Marine Glossary

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air-mass classification

Air masses are classified according their source regions. Four such regions are generally recognized:

(1) equatorial (E), the area between the north and south trades;

(2) tropics (T), the trade wind and lower temperate regions;

(3) polar (P), the higher temperate latitudes; and

(4) arctic or antarctic (A), the north or south polar regions of ice and snow.

This classification is a general indication of relative temperature, as well as latitude of origin.

Air masses are further classified as maritime (m) or continental (c), depending upon whether they form over water or land. This classification is an indication of the relative moisture content of the air mass.

Tropical air, then, might be designated maritime tropical (mT) or continental tropical (cT). Similarly, polar air may be either maritime polar (mP) or continental polar (cP). Arctic/antarctic air, due to the predominance of land masses and ice fields in the high latitudes, is rarely maritime arctic (mA). Equatorial air is found exclusively over the ocean surface and is designated neither (cE) nor (mE), but simply (E).

A third classification sometimes applied to tropical and polar air masses indicates whether the air mass is warm (w) or cold (k) relative to the underlying surface. Thus, the symbol mTw indicates maritime tropical air which is warmer than the underlying surface, and cPk indicates continental polar air which is colder than the underlying surface. The w and k classifications are primarily indications of stability (i.e., change of temperature with increasing height).

If the air is cold relatively to the surface, the lower portion of the air mass is being heated, resulting in instability (temperature markedly decreases with increasing height) as the warmer air tends to rise by convection. Conversely, if the air is warm relative to the surface, the lower portion of the air mass is cooled, tending to remain close to the surface. This is a stable condition (temperature increases with increasing height).

WXT G140 shows typical air masses of North America and adjacent ocean areas and similar plots for waters adjacent to Great Britain.