climatology and meteorology

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air mass

an extensive body of the atmosphere in which physical properties, particularly temperature and humidity, exhibit only small and continuous differences in the horizontal plane; it may extend over an area of several million square kilometers and over a height of several kilometers.

air temperature

the ambient temperature indicated by a thermometer exposed to the air but sheltered from direct solar radiation, or placed in an instrument shelter 1.5 to two meters above ground; also called surface temperature.

air thawing index

the cumulative number of degree-days above 0°C for the air temperature during a given period.


a non-dimensional, unitless quantity that measures how well a surface reflects solar energy; ranges from 0 - 1; a value of 0 means the surface is a “perfect absorber,” where all incoming energy is absorbed, a value of 1 means the surface is a “perfect reflector,” where all incoming energy is reflected and none is absorbed.

Alberta low

a low centered on the eastern slope of the Canadian Rockies in the province of Alberta, Canada.

Aleution low

the low pressure center located near the Aleutian Islands on mean charts of sea level pressure during the winter; it represents one of the main centers of action in the atmospheric circulation of the northern hemisphere.


a principal middle level cloud type (cloud genus), white or gray, or both white and gray, which occurs as a layer or patch with a wave aspect, the elements of which appear as laminae, rounded masses, rolls, etc; these elements usually are sharply outlined, but they may become partly fibrous or diffuse; they may or may not be merged, and they generally have shadowed parts; most of the regularly arranged altocumulus elements have an apparent width of 1 - 5 degrees.


a principal middle level cloud type (cloud genus) in the form of a grayish (gray) or bluish cloud sheet or layer of striated, fibrous, or uniform appearance, totally or partly covering the sky; the layer has parts thin enough to reveal the position of the sun; halo phenomena do not usually occur.

anabatic wind

any wind blowing up a steep incline or mountain; the opposite of katabatic wind.


in synoptic meteorology, a detailed study of the state of the atmosphere over a particular region based on the actual observations.