| A monsoon is a seasonal shift in the prevailing wind direction, that usually
brings with it a different kind of weather. The persistent wind flow is driven by a warm
air mass with low pressure at the surface that forms over the continent as it is warmed
by the sun. Air from the relatively higher pressure air mass over the ocean flows toward the
low pressure over land. Probably the most famous monsoon is the Indian
summer monsoon, as it affects such a large portion of Asia. In May and June of each year, the dry northerly wind flow over India changes direction,
and warm humid air from the Indian Ocean flows from the south, gradually overspreading the
Indian subcontinent. Widespread torrential rains, and even severe thunderstorms, accompany
the "onset" of the monsoon. The Indian Ocean version of the hurricane, called a "cyclone", can
also occur and move ashore in association with the onset of the monsoon.
These cyclones have at times killed thousands of people who live in the low-lying areas
along the eastern coast of India and Bangladesh. A weaker version of this monsoon occurs
over the normally dry southwestern United States in the later summer when more humid air,
accompanied by thunderstorms, invades the region.
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