Monsoons

The word monsoon is derived from the Arabic word 'mausim', meaning season. A monsoon is a wind pattern that reverses direction on a seasonal basis. The term was originally applied to monsoonal winds in the Arabian Sea and Indian Ocean. The word is also used to label the season in which this wind blows from the southwest in India and adjacent areas that is characterized by very heavy rainfall, and specifically the rainfall that is associated with this wind.

Monsoons are caused by the fact that land heats up and cools down more quickly than water, owing to the difference in their specific heat. Thus, in summer, land reaches a higher temperature than the ocean. The hot air over the land tends to rise, creating an area of low pressure. This creates a constant wind blowing toward the land which entrains the humid oceanic marine layer. Associated rainfall is caused by the moist ocean air being diverted upward by mountains, which causes cooling, and in turn, condensation.

In winter, the land cools off quickly, but the ocean retains heat longer. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean while a large area of high pressure is formed over the land, intensified by wintertime radiational cooling.