However, the term monsoon refers not only to the summer rains but to the entire cycle that consists of both summer moist onshore winds and rain from the south as well as the offshore dry winter winds that blow from the continent to the Indian Ocean.
The Arabic word for season, mawsin, is the origin of the word monsoon due to their annual appearance. Although the precise cause of the monsoons is not fully understood, no one disputes that air pressure is one of the primary factors. In the summer, a high pressure area lies over the Indian Ocean while a low exists over the Asian continent. The air masses move from the high pressure over the ocean to the low over the continent, bringing moisture-laden air to south Asia.
During winter, the process is reversed and a low sits over the Indian Ocean while a high lies over the Tibetan plateau so air flows down the Himalaya and south to the ocean. The migration of trade winds and westerlies also contributes to the monsoons.
Smaller monsoons take place in equatorial Africa, northern Australia, and, to a lesser extent, in the southwestern United States.
Almost half of the world's population lives in areas affected by the monsoons of Asia and most of these people are subsistence farmers, so the coming and goings of the monsoon are vital to their livelihood to grow food to feed themselves. Too much or two little rain from the monsoon can mean disaster in the form of famine or flood.
The wet monsoons, which begins almost suddenly in June, are especially important to India, Bangladesh, and Myanmar (Burma). They are responsible for almost 90 percent of India's water supply. The rains usually last until September.