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.
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.