A monsoon is a seasonal reversal of wind direction that brings a dramatic change in rainfall. The classic example is the South Asian monsoon, where winter winds blow from the dry interior toward the ocean and summer winds blow from the ocean toward the land, delivering heavy rain.

The shift is driven by the differential heating of land and ocean. In summer the land becomes much warmer, drawing moist air inland. Mountains enhance the rainfall through orographic lift. Other monsoon systems exist in Australia, West Africa, and the southwestern United States.

Timing and intensity of the monsoon determine agricultural success or failure for hundreds of millions of people. Flooding and landslides are common hazards during the wet phase. Climate models and observations are used to forecast the onset and strength of each year’s monsoon.

• Seasonal wind reversal
• Brings distinct wet and dry periods
• Driven by land-ocean temperature contrast
• Critical for regional water supply