Monsoons are measured by calculating the average rainfall or wind speed. Prominent monsoon systems include the West African monsoon, the North American monsoon, the Asian-Australian monsoon and the Indian Summer monsoon.
The West African monsoon spans from June to September and is located in western Africa. The North American monsoon spans from July to September, creating storms and heavy rain in the southwestern part of the United States. In 1686, Edmond Halley was the first to explain how monsoons formed by speculating that the difference in land and sea temperatures produced air circulations. Monsoons affect humans greatly, as shown when the lack of monsoons in India from 1876 to 1879 produced famines due to severe drought.