Areas where air masses form are known as source regions. The nature of the source region largely determines the temperature and humidity characteristics of an air mass.
There are two essential features which make an ideal source region. First, it must be an extensive and broadly uniform surface of the earth.
If a region has irregular topography or it has a surface consisting of both water and land, then it is not considered satisfactory. Secondly, the area should have a comparatively gentle and divergent air flow so that air will stay over the region for a considerably long period of time to reach some measure of equilibrium with the surface.
Therefore the regions with high barometric pressure and low barometric gradients are the ideal source regions. In other words, the source regions are characterized by stationary or slow moving anticyclones with their extensive areas of calms or light winds.
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Cyclonic areas being characterized by converging surface winds do not favour the formation of air masses. Conditions which produce air masses are well developed over the tropical seas and hot deserts during much of the year and over the Arctic region, particularly in winter. These areas may be considered as the primary source regions of air masses.
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There are other extensive regions with uniform surface over which air flow is generally not stagnant. If the air that passes over such a region can be transformed rapidly, it may acquire unique characteristics. Such regions are referred to as secondary source regions.
Primary source regions may be either warm or cool because the air may be stagnant for longer periods to establish equilibrium. But the secondary source regions should either be relatively warmer than the overlying air, or otherwise extensive.
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It is noteworthy that no major source regions are found in the middle latitudes. They are generally confined to tropical or polar locations. Since regions in the middle latitudes are dominated by cyclones and other types of storms, they do not possess the homogeneous conditions so essential for a source region.
Examples of primary source regions are the following: the tropical Atlantic Ocean around Bermuda, the tropical Pacific Ocean around Hawaii, the Sahara Desert region, and the interior of Siberia. Some of the secondary source regions are: the North Pacific Ocean between Siberia and Canada, North Atlantic Ocean between Canada and Northern Europe, and arid south-west of the United States.