airmass classification

A system used to identify and to characterize the different air masses according to a basic scheme.

A number of systems have been proposed, but the Bergeron classification has been the most widely accepted. In this system, air masses are designated first according to the thermal properties of their source regions: tropical (T); polar (P); and less frequently, arctic or antarctic (A). For characterizing the moisture distribution, air masses are distinguished as to continental (c) and maritime (m) source regions. Further classification according to whether the air is cold (k) or warm (w) relative to the surface over which it is moving indicates the low-level stability conditions of the air, the type of modification from below, and is also related to the weather occurring within the air mass. This outline of classification yields the following identifiers for air masses: cTk, cTw, mTk, mTw, cPk, cPw, mPk, mPw, cAk, mAk, mAw; the last of which is never used. H. C. Willett, in his classification, introduces further distinction between stable (s) and unstable (u) conditions in upper levels. Some authors may include equatorial (E), monsoon (M), or superior air (S) in their classifications. Others prefer to omit the arctic (A) type and describe all air masses on the basis of polar and tropical air, separated by the polar front.
Byers, H. R. 1944. General Meteorology. p. 247.
Hewson, E. W., and R. W. Longley 1944. Meteorology, Theoretical and Applied. p. 249.
Willett, H. C. 1944. Descriptive Meteorology. 183–191.
Palmén, E. 1951. Compendium of Meteorology. 599–620.

• INDEX
• A
• B
• C
• D
• E
• F
• G
• H
• I
• J
• K
• L
• M
• N
• O
• P
• Q
• R
• S
• T
• U
• V
• W
• X
• Y
• Z
• 0
• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9

1. A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region.
   An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent. The stagnation or long-continued motion of air over a source region permits the vertical temperature and moisture distribution of the air to reach relative equilibrium with the underlying surface.
   See airmass classification.
   
2. In radiation, the ratio of the actual path length taken by the direct solar beam to the analogous path when the sun is overhead from the top of the atmosphere to the surface.
   Extrapolation of surface measurements to zero air mass was the original method for estimating the value of solar irradiance at the top of the atmosphere.
   
3. 
   See optical air mass.