Every summer, southern Asia and especially India, is drenched by rain that comes from moist air masses that move in from the Indian Ocean to the south. These rains, and the air masses that bring them, are known as monsoons.
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.
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. Usually, the term monsoon is used to refer to the rainy phase of a seasonally-changing pattern, although technically there is also a dry phase.
The major monsoon systems of the world consist of the West African and Asia-Austrlian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated.
The term was first used in English in British India (now India, Bangladesh and Pakistan) and neighbouring countries to refer tothe big seasonal winds blowing from the Bay of Bengle and Arabian Sea in the southwest bringing heavy rainfall to the area