An air mass is a body of air which exhibits similar characteristics over a wide area, usually spanning several hundred miles.  An air mass will have consistent temperature gradients, i.e. the temperature at a given altitude in one part will be the same at that alitude in another part of the air mass.  Likewise, similarities of pressure & humidity will be found throughout the region.  Air bodies of different temperatures, humidity, and pressure mix surprisingly little, so these air masses remain quite distinct over days or weeks, even as they move from area to area, and thus have long term effects on weather conditions.

Air masses are classified by their place of origin.  There are four primary types of air mass:
Tropical air masses are warm, polar air masses are cold; continental air masses are dry, maritime air masses are moist.
Additional classifications of "cA", continental Arctic for extremely cold cP that occasionally comes south and "mE", maritime Equatorial for very hot and humid air from the Equator, are occasionally used, but they are more names for the extreme variations of cP and mT air masses than truly useful classifications.

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