Monsoons are formed when the sun heats the atmosphere and causes variations in temperature between the oceans and land masses. At certain times of year, land heats faster than water in the oceans, and warmer air rises. Monsoons are a seasonal reversing of wind direction that result from the atmosphere attempting to balance cool and warm air.
The change in prevailing-wind direction significantly alters weather in the localised area. Monsoons often cause long periods of rain in tropical and subtropical regions. The rains are important for agriculture in areas affected by monsoons. A monsoon season that is drier than normal can result in crop failures and drought. Extremely heavy monsoon rains can cause flooding, significant structural damage and loss of life.
Monsoons occur most often in Asia, including Japan, China, India and Southeast Asia. They also occur in central Africa. Smaller monsoons impact parts of the Southwest in the United States, moderating the severe dryness of New Mexico and Arizona.
The term "monsoon" originated in India and nearby countries in reference to seasonal winds from the Arabian Sea and the Bay of Bengal that brought heavy rains. Geologists believe that the first strong monsoons occurred about eight million years ago.