My Science School

            Parts of India and Southeast Asia have a monsoon climate. In these areas, it changes very suddenly from a wet to a dry season, according to the direction of the prevailing wind. The dry period is extremely hot, and the powerful monsoon winds that blow in from the sea bring torrential rain, often without warning. Such violent extremes of weather can make daily life very difficult, with heavy flooding, damage to property and loss of life commonplace.

            A monsoon often brings about thoughts of torrential rains, similar to a hurricane or typhoon. But there is a difference: a monsoon is not a single storm; rather, it is a seasonal wind shift over a region. The shift may cause heavy rains in the summer, but at other times, it may cause a dry spell.

            A monsoon (from the Arabic mawsim, which means “season”) arises due to a difference in temperatures between a land mass and the adjacent ocean, according to the National Weather Service. The sun warms the land and ocean differently, according to Southwest Climate Change, causing the winds to play "tug of war" eventually switching directions bringing the cooler, moister air from over the ocean. The winds reverse again at the end of the monsoon season. 

            A wet monsoon typically occurs during the summer months (about April through September) bringing heavy rains, according to National Geographic. On average, approximately 75 percent of India's annual rainfall and about 50 percent of the North American monsoon region (according to a 2004 NOAA study) comes during the summer monsoon season. The wet monsoon begins when winds bringing cooler, more humid air from above the oceans to the land, as described above.

            A dry monsoon typically occurs between October and April. Instead of coming from the oceans, the winds tend to come from drier, warmer climates such as from Mongolia and northwestern China down into India, according to National Geographic. Dry monsoons tend to be less powerful than their summer counterparts. Edward Guinan, an astronomy and meteorology professor at Villanova University, states that the winter monsoon occurs when "the land cools off faster than the water and a high pressure develops over the land, blocking any ocean air from penetrating." This leads to a dry period. 

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