air mass

air mass A large body of air, whose properties (temperature, humidity, and lapse rate) are essentially homogeneous over an area several hundred kilometres across. Fronts form the boundaries between air masses with differing properties.

The nature of air masses is determined by three factors: the properties of the source region; the age of the air mass; and the changes that occur as it moves across the globe. The principal source regions are the semipermanent high-pressure zones (the subtropical and polar anticyclones), together with the continental anticyclones that develop in winter. Primary classification is based on temperature, giving Arctic (A), polar (P), or tropical air (T); and on the nature of the surface in the source region: maritime (m), or continental (c), the latter symbols being used as a prefix:

mA	maritime arctic air
cA	continental arctic air
mP	maritime polar air
cP	continental polar air
mT	maritime tropical air
cT	continental tropical air

Various additional and secondary types are recognized, in particular equatorial air (E), Antarctic air (A or occasionally AA), and Mediterranean air. Air masses may be modified as they move away from their source regions. If continental Arctic (cA) air crosses the sea, for example, it becomes maritime Arctic (mA) air.

Additional classifications are sometimes used to indicate how in the northern hemisphere (in particular), polar air may move towards the Equator and acquire a particular character and then subsequently be drawn back towards the pole. A suffix indicates whether the air is warmer (w) or colder (k) than the surface. The former becomes more stable, and the latter more unstable. Older works may use the concept of returning air (r) and these also often reverse the order of the symbols, with Tc for continental tropical or rPm for returning maritime polar air
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