A monsoon is a seasonal shift in wind direction in a region. Monsoons are typically associated with summer seasons of heavy rain in regions near the equator, such as in South Asia and West Africa, but can also occur in Western Europe and the American Southwest.

In general, monsoons occur when the center of continental landmasses heat up during summertime more quickly than nearby waters. Air masses over this hot land begin to warm and rise higher and higher, resulting in a zone of low air pressure. Offshore high-pressure systems push moist air inland towards the low-pressure system (SF Fig. 3.2 A). If this moist air mass lifts over mountains and becomes cooled, the moisture condenses and falls as precipitation—typically heavy rain during summer monsoons in the tropics (SF Fig. 3.2 B).

Monsoons occur in some of the world’s most densely populated countries like India, Sri Lanka, Bangladesh, Taiwan, Singapore, and the Philippines. Monsoons can have both negative and positive effects. Flooding caused by monsoon rains can destroy property and crops (SF Fig. 3.2 C). However, seasonal monsoon rains can also provide freshwater for drinking and crop irrigation.