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Air Masses

The most common definition of an air mass is a large body of air, relatively homogeneous with respect to temperature, humidity, and vertical lapse rates. While this definition is generally valid, it is true only for air masses that remain within their source regions. Once in motion, air masses begin to modify, in some cases quickly, especially if they move significant latitudinal distances. Eventually, modification renders the air mass unrecognizable with respect to its source region.

Source Regions

Air masses form in regions of generally uniform surfaces where winds are typically light for extended periods of time, usually days to weeks. Over periods of time, the overlying air takes on the properties of the underlying surface through radiative and convective processes. These regions tend to be located near the centers of semipermanent high-pressure centers due to the weak pressure gradients found in these regions. Climatologically, these semipermanent highs are located in the polar and subtropical latitudes.

In the Northern Hemisphere there is little land in the polar latitudes, so in most cases air mass source regions are located over pack ice. The thermal character of pack ice is much different from that of snow-covered land, with the underlying Arctic Ocean exchanging heat with the overlying atmosphere. As a result, winter polar air masses, with the Siberian exception, are milder than in the Southern Hemisphere. For this reason, air masses that form in the “High Arctic” are commonly referred to as polar, while only Antarctic and Siberian air masses are considered truly Arctic in nature.

There is also much variability among subtropical source regions due to land cover type. In general, land surfaces within subtropical highs are desert-like, with very little water available. As a result, more of the incoming solar radiation that reaches the surface is available to heat the air, so dry bulb temperatures are quite high and dew point temperatures quite low. The result is very low relative humidity and great drying power of the air. Ocean surfaces, on the other hand, convert a significant proportion of solar irradiance to latent heat, which leaves much less sensible heat to increase the air temperature.

Air Mass Types

Based on common reference to air mass source regions as polar, subtropical, land covered (continental), or maritime, four primary air mass types are recognized: (1) continental polar (cP), (2) maritime polar (mP), (3) continental tropical (cT), and (4) maritime tropical (mT). Continental arctic (cA) air is usually considered a special case of cP air, while maritime arctic air (mA) is an air mass rarely recognized because for air to be considered truly arctic, it must develop over an ice-covered surface.

This classification scheme has become popular among geographers because of its simplicity, which has been considered an important consideration in introductory courses. For example, there are cold air masses originating in the higher latitudes that are either moist (mP) or dry (cP and cA), depending on whether the surface type over which they formed was ocean or land, and warm air masses that develop in the lower latitudes that are either moist (mT) or dry (cT).

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