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The major climate groups are discussed in this classification. Group I includes the tropical air mass source regions and the equatorial trough or convergence zone. Equatorial is here defined as pertaining to the latitude zone within a few degrees north and south of the equator; tropical is defined as referring to the two latitude zones included within a few degrees north and south of the tropics of Cancer and Capricorn. Climates of Group I are controlled by the dynamic subtropical high-pressure cells, or anticyclones, which are regions of air subsidence and are basically dry, and by the great equatorial trough of convergence that lies between them. Though it is true that air of polar origin occasionally invades the tropical and equatorial latitudes, it may be said of the climates of Group I that they are almost wholly dominated by tropical and equatorial air masses.

Climates of Group II are in a zone of intense interaction between unlike air masses: the polar front zone. Tropical air masses moving poleward and polar air masses moving equatorward are in conflict in this zone, which contains a procession of eastmoving cyclonic storms. Locally and seasonally either tropical or polar air masses may dominate in these regions, but neither has exclusive control.

Climates of Group III are dominated by polar and arctic (including Antarctic) air masses. The two polar continental air mass source regions of northern Canada and Siberia fall into this group, but there is no southern hemisphere counterpart to these continental centres. In the belt of the 60th to 70th parallels, air masses of arctic origin meet polar continental air masses along an arctic front zone, creating a series of east-moving cyclones.

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