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How do monsoons affect rainfall in South and Southeast Asia?

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A map of the continent of Asia, including its countries, boundaries, and capital cities, as well as parts of Europe, Africa, and surrounding oceans. — Asia Map showing the countries and national capitals of Asia.

Monsoons are major wind systems that seasonally reverse direction, fundamentally shaping the climate and rainfall patterns of South and Southeast Asia. These systems are driven by temperature differences between land and sea, causing winds to blow from high-pressure areas (heat sinks) to low-pressure areas (heat sources).1

The Southwest (Wet) Monsoon

Prevailing from approximately May to September, the southwest monsoon is the primary source of precipitation for the region. As air flows from the sea toward the continent, it gathers moisture and rises, leading to condensation and heavy rainfall. In much of South Asia and mainland Southeast Asia, this period accounts for 80 to 90 percent of total annual precipitation. This rainfall is vital for agriculture and water supply, though its intensity can vary annually, sometimes leading to flooding or drought.23145

The Northeast (Dry) Monsoon

Occurring roughly from November to March, the northeast monsoon brings relatively dry, cool air from the land to the sea. While generally dry, this wind can produce significant rainfall on windward maritime coasts—such as southeastern India, the east coast of Malaysia, and the Philippines—as the air gathers moisture while passing over warmer waters.42