A monsoon is a seasonal change in the direction of the prevailing, or strongest, winds of a region. Monsoons cause wet and dry seasons, and are usually associated with parts of Asia, but they can occur in other parts of the world as well. The term "monsoon" is traditionally used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator.

A monsoon is caused by a seasonal shift in the winds, which is due to the temperature difference between the land and the water as seasons change. For example, at the beginning of summer, the land warms up faster than bodies of water. Monsoon winds always blow from cold to warm. In the summer, warm air rising off the land creates conditions that reverse the direction of the wind.

The major monsoon systems of the world consist of the West African, Asia–Australian, the North American, and South American monsoons. The North American Monsoon is a seasonal change in wind that occurs as the summer sun heats the land of North America. During much of the year, the strongest winds over northwestern Mexico, Arizona, and New Mexico are dry air blowing from the west. As the land heats up in the summer, the wind begins to change direction and blows from the south. This new wind blows moist air from the Pacific Ocean and Gulf of California into the region, resulting in a wet season.

Monsoons can cause heavy rainfall, humidity, and distinct wet and dry seasons that can last for months. The monsoons that cause the heaviest rainfall are summer monsoons near the Indian Ocean. Warm water in the ocean evaporates, rising into the air. This causes the wind to change direction and moisture blows toward the land in countries such as India and Sri Lanka. The warm, moist air then condenses and becomes rain.