MONSOON WINDS

MONSOON WINDS Derived from the Arabic word 'mausim', meaning season, 'monsoon' is applied to winds whose direction is reversed completely from one season to the next. Land masses of Asia and North America powerfully control the temperature and pressure conditions in the northern hemisphere. As pressure conditions control winds, these areas also develop wind systems, quite inde­pendent of the belted wind system in the southern hemi­sphere.

Summer Monsoon During summer, a 'thermal' or 'heat' low is developed over southern Asia in the lower levels of the atmosphere. It is a cyclone with a considerable air flow. From the Indian Ocean and the south-western Pacific warm, humid air moves northward and north­westward into Asia passing over India, Indo-China and China. This air flow accompanied by heavy rainfall consti­tutes the summer monsoon in southeast Asia.

Winter Monsoon Reverse air flow from that of sum­mer takes place in winter in Asia. The land area is dominated by a strong centre of high pressure from which there is an outward flow of air. Blowing southward and south-eastward towards the equatorial oceans, the winter monsoon brings dry, clear weather for several months. In North America, there is an alternation of temperature and pressure conditions between winter and summer, but the differences are not as remarkable as in southeast Asia. Australia also shows a monsoon effect but it reverses the conditions of Asia, being in the southern hemisphere.

GLOBAL CIRCULATION SYSTEMS The surface wind systems represent only a small part of the circulation pattern. There is an upper air flow also-mainly westerlies flowing in a complete circuit about the earth from latitude 25° almost to the poles, and equatorial easterlies between the high-pressure ridges.

JET STREAM The term was introduced in 1947 by Swedish-born US meteorologist Carl-Gustaf Rossby, and it is used to describe a very strong steady westerly wind blowing at high altitudes (6,000 to about 1400 metres above the earth's surface) just below the tropopause. It is usually confined to a narrow band and its speed reaches up to 350­450 kmph. The highest streams occur during winter. There are two main jet streams: (a) polar front jet stream, irregular in its location and commonly discontinuous, (b) subtropical jet stream (between 20° and 30° latitudes, north and south)-it is fairly consistent for a given season. In the northern hemisphere, the strongest jets flow across Japan and the United States. The jets have an Important role in the weather changes. High altitude flying has to take them into account.