Monsoon (Arabic mauism,”season”), wind that changes direction with the change of seasons. The monsoon prevails mainly in the Indian Ocean. It blows from the southwest, generally from April to October, and from the opposite direction, the northeast, from October to April. The southwest, or summer, monsoon is usually accompanied by heavy rain in areas of India and the East Indies, constituting the dominant climate event of the area. The appearance of this wind pattern over geological time has been linked, through sedimentary evidence, to the uplift of the Himalayas and the Tibetan Plateau (Qing Zang Gaoyuan) as the Indian subcontinent began to collide with the Asian crustal plate about 20 million years ago. The northern land mass was high enough by about 6 million years ago to cause air rising from the southern land mass to be replaced by the monsoon, establishing this wind pattern.

Monsoons, in weaker form, also occur in other parts of the world.