Monsoons occur throughout the world, but are most heavily concentrated in the Indian Ocean region. A monsoon occurs when the winds change direction due to a seasonal change. They always blow from cold areas to warm areas and do not necessarily bring moisture with them.
In India and Southeast Asia, the summer monsoon occurs between April and September in conjunction with heavy rainfall. With the end of winter, moist air from the Indian Ocean is borne on wind toward India, along with Bangladesh, Myanmar and Sri Lanka. This monsoon ushers in extremely heavy precipitation and humidity. It also often causes severe flooding.
Monsoons affect other areas as well. For instance, the Asian-Australian monsoon blows from the Pacific Coast of Russia to Australia.
The North American monsoon, which usually occurs midsummer, originates in both the Gulf of California and the Gulf of Mexico. Winds from these regions meet in central Mexico over the Madre Occidental Mountains, where the monsoon brings much needed moisture. Eventually the monsoon moves north into Texas, New Mexico and Arizona.
The western sub-Saharan monsoon of Africa occurs in February during seasonal shifts in wind. This monsoon reaches western Africa in late June and continues south by the time October arrives.