The “Monsoon”, from the Arabic “mauism” meaning “season”, defines a wind that changes direction with the seasons. Monsoons develop as a result of changing patterns of atmospheric pressure caused by the varied heating and cooling rates of continental landmasses and oceans. The strongest and most well known monsoons are those which affect India and Southeast Asia. The summer monsoon, which blows southwesterly across the Indian Ocean, is extremely wet. The winter monsoon, in contrast, blows northeasterly and is generally dry.
During May and June of each year, the summer monsoon arrives with persistent southerly wind flow driven by a warm air mass with low pressure at the surface that forms over southern Asia as it is warmed by the sun. Air from the relatively higher pressure air mass over the Indian and tropical western Pacific Ocean flows northward toward the low pressure over land, bring with it torrential rains. A late arrival of the monsoon can be bad for agriculture, as the monsoon rains are necessary for summer crops.
In India, 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, large hail and tornadoes can accompany the onset of the summer monsoon.
The Indian Ocean version of the hurricane, which is traditionally called a “cyclone” in the Indian Ocean, can also form and move ashore in association with the onset of the monsoon. These cyclones have at times killed many thousands of poor people who live in the low-lying areas along the eastern coast of India and Bangladesh.
A weaker and much more localized monsoon occurs over the normally dry southwestern United States in the late summer when more humid air, accompanied by thunderstorms, overspreads the region.
India and Southeast Asia lie in between the centres of the tropical and subtropical climate zones. For much of the year, and particularly during winter, northeast trade winds blow across the region, from subtropical high pressure to equatorial low pressure. These winds originate from the continental interiors and are generally dry. During the summer months however, the large landmasses of Asia and the Indian subcontinent heat up, generating a seasonal continental region of low pressure. Airflow reverses and wind blows southwesterly across the Indian Ocean, accumulating considerable moisture which is deposited as heavy rainfall during the wet season from May to September.
Scientists have linked the development of the monsoonal wind phenomenon over India during Earth History to the uplift of the Himalayas and the Tibetan Plateau, which occurred about 20 million years ago when India collided into the Asian continent.
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