Terms used by meteorologists, forecasters, weather observers, and
in weather forecasts
Compiled from several sources
Ablation- The process of being removed. Snow
ablation usually refers to removal by melting
Absolute Humidity- The density of water vapor. It is the mass of
the water vapor divided by the volume that it occupies.
Accretion- Growth of precipitation particles by collision of ice
crystals with supercooled liquid droplets which freeze on impact
Accessory Clouds- Clouds that are dependent on a larger cloud
system for development and continuance. Accessory clouds associated with the
thunderstorm include roll, shelf, mammatus, and wall clouds.
Acid Rain- Cloud or rain droplets containing pollutants, such as oxides
of sulfur and nitrogen, to make them acidic (e.g. pH < 5.6).
Additive data- A group of coded remarks in a weather observation
that includes pressure tendency, amount of precipitation, and maximum/minimum
temperature during specified periods of time.
Adiabatic- changes in temperature caused by the expansion (cooling)
or compression (warming) of a body of air as it rises or descends in the
Adiabatic Process- The change of temperature of air without transferring
heat. In an adiabatic process compression results in warming, and
expansion results in cooling.
Advection- The horizontal transport of air, moisture or other
atmospheric properties. Commonly used with temperatures, i.e., "warm air
Advection Fog- a type of fog that results from the advection of moist
air over a cold surface and the cooling of the air to its dew point that
follows; this type of fog is most common in coastal regions.
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.
Aerosol- Particles of matter, solid or liquid, larger than a molecule
but small enough to remain suspended in the atmosphere (up to 100� m diameter). Natural origins include salt particles from sea spray
and clay particles as a result of weathering of rocks. Aerosols can also
originate as a result of man's activities and in this case are often considered
Aerovane- Aerovanes are commonly used at many weather stations and
airports to measure both wind direction and speed. They are similar to wind
vanes and cup anemometers except have three-bladed propellers attached to the
end of the vane.
AFOS- the Automation of Field Operations and Services; AFOS is the
computer system that links National Weather Service offices and other computer
networks, such as the NOAA Weather Wire, to transmit weather information.
AGL- above ground level.
Air- the mixture of gases that make-up the earth's atmosphere.
Air Mass- A large body of air that has 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.
Air Parcel- An imaginary small body of air that is used to explain
the behavior of air. A parcel is large enough to contain a very great number of
molecules, but small enough so that the properties assigned to it are
approximately uniform throughout.
Air Pollution- The existence
in the air of substances in concentrations that are determined unacceptable.
Contaminants in the air we breathe come mainly from manufacturing industries,
electric power plants, automobiles, buses, and trucks.
Air Pressure- (atmospheric pressure) air pressure is the force exerted
on a surface by the weight of the air above it. The internationally recognized
unit for measuring this pressure is the kilopascal.
Airstream- A significant body of air flowing in the same general
Albedo- The percentage of light reflected by an object. Snow covered
areas have a high albedo (0.9 or 90%) due to their white color.
Alberta Clipper- A small, fast-moving low-pressure system that forms in
western Canada and travels southeastward into the United States. These storms,
which generally bring little precipitation, generally precede an Arctic air
Altimeter- An active instrument (see active system) used to measure the
altitude of an object above a fixed level.
Altimeter setting- That 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- Height expressed as the distance above a reference point,
which is normally sea level or ground level.
Altocumulus- Mid-altitude clouds with a cumuliform shape.
clouds with a flat sheet-like shape.
Anabatic- wind flowing up an incline, such
as up a hillside; upslope wind.
Anafront- A front at which the warm is ascending the frontal surface up
to high altitudes.
Anemometer- An instrument that measures wind speed.
Aneroid barometer- An instrument built around a metal structure that
bends with changing air pressure. These changes are recorded on a pointer that
moves back and forth across a printed scale.
Angular Momentum- the energy of motion of a spinning body or mass of air
Angular Velocity- the rate at which a spinning body rotates.
Anomaly- The deviation of (usually) temperature or precipitation in a
given region over a specified period from the normal value for the same
Anticyclone- A large body of air in which the atmospheric pressure is
higher than the pressure in the surrounding air. The winds blow clockwise
around an anticyclone in in the Northern Hemisphere.
Anticyclonic- describes the
movement of air around a high pressure, and rotation about the local
vertical opposite the earth's rotation. This is clockwise in the Northern
Anvil Cloud- 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 Crawler - 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 - A large overshooting top or penetrating top.
Anvil Rollover - 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 - Frequent (often continuous or nearly continuous),
localized lightning discharges occurring from within a thunderstorm
Arctic Air- a mass of very cold, dry air that usually originates over
the Arctic Ocean north of Canada and Alaska.
Arctic High- a very cold high pressure that originates over the Arctic
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.
Aridity- A general term used to describe areas suffering from lack
of rain or drought. More specifically, a condition in which evaporation exceeds
ASOS- Automated Surface Observing System. This system observes sky
conditions, temperature and dewpoint, wind direction and speed, and barometric
pressure, and precipitation.
Atmosphere- The mass of air surrounding the earth and bound to it more
or less permanently by the earth's gravitational attraction.
