What Causes a Monsoon?

causes-monsoon Credit: Clay Gilliland/CC-BY-SA 2.0

A monsoon is caused when a low-pressure area built up over a hot landmass reacts with a high-pressure zone over a cool ocean, sending moisture-laden wind toward the low-pressure zone. Once over the landmass, the ocean air rises and forms rain clouds. Dense cloud formation and heavy rains are especially likely to occur if there are higher elevations like with inland mountains.

Most of the year, wind movement occurs from land toward the ocean. This keeps the air dry and sometimes causes droughts. In sub-tropical and tropical areas, the convection of the intense heat throughout the summer months produces a low-pressure zone over the land. Due to differential heating, the ocean itself and the dense, humid air above the ocean remains cooler. A cyclical pattern is formed by the cool ocean air rushing into the low-pressure zone and the warm air over the landmass rising and flowing outward to replace the ocean air. When the land sufficiently cools in the fall, the rainfall diminishes and the wind once again changes direction.

One of the most dramatic examples of seasonal monsoons occurs in India. When the heavy humid air reaches the obstacle of the Himalayan Mountains on its way toward the low-pressure zone over the Tibetan Plateau, the result is extremely heavy rainfall that typically causes widespread flooding and crop damage.