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NOAA - Meteorology glossary
Category: Sciences > Meteorology
Date & country: 14/10/2013, US
Words: 671


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.

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.

Accretion
Growth of precipitation particles by collision of ice crystals with supercooled liquid droplets which freeze on impact

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 atmosphere.

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 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.

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 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.

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.

Airstream
A significant body of air flowing in the same general circulation.

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 mass.

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.

Altostratus
Mid-altitude 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 or water.

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 region.

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 Hemisphere.

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).

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 anvil.

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 Ocean.

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 precipitation.

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 specific point.

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 started.

Aurora Borealis
Also known as the northern lights

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.

Avalanche
a large mass of rapidly moving snow down a steep mountain slope.

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 direction.

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.

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 atmospheric pressure.

Barometer
An instrument for measuring atmospheric pressure.

Barometric pressure
The actual pressure value indicated by a pressure sensor.

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 thunderstorm development.

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.

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 conditions.

Blizzard warning
Issued when blizzard condition are expected or are occurring.

Blocking High
A 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..

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 statute miles.

Blustery
Descriptive term for gusty winds that accompany cold weather.

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.

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 temperatures.

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 sky.

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 supercell development.

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.

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 seen visually.

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 clouds.

Ceilometer
A device used to evaluate the height of clouds or the vertical visibility into a surface-based obscuration.

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

Celsius
a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

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

Chinook Wind
A strong downslope wind that causes the air to warm rapidly as a result of compressive heating; called a foehn wind in Europe.