

WATER CYCLE

The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

WATERSPOUT

A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or towering cumulus clouds and is the water equivalent of a landspout.

WATER VAPOR (H2O)

Water in gaseous form. It is one of the most import constituents of the atmosphere. Due to its molecular content, air containing water vapor is lighter than dry air. This contributes to the reason why moist air has a tendency to rise.

WAVE(S)

In general, any pattern with some roughly identifiable periodicity in time and/or space. It is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. In meteorology, this applies to atmospheric waves, such as long waves and short waves. In oceanography, this applies to waves generated by mechanical means, such as currents, turbidity, and the wind.

WAVE CYCLONE

A cyclone which forms and moves along a front. The circulation around the cyclone's center produces a wavelike deformation on the front. May also be call a migratory cyclone or low.

WAVE LENGTH

The least distance between particles moving in the same phase of oscillation of a wave. In oceanography, it is the horizontal distance between the highest parts of two successive wave crests above the still water level, separated by a trough that is below the still water level, and it is measured in meters.

WAXING CRESCENT MOON

The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

WAXING GIBBOUS MOON

The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

WEATHER

The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind,

WEATHERING

The decay and breakup of rocks on the earth's surface by natural chemical and mechanical processes. The mechanical action includes large changes of temperature, extreme temperatures, frost, or the impact of wind borne sand or water. Chemical action includes the chemical reactions between atmospheric constituents in a moist environments or in rain water. Biological agents are mainly fungi which attack organic material.

WEATHER SURVEILLANCE RADAR (WSR-88D)

The newest generation of Doppler radars, the 1988 Doppler weather radar. The radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm.

WEATHER VANE

Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses.

WEDGE

Primarily refers to an elongated area of shallow high pressure at the earth's surface. It is generally associated with cold air east of the Rockies or Appalachians. It is another name for a ridge, ridge line, or ridge axis. Contrast with a trough. Wedge is also a slang term for a large, wide tornado with a wedge-like shape.

WESTERLIES

Usually applied to the broad patterns of persistent winds with a westerly component. It is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere. Near the earth's surface, the westerlies extend from approximately 35 to 65 degrees latitude, while in the upper levels they extend further polarward and equatorward.

WEST VIRGINIA HIGH

An area of stagnant high pressure located over West Virginia during Indian Summer.

WET BULB DEPRESSION

Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

WET BULB THERMOMETER

A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

WHIRLWIND

A small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear, dry, hot afternoons. Often called a dust devil when visible by the dust, dirt or debris it picks up. Also slang for a landspout or a tornado.

WHITEOUT

When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.

WIND

Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

WIND CHILL INDEX

The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.

WIND DIRECTION

The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc.).

WIND SHEAR

The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.

WIND SHIFT

The term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. It may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt.

WIND SPEED

The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

WIND VANE

An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.

WINDWARD

The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.

WIND WAVE

An ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its fetch area, it is considered a swell.

WINTER

Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August in the Southern Hemisphere.

WINTER STORM

Any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice.

WORLD METEOROLOGICAL ORGANIZATION (WMO)

From weather prediction to air pollution research, climate change related activities, ozone layer depletion studies and tropical storm forecasting, the World Meteorological Organization coordinates global scientific activity to allow increasingly prompt and accurate weather information and other services for public, private and commercial use, including international airline and shipping industries. Established by the United Nations in 1951, it is composed of 184 members.

X-RAYS

The portion of the electromagnetic spectrum that has a very short wave length. It has a wave length longer than gamma rays, yet shorter than visible light. X-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization.

YEAR

The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.

YELLOW SNOW

Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

ZENITH

The point which is elevated 90 degrees from all points on a given observer's astronomical horizon. The point on any given observer's celestial sphere that lies directly above him. The opposite of nadir.

ZODIAC

The position of the sun during the course of the year as it appears to move though successive constellations. Also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except Venus and Pluto.

ZONAL FLOW

The flow of air along a latitudinal component of existing flow, normally from west to east.

ZONAL INDEX

The measure of the strength of the westerly winds of the middle latitudes. It is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind.

ZULU TIME

One of several names for the twenty-four hour time which is used throughout the scientific and military communities.

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