A typical example of this are the bands of westerly winds in the Northern and Southern Hemispheres (see Figure 1) which are responsible for the relatively even distribution of precipitation over the year in the middle latitudes. In the winter it also covers parts of the subtro-pics (e.g., the Mediterranean), so that most precipitation in these areas falls in winter.
The seasonal variation in transportation of air masses is linked with the position of the Intertropical Convergence Zone (ITCZ) or with the varying warming of land and sea. This process is known as the monsoon. The Indian monsoon is the most distinctive. At the beginning of June the first warm air masses from the southwest cross to the Indian subcontinent. This pattern of currents continues until the beginning of October and is accompanied by intensive rainfall (see also Figure 2). In winter the northeast trade winds bring dry air from central Asia and precipitation tends to zero (see also Figure 3), as can be seen from the annual course of precipitation for Mumbai (see Figure 5). A similar pattern can be observed on the west coast of Africa (Sierra Leone, Liberia) and in Southeast Asia.
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