Atmospheric Pressure- (also called air pressure or barometric pressure)
The pressure asserted by the mass of the column of air directly above any
Atmospheric Stability- An indication of how easily a parcel of air is
lifted. If the air is very stable it is difficult to make the parcel
rise. If the air is very unstable the parcel may rise on its own once
Aurora Borealis- Also known as the northern lights - The luminous,
radiant emission from the upper atmosphere over middle and high latitudes, and
centered around the earth's magnetic poles. These silent fireworks are often
seen on clear winter nights in a variety of shapes and colors.
Automated Weather Station- An unmanned station with various sensors
that measure weather elements such as temperature/wind/pressure and transmit
these readings for use by meteorologists.
VHRR- Advanced Very High Resolution Radiometer. Main sensor on U.S.
polar orbiting satellites.
Avalanche- a large mass of rapidly moving snow down a steep mountain
AVN- Aviation Model generated every 12
hours by NCEP.
AWIPS- Advanced Weather Information Processing
System. New NWS computer system integrating graphics, satellite and radar
imagery. The successor to AFOS.
Back Door Cold Front- A front that moves east to west in direction
rather than the normal west to east movement. For instance, one that enters
Southern New England from the Gulf of Maine.
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
Back-sheared Anvil- 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.
Backing Wind- Wind which shifts 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). Backing winds with height are indicative of
cold air advection (CAA). The opposite of veering winds.
Ball lightning- A relatively rarely seen form of lightning,
generally consisting of an orange or reddish ball of the order of a few cm to
30 cm in diameter and of moderate luminosity, which may move up to 1 m/s
horizontally with a lifetime of a second or two.
Barber Pole- 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.
high pressure area (anticyclone), often aloft, that remains nearly stationary
or moves slowly compared to west-to-east motion. It blocks the movement
eastward movement of low pressure areas (cyclones) at its latitude..
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.
Barogram- The graphic record of pressure produced by a barograph.
Barograph- An instrument that provides a continuous record of
Barometer- An instrument for measuring atmospheric pressure.
Barometric pressure- The actual pressure value indicated by a pressure
Barometric Tendency- The amount and direction of change in
barometer readings over a three-hour period.
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
Bear's Cage- 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 Scale- A scale that indicates the wind speed using the effect
wind has on certain familiar objects.
Beaver('s) Tail- 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.
Black Ice- thin, new ice that forms on fresh water or dew covered
surfaces; it is common on roadways during the fall and early winter and appears
"black" because of its transparency.
Blizzard- Includes winter storm conditions of sustained winds or
frequent gusts of 35 mph or more that cause major blowing and drifting of
snow, reducing visibility to less than one-quarter mile for 3 or more
hours. Extremely cold temperatures often are associated with dangerous
Blizzard warning- Issued when blizzard condition are expected or
Blowing Dust- dust that is raised by the wind to moderate heights above
the ground to a degree that horizontal visibility decreases to less than seven
miles. Visibilities of 1/8 mile or less over a widespread area are
criteria for a Blowing Dust Advisory.
Blowing Sand- Sand particles picked up from the surface of the
earth by the wind to moderate heights above the ground, reducing the reported
horizontal visibility to less than 7 statute miles.
Blowing Snow- Wind driven snow that reduces visibility to six miles
or less causing significant drifting. Blowing snow may be snow that is falling
and/or loose snow on the ground picked up by the wind.
Blowing spray- Water droplets torn by the wind from a body of
water, generally from the crests of waves, and carried up into the air in such
quantities that they reduce the reported horizontal visibility to less than 7
Blustery- Descriptive term for gusty winds that accompany cold
Bomb Cyclone- An extratropical area of low pressure in which the central
pressure drops at least 24 millibars in 24 hours.
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.
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 - especially in the left (usually northern)
end, where the circulation exhibits cyclonic rotation.
Box (or Watch Box) - A severe thunderstorm or tornado watch.
Breezy- Wind in the range of 15 mph to 25 mph with mild or warm
Brisk- Wind in the range of 15 to 25 mph when the temperature is cold.
Broken Clouds- Clouds which cover between 5/8ths and 7/8ths of the
Buoyancy- That property of an object that enables it to float on
the surface of a liquid, or as in the case with air parcels, to ascend and
remain freely suspended in the atmosphere.
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.
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
Bust- An inaccurate forecast, usually a situation in which
significant weather is expected, but does 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
CAA- Cold Air Advection
Calm- the absence of apparent motion in the air.
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.
CAPE- Convective Available Potential Energy. A measure of the
amount of energy available for convection. CAPE is directly related to the
maximum potential vertical speed within an updraft; thus, higher values
indicate greater potential for severe weather. Observed values in thunderstorm
environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme
cases may exceed 5,000 j/kg. However, as with other indices or indicators,
there are no threshold values above which severe weather becomes imminent.
Cb- Cumulonimbus cloud
Ceiling- The height of the lowest layer of broken or overcast
Ceilometer- A device used to evaluate the height of clouds or the
vertical visibility into a surface-based obscuration.