Because pressure conditions control winds, it is obvious that these areas must develop wind systems relatively independent of the belted system of earth winds so well illustrated in the southern hemisphere.
In summer, southern Asia develops a cyclone into which there is a considerable flow of air. This may be a heat low, or thermal low, limited to the lower levels of the atmosphere. From the Indian Ocean and the southwestern Pacific warm, humid air moves northward and northwestward into Asia, passing over India, Indochina, and China. This airflow constitutes the summer monsoon, which is accompanied by heavy rainfall in southeastern Asia.
In winter, Asia is dominated by a strong centre of high pressure, from which there is an outward flow of air reversing that of the summer monsoon. Blowing southward and southeastward toward the equatorial oceans, this winter monsoon brings dry, clear weather for a period of several months.
North America does not have the remarkable extremes of monsoon winds experienced by southeastern Asia, but there is nevertheless an alternation of temperature and pressure conditions between winter and summer. Wind records show that in summer there is a prevailing tendency for air originating in the Gulf of Mexico to move northward across the central and eastern part of the United States, whereas in winter there is a prevailing tendency for air to move southward from sources in Canada. Australia, too, shows a monsoon effect, but being south of the equator it reverses the conditions of Asia.
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