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Air masses are classified according to two categories of generalized source regions:

• (1) latitudinal position on the globe, which primarily determines thermal properties and
• (2) underlying surface-continents or oceans-determining the moisture content.

With respect to latitudinal position, five types of air masses are as follows:

Air Mass Symbol Source Region
Arctic A Arctic ocean and fringing lands
Antarctic AA Antarctica
Polar P Continents and oceans, 50°, 60° N and S.
Tropical T Continents and oceans, 20°-35° N and S.
Equatorial E Oceans close to equator

With respect to type of underlying surface, two further subdivisions are imposed on the preceding types as follows:

Air mass Symbol Source Region
Maritime m Oceans
Continental c Continents

By combining types based on latitudinal position with those based on underlying surface a list of six important air masses results; these are listed in (the following tables). The table gives typical values of temperature and specific humidity at the surface, although a wide range in these properties may be expected, depending on season. The equatorial air mass, mE, holds about 200 times as much water vapour as the extremely cold arctic and Antarctic air masses, cA and cAA. The maritime tropical air mass mT and maritime equatorial mE air mass are quite similar in temperature and water vapour content. Both are capable of very heavy yields of precipitation. The continental tropical air mass cT has its source region over subtropical deserts of the continents. Although typically it has a substantial water vapour content, it has low relative humidity when highly heated during the daytime. The polar maritime air mass mP, with specific humidity of 4 to 5 g/kg in winter, originates over middle-latitude oceans. It has a high relative humidity and can yield substantial precipitation.

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