A monsoon is a wind system wherein the prevailing winds reverse in direction based on the season. A monsoon is traditionally defined by the change in the wind as well as the change in the precipitation that comes along with it.
A monsoon is regionally specific, and it usually only occurs in tropical and subtropical areas that are either east, south or southeast of a large land mass in the Northern Hemisphere. Monsoons are particularly intense in southeast Asia. There are some monsoon storms that occur in North America, but they are comparatively weaker than those in other regions.
The reason for southern Asia monsoons is that in the summer, there is a strong low pressure center, into which winds from adjacent oceans begin to blow. Being both warm and moist, the winds bring a combination of heavy rainfall and storms.
Monsoons are the cause of dry seasons as well. In the winter, Asia has a strong high-pressure area. The wind blows from land to sea, which creates a winter monsoon that brings many months of clear, dry and relatively cold weather.
Monsoons range in severity based on location and the season. According to HowStuffWorks, Cherrapunji, India, one of the wettest places on the planet, can get over a 1,000 inches of rain in a year.