Weather Terms and Definitions

This page provides some basic weather terms and definitions which may appear on our site. At the end of some terms are links to additional information.


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-A-

Weather FAQ:
AC - Anticipated Convection



Weather FAQ:
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.


Weather FAQ:
Accessory Cloud



Weather FAQ:
Advection



Weather FAQ:
Advisories/Statements/Watches/Warnings

  • An advisory is for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property.
  • A statement generally provides additional or followup information to an existing weather condition.
  • A watch is used when the risk of a hazardous weather event has increased significantly, but its occurrence, locations, and/or timing is still uncertain. It is intended to provide advance notice of possible inclement weather.
  • A warning is used for conditions posing an immediate threat to life or property. Depending on the type of warning, you should take immediate, appropriate action.


Weather FAQ:
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.
  • See popcorn convection.
  • 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.


Weather FAQ:
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.


Weather FAQ:
Anticyclonic Rotation

  • Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.


Weather FAQ:
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 Crawler
  • [Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil.
  • They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.
Anvil Dome
Anvil Rollover
Anvil Zits
  • [Slang], frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.


Weather FAQ:
AP - Anomalous Propagation

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


Weather FAQ:
Apparent Temperature

  • A measure of the health risk due to various combinations of high temperature and humidity. The higher the number, the greater the possibility of heat exhaustion or heat stroke.

  Wikipedia: Apparent Temperature



Weather FAQ:
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.


Weather FAQ:
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.


Weather FAQ:
Atmospheric Pressure

Weather FAQ:
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 120 hours only from the 0000 and 1200 runs.
  • At 0600 and 1800, the model is run only out to 72 hours.



-B-

Weather FAQ:
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.


Weather FAQ:
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.


Weather FAQ:
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.
  • See Figure 7.


Weather FAQ:
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.


Weather FAQ:
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.


Weather FAQ:
Barometric Pressure

  • The air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure.
  • Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool.
  • High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather.
  • For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure.
  • In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.

  Wikipedia: Atmospheric/Barometric Pressure



Weather FAQ:
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 - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface.
  • In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.
  • As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere.
  • While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.


Weather FAQ:
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.


Weather FAQ:
Beaufort Wind Scale

  • First used in the early nineteenth century by the British Navy. For every number on the scale, there is a descriptive term such as light air, strong breeze, fresh gale, etc.
  • See Beaufort Wind Scale.

  Wikipedia: Beaufort Wind Scale



Weather FAQ:
Beaver 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.


Weather FAQ:
Blue Watch (or Blue Box)

  • [Slang] A severe thunderstorm watch.


Weather FAQ:
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.


Weather FAQ:
Bow Echo

  • A radar echo which is linear but bent outward in a bow shape (Figure 1). 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 - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.


Weather FAQ:
Box (or Watch Box)



Weather FAQ:
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.


Weather FAQ:
Bulk 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.


Weather FAQ:
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.


Weather FAQ:
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 (Figure 2).
  • 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.





Most of the definitions on this page were borrowed from El Dorado Weather


The "Weather FAQ:" icons on this page are © CarterLake.org
Some of the definitions on this page were borrowed from CarterLake.org


Some of the definitions on this page were borrowed from Davis Instruments