Monsoon

History

The word "monsoon" appears to have originated from the Arabic language|Arabic word "موسم" (''mausem''), which means ''season''. It is most often applied to the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other.

Processes

Monsoons are caused by the fact that land heats up and cools down more quickly than water. Thus, in summer, land reaches a higher temperature than the ocean. The hot air over the land tends to rise, creating an area of Low (atmospheric)|low pressure. This creates an extremely constant wind blowing toward the land. Associated rainfall is caused by the moist ocean air being diverted upward by mountains, which causes cooling, and in turn, condensation. In winter, the land cools off quickly, but the ocean retains heat longer. The hot air over the ocean rises, creating a low pressure area and a breeze from land to ocean. Because the temperature difference between the ocean and land is less than in summer, the winter monsoon wind is not as constant. Monsoons are similar to sea breezes, but they are much larger in scale, stronger, and are more constant and fast.

Monsoon systems

As monsoons have come to be better understood, the definition has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia, and Africa and the adjacent seas and oceans. It is within these regions that the most vigorous and dramatic cycles of weather events on Earth takes place. Even more broadly, in the geological past, it is now understood, monsoon systems must have always accompanied the formation of supercontinents such as Pangea, with their extreme continental climates.

North American Monsoon

The North American monsoon occurs from mid July into September and affects Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of southern California but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast). The North American monsoon is associated an area of High pressure area|high pressure called the subtropical ridge that moves northward during the monsoon. At the surface, the monsoon is associated with a weak highs that form over the Four Corners (United States)|four corners region and the southern Great Plains and a low pressure area|thermal low that forms over Baja California. The Bermuda high also plays some role in the monsoon. The placement of the high and low pressure areas causes the wind to shift from westerly to southerly. Moisture is drawn northward from the Gulf of California into northern Mexico and the Southwest United States. Rainfall during the monsoon occurs as thunderstorms over the southern Rocky Mountains and adjacent areas. This season of almost daily thunderstorms is often called the ''Southwest'', ''Mexican'' or ''Arizona'' monsoon. It is also sometimes called the ''Desert Monsoon'' as a large part of the affected area is desert. As much as 70% of rainfall in the region occurs during the summer monsoon. Many desert plants are adapted to take advantage of this wet season in the usually dry area. Because of the danger of being caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.

Northeast Winter Monsoon (Asia)

In Asia, the northeastern winter monsoons take place from December to early March. The temperature over central Asia is lower, creating a zone of high pressure there. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across south Asia, creating dry air streams which produce clear skies over India from the months of November to May. Meanwhile, a low pressure system develops over northern Australia and winds are directed toward Australia. During the Northeast Winter Monsoon, Australia and southeast Asia receive large amounts of rainfall.

Southwest Summer Monsoon

The Southwestern Summer Monsoons occur from June to September, the southwest monsoon is drawn towards the Himalayas, creating winds blowing rain clouds towards India, which receive up to 10,000 mm of rain in some areas.

See also

Climate of India#Monsoons|Climate of India

Reference and external links

Initial text from the Goddard Space Flight Center's public domain http://daac.gsfc.nasa.gov/

http://geography.asu.edu/azclimate/aboutmon.htm- Arizona State University

http://earthobservatory.nasa.gov/Newsroom/Campaigns/NAME_Mission.html

http://www.pbs.org/wnet/nature/monsoon/html/intro.html