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For the band Monsoon see Sheila Chandra.

A monsoon is a (wind) pattern that reverses direction on a seasonal basis. The term was originally applied to monsoonal winds in the Arabian Sea and Indian Ocean. The word is also used to label the season in which this wind blows from the southwest in India and adjacent areas that is characterized by very heavy rainfall, and specifically the rainfall that is associated with this wind.

In terms of total precipitation, total area covered and the total number of people affected, the monsoons affecting the Indian Subcontinent dwarf the so-called North American monsoon.

History

Monsoon is derived from the Arabic word "موسم" (mausem), which means season. In English, the term was borrowed more directly from the Portuguese, monção, and possibly via early modern Dutch, monson. The term arose in the 16th century during the rise in navigation across the Indian Ocean, because the monsoonal winds were so critical to sailing:

In Goa they stayed till the Monson, or time of the windes came in to sayle for China. 1598 W. PHILIP tr. J. H. van Linschoten Disc. Voy. E. & W. Indies I. xcii. 143/1.

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.

The monsoon is very relevant to the people of South India, especially the people of Kerala state. But environmental degradation has weakened or changed the monsoon system prevalent for many centuries.

Processes

Monsoons are caused by the fact that land heats up and cools down more quickly than water, owing to the difference in their specific heat. 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 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.

Monsoon systems

As monsoons have become 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 take place.

Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of supercontinents such as Pangea, with their extreme continental climates.

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, and are drawn towards the Himalayas, creating winds blowing rain clouds towards India, some areas of which receive up to 10,000 mm of rain.

Indian Ocean Monsoon

The southwest monsoon begins around June and dies down by September. It generally begins in the coastal state of Kerala on June 1 and takes about a month to reach the rest of India. The monsoon accounts for 80 percent of the rainfall in the country. Indian agriculture (which accounts for 25 percent of the GDP and employs 70 percent of the population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy.

It also has a big effect on the natural environment. People in this area love the monsoon, especially children. The monsoon is welcomed with celebration because it cools and cleanses the city. In large cities traffic is usually brought to a standstill and daily life is usually impacted during the periods of heaviest rainfall. The most noticeable change that people make is in their attire. In major cities such as Kolkata a sturdy umbrella, a mackintosh/ waterproof jacket and plastic shoes (because leather rots when it is wet), are often worn, especially by visitors. In rural areas such as in the hills, home to the two wettest places on Earth, the attire is simpler. They usually dress in normal clothes with a long basket like hat covering the head and back. It is woven out of a type of grass.

Occasionally, the Indian Ocean monsoon is called the Southeast Asian monsoon.

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 with an area of high pressure called the subtropical ridge that moves northward during the monsoon. At the surface, the monsoon is associated with weak highs that form over the four corners region and the southern Great Plains and a 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 cause 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. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.

See also

• Climate of India

Reference and external links

• Initial text from the Goddard Space Flight Center's public domain Distributed Active Archive Center
• About the North American Monsoon - Arizona State University
• North American Monsoon Experiment
• Meet the Indian monsoons at PBS.org

• Weather hazards
• Winds
• Arabic words
• Flood