| Section A | A index | 351-359 of 917 terms |
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airlight—Light scattered into quasi-horizontal viewing directions by the atmosphere's molecules and small particles (usually excluding fog and raindrops). On clear to partly cloudy days, airlight is dominated by singly scattered sunlight, but it also includes multiply scattered sunlight from clouds, the surface, and the clear sky. A more restrictive definition limits airlight to clear days; a less restrictive one does not limit the elevation angles at which airlight is seen. As haze concentrations increase, airlight radiances increase near the horizon, although not without limit. Increased airlight reduces a distant object's contrast, thus reducing its visibility. At sufficiently large horizontal optical thicknesses, airlight reduces this contrast below the threshold contrast, and the object is then visually indistinguishable from its surroundings. Airlight and the artistic term aerial perspective are related, since airlight causes the reduced contrast associated with increased distance to an object. Airlight is distinct from airglow that originates in the upper atmosphere.
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airmass analysis—In general, the theory and practice of surface synoptic chart analysis by the so- called Norwegian methods, which involve the concepts of the polar front and of the broad-scale air mass that it separates. Airmass analysis of surface charts may be said to consist of 1) determining the extent, the physical and stability properties, the movements, and the modifications of each of the air masses on the chart; 2) locating with some precision the fronts separating the air masses, and analyzing the structure and motion of the fronts; 3) analyzing wave perturbations on the fronts; and 4) describing and explaining the weather on the basis of the above factors.
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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.
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airmass climatology—The representation of the climate of a region by the frequency and characteristics of the air masses under which it lies; basically, a type of synoptic climatology. It is a development of the representation of the weather associated with winds of different directions (thermal wind roses, etc.) by taking account of the source and trajectory of the air, and it gives a more dynamic picture of the climate than do monthly averages. The first detailed study was by E. Dinies (1932). Dinies gave tables of average temperature, humidity, and cloudiness in Germany associated with eight classes of air in winter and summer (Durst 1951). Similar tables have since been constructed for many parts of the world.
Dinies, E., 1932: Luftkörper-Klimatologie. Aus. d. Arch. dtsch. Seew, 50, No. 6
Durst, C. S., 1951: Compendium of Meteorology, 967–975.
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airmass modification—The change of characteristics of an air mass as it moves away from its region of origin. For example, maritime air at midlatitudes that originally has high humidity and cool temperatures can be modified as it blows onshore over coastal mountains, where orographic precipitation causes the air to become drier and warmer by the time it reaches the lee side of the mountains. The rate of airmass modification depends on the differences between its original characteristics and those of the new surface over which it flows.
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airmass source region—Region where air masses originate and acquire their horizontal homogeneous properties of temperature and moisture. Horizontal homogeneity of the air mass is produced by prolonged contact (days to weeks) with the underlying surface. Main source regions are those in which the permanent or semipermanent anticyclones occur.
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