<P> In meteorology, an air mass is a volume of air defined by its temperature and water vapor content . Air masses cover many thousands or billions of square miles, and adapt to 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, as well as modification . </P> <P> 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). 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). For instance, an air mass originating over the desert southwest of the United States in summer may be designated "cT". An air mass originating over northern Siberia in winter may be indicated as "cA". </P> <P> The stability of an air mass may be shown using a third letter, either "k" (air mass colder than the surface below it) or "w" (air mass warmer than the surface below it). An example of this might be a polar air mass blowing over the Gulf Stream, denoted as "cPk". Occasionally, one may also encounter the use of an apostrophe or "degree tick" denoting that a given air mass having the same notation as another it is replacing is colder than the replaced air mass (usually for polar air masses). For example, a series of fronts over the Pacific might show an air mass denoted mPk followed by another denoted mPk' . </P> <P> Another convention utilizing these symbols is the indication of modification or transformation of one type to another . For instance, an Arctic air mass blowing out over the Gulf of Alaska may be shown as "cA - mPk". Yet another convention indicates the layering of air masses in certain situations . For instance, the overrunning of a polar air mass by an air mass from the Gulf of Mexico over the Central United States might be shown with the notation "mT / cP" (sometimes using a horizontal line as in fraction notation). </P>

The one term that does not describe an air mass is