A monsoon is a seasonal reversal in wind patterns over a region. The word “monsoon” comes from the Arabic word mausim, meaning “season.” The seasonal wind shift is usually accompanied by a dramatic change in precipitation.
The best known example of a monsoon occurs over the Indian subcontinent. In summer, the sun warms the land and the air above it. With cooler air lying over the oceans that surround the subcontinent, a horizontal pressure gradient is established that generates winds directed from the ocean to land.
The air flowing over the water remains over the ocean a long time, which causes it to gain moisture. This wind brings humid air inland. The solar heating over land triggers convection that produces rain. Orographic lifting of air by the Himalaya Mountains generates more rain.
The summer monsoon is a wet season over land. These rains can cause major flooding, but they are vital to agriculture and the economy.
During the winter season, the air above the land cools faster than the air over the water, establishing a pressure gradient force from land to sea. The winds are therefore reversed from the summer monsoon flow — from land to sea instead of from sea to land. Sinking air above the land suppresses cloud development and precipitation. The winter monsoon is a dry season.
Other parts of the world also have monsoons. The North American monsoon, also called the Arizona monsoon, occurs over the southwestern United States and northwestern Mexico. It results in a rainy season that begins in July and typically lasts until mid-September, and then the drier weather regime gets re-established.