1. The anticyclonic systems. Anticyclonic systems have stagnant or slow-moving air, which allows time for air to adjust its heat and moisture content to that of the underlying surface. These anticyclones have a divergent airflow that spreads the properties horizontally over a large area; tur-bulence and convection distribute these proper-ties vertically. Subsidence (downward motion), another property of anticyclones, is favorable for lateral mixing, which results in horizontal or layer homogeneity. Warm highs, such as the Bermuda and Pacific highs, extend to great heights because of a lesser density gradient aloft and thereby produce an air mass of relatively great vertical extent. Cold highs, such as the Siberian high, are of moderate or shallow vertical extent and produce air masses of moderate or shallow height.

2. Cyclonic systems. Cyclonic systems are not conducive to air mass formation because they are characterized by greater wind speeds than an-ticyclonic systems. These wind speeds prevent cyclonic systems from stabilizing. An exception is the stationary heat low. 

3. Belts of convergence. Belts of convergence are normally not conducive to air mass forma-tion since they have essentially the same proper-ties as cyclonic systems. However, there are two areas of convergence where air masses do form. 

These are the areas over the north Pacific, between Siberia and North America, and the Atlantic, off the coast of Labrador and New-foundland. These two areas act as source regions for maritime polar air.

The ideal condition for the production of an air mass is the stagnation of air over a uniform surface (water, land, or ice cap) of uniform temperature and humidity. The length of time an air mass stagnates over its source region depends upon the surrounding pressures. From the surface up through the upper levels, such air acquire  definite properties and characteristics. The resulting air mass becomes virtually homogeneous throughout, and its properties become uniform at each level. In the middle latitudes, the land and sea areas with the associated steep latitudinal temperature gradient are generally not homo-geneous enough for source regions. These areas act as transitional zones for air masses after they have left their source regions.

The source regions for the worlds air masses are shown in figure 4-1-1. Note the uniformity of the underlying surfaces; also note the relatively uniform climatic conditions in the various source regions, such as the southern North Atlantic and Pacific Oceans for maritime tropical air and the deep interiors of North America and Asia for con-tinental polar air.

The characteristics of an air mass are acquired in the source region, which is the surface area over which the air mass originates. The ideal source region has a uniform surface (all land or all water), a uniform temperature, and is an area in which air stagnates to form high-pressure systems. The properties (temperature and moisture content) an air mass acquires in its source region are dependent upon a number of factorsthe time

Figure 4-1-1.Air mass source regions.

of year (winter or summer), the nature of the underlying surface (whether land, water, or ice covered), and the length of time it remains over its source region.

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