  250px|thumb|right|different air masses which affect north america, as well as other continents, tend to be separated by frontal boundaries in meteorology, an air mass is a volume of air defined by its temperature and water vapor content. air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. they are classified according to latitude and their continental or maritime source regions. colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. continental and superior air masses are dry while maritime and monsoon air masses are moist. weather fronts separate air masses with different density (temperature and/or moisture) characteristics. once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. classification schemes tackle an air mass' characteristics, and well as modification. ==classification and notation== thumb|right|250px|source regions of global air masses the bergeron classification is the most widely accepted form of air mass classification, though others have produced more refined versions of this scheme over different regions of the globe. air mass classification involves three letters. the first letter describes its moisture properties, with c used for continental air masses (dry) and m for maritime air masses (moist). the second letter describes the thermal characteristic of its source region: t for tropical, p for polar, a for arctic or antarctic, m for monsoon, e for equatorial, and s for superior air (an adiabatically drying and warming air formed by significant downward motion in the atmosphere). the third letter is used to designate the stability of the atmosphere. if the air mass is colder than the ground below it, it is labeled k. if the air mass is warmer than the ground below it, it is labeled w. for instance, an air mass originating over the desert southwest of the united states in summer may be designated "ct". an air mass origina
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