Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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

Advection - The horizontal movement of things like temperature and moisture by the wind; transport of an atmospheric property by the wind.

Air-mass - A body of air that has similar temperature and moisture characteristics. A Continental Polar air mass would be dry and cool while a maritime tropical air mass would be moist and warm.

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.

Air Pressure - The weight of the air above a given point. Measured with a barometer.

Alberta Clipper
- Low pressure forms east of the Rockies in Alberta, Canada and usually stays well north of Colorado. These storms are named "clipper' because of their speed of 30 to 50 mph as they race eastward. Producing 1-6 inches of dry fluffy snow over the Northern Plains and Great Lakes, these storms rarely produce snow in Colorado, but can create strong Chinook winds over the northern Front Range.

Algorithm - A computer program (or set of programs) that is designed to systematically solve a certain kind of problem. WSR-880 radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.

American Meteorological Society (AMS) - The AMS promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. For more information, contact the AMS with headquarters in Boston, Mass.

Anemometer - An instrument used to measure wind speed.

Anti-Cyclone - A region of higher pressure usually dictated by calm, fair weather. Also called a "high," the wind associated with this system circulates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. A blue "H" on a weather map.

Anti-cyclonic 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.

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 - 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, and mushroom.

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

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.

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.

Autumnal Equinox - A day of equal day and night that marks the beginning of fall and occurs when the sun passes directly over the equator.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society