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monsoon

A name for seasonal winds first applied to the winds over the Arabian Sea, which blow for 6 months from the northeast (northeast monsoon) and for 6 months from the southwest (southwest monsoon), but now extended to other parts of the world.

The primary cause is the much greater annual variation of temperature over large land areas compared with the neighboring ocean surfaces, causing an excess of pressure over the continents in winter (see Siberian High) and a deficit in summer (Asian Low). Hence the winds are behaving like large scale sea and land breezes.

Other factors such as the relief features of the land also have a considerable effect. View Pressures and Winds over the Indian ocean in the World Atlas to see the correlation, and see section Global winds of that book for an illustration. An extended discussion of this wind pattern is given at the end of this note. See also statistics of monsoon winds in G162.

Monsoons are strongest on the southern and eastern sides of Asia, but monsoons also occur on other coasts when the prevailing atmospheric circulation is not strong enough to inhibit them. Monsoon type winds have been identified on the west and gulf coasts of the US, in northern Australia, in Africa (except in the Mediterranean. In Europe, the prevailing west to northwest winds have also been described as monsoons.

In India the term is popularly applied chiefly to the southwest monsoon and to the rain which it brings.

See also brisa and elephanta.

See also World Atlas, under global winds. Global winds are generally distinguished from local winds, but the monsoon must be considered a hybrid of sorts.

Notes on Indian Ocean Monsoons

The monsoon winds of the N Indian Ocean, China Seas and Eastern Archipelago are the worlds most famous example of seasonal winds. They result from consistent seasonal changes in atmospheric pressure on land versus over the neighboring oceans. Here the primary change is the dominance of the Siberian High over Asia in the winter to the Asiatic Low during the summer when the continent is heated (see Action Centers in the World Atlas under Global winds).

In the NH winter an intense High develops over the cold Asian continent and from around October or November to March a persistent NE monsoon wind blows over the N Indian Ocean and South China Sea a direct consequence of the clockwise circulation around the High. Over the W Pacific Ocean the wind is NNE. The winds are generally moderate to fresh but can reach gale force locally as surges of cold air move S and particularly where funneling occurs (Taiwan Strait, Palk Strait, etc.). Weather is generally cool, fair and with well-broken cloud through the coasts of S China and Vietnam are frequently affected by extensive low cloud and drizzle. The NE Monsoon winds may extend across the equator changing direction to N or NW to become the N Monsoon off E Africa and the NW Monsoon of N Australian waters.

In the NH summer pressure over Asia falls with lowest pressure near the W Himalayas. The counterclockwise circulation around the Asiatic Low gives persistent SW Monsoon winds from May to September or October over the N Indian Ocean and South China Sea, and SSW or S winds over the W Pacific Ocean. Winds are generally fresh to strong and raise considerable seas. Warm humid air gives much cloud and rain on windward coasts and islands.

Similar regular and persistent winds, also known by the name of 'monsoon' occur in other parts of the world, although the areas affected are by comparison far more limited. An example is the Gulf of Guinea where a SW Monsoon wind blows from June to September. The seasons of the principal monsoons and their average strengths are shown in G162.

Sailing directions throughout the monsoon regions and especially the China Sea have been well known since the early 1800s due to extensive early China trade. In the China Sea the south-west monsoon begins in the middle or end of April and extends to the beginning or middle of October, being at its greatest strength in June, July and August and varying from SSW to SSE in June and July. North-east of Formosa the monsoon is usually a gentle southerly breeze from June to September, with occasional light summer gales under blue skies, but in the Eastern Sea July brings westerly winds, and August and early September southerly ones.

The north-east monsoon commences in early October in the northern part of the China Sea, but not till November does it blow steadily in the southern half. Usually the change of the monsoon is forecast by a ten-day gale. In December and January the monsoon blows at great strength accompanied by rain and a turbulent sea, though by March it has moderated. April is usually regarded as the finest month off the China coast. In the Eastern Sea the north-east monsoon blows for virtually nine months, from the middle of September to the middle of June. In each type of monsoon heavy weather is experienced at times with sudden and severe squalls and torrential rain.

But the severest weather of the region is, of course, from typhoons — a Chinese word meaning great wind. They chiefly occur between May and October or November and are most frequent in the vicinity of Luzon, Hainan Island, and the south-west corner of Japan. Only a few find their way up Formosa Strait. Average numbers for an active year might be: July-4; August-8; September-12; October-7. Full statistics for typical years can be seen in G193 (see under Western North Pacific).

See also monsoon trof



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