The word monsoon has been derived from the Arabic word ‘mausim’ which means season. Thus, the monsoon winds refer to a system of winds in which there is a complete reversal of direction with the change of season. These winds blow from sea to land during the summer and from land to sea during the winter season. The monsoon winds blow over India, Pakistan, Bangladesh, Myanmar, Sri Lanka, Arabian Sea, Bay of Bengal, south-eastern Asia, northern Australia, China and Japan. Some parts of Africa and North America also experience monsoon winds.
Traditionally, monsoon winds were considered as land and sea breezes on a large scale. In 1686 the famous Englishman Sir Edmund Halley explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating. Accordingly, Halley conceived summer and winter monsoons depending upon the season. In summer the sun shines vertically over the Tropic of Cancer resulting in high temperature and low pressure in Central Asia while the pressure is still slightly high over Arabian Sea and Bay of Bengal. This induces air from sea to land and brings heavy rainfall to India and her neighbouring countries. In winter the sun shines vertically over the Tropic of Capricorn. The north western part of India grows colder than Arabian Sea and Bay of Bengal and the of the monsoon is reversed.
Halley’s classical theory, based on differential heating of land and water, as the main driving force of the monsoon winds dominated the scene for about three centuries. However, the monsoons do not develop equally everywhere and the thermal concept of Halley fails to explain the intricacies of the monsoons.
Flohn of the German Weather Bureau, while rejecting the classical theory of origin of monsoons suggested that the tropical monsoon of tropical Asia is simply a modification of the planetary winds of the tropics. He thinks of the thermal low of northern India and the accompanying monsoon as simply an unusually great northward displacement of the Northern Inter-Tropical Convergence Zone (NITCZ). The seasonal shift of the ITCZ has given the concept of Northern Inter-Tropical Convergence Zone (NITCZ) in summer (July) and Southern Inter-Tropical Convergence Zone (SITCZ) in winter (Jan.). The fact that the NITCZ is drawn to about 30’N latitude may be associated with the unusually high temperature over north India. According to this interpretation the main westerly current of the monsoon is simply the expanded equatorial westerlies which lie embedded in the great mass of tropical easterlies or the trade winds. NITCZ is the zone of clouds and heavy rainfall.
In summer of the northern hemisphere, a deep and widespread surface pressure trough extends across northern India-Pakistan. s is a part of the planetary intertropical convergence zone. It reaches maximum poleward displacement in the summer season. To the south of this trough there is a deep current of maritime tropical air which is known as the southwest monsoon. At the end of the summer season of the northern hemisphere, ITCZ moves South of Equation and assumes the form of SITCZ. e areas in the northern hemisphere which were experiencing the equatorial westerlies in the summer season come under the influence of the north-easterly trade winds which are termed as the north-east monsoons. The north-east trade winds are deflected when they cross the equator and give rise to equatorial westerlies in the southern hemisphere. These westerlies blow in a north-east to south-west direction and replace the trade winds in the southern hemisphere between equator and SITCZ. These winds are termed as the summer monsoons of the southern hemisphere.
Monsoon winds, all over the world, are supposed to the affected by El Nino and La Nino. El Nine is a warm subsurface current wing between 36’S and 30’S latitudes near the Peruvian coast. El Nino leads to poor monsoon rainfall in India and other parts of the world.
After an EL-Nino, weather conditions return to normal. However, sometimes trade winds become so strong that they cause abnormal accumulation of cold water in the central and eastern Pacific region. This event is called La Nina which brings heavy monsoon showers in India and other adjoining countries of the monsoon region.
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Land and Sea Breezes
Land and sea breeze influence only a narrow strip 20 to 30 km wide along the coast. During daytime when the sun shines, land gets more heated than the adjoining sea and develops low pressure. e adjoining sea is still cooler and develops a comparatively high pressure. Cool breeze called ‘sea breeze’ starts blowing from high pressure area of sea to low pressure area of land.
At night, land becomes cooler than sea due to rapid radiation. s results in high pressure over the land and low pressure over the sea. Air starts blowing from land to sea and is known as land breeze.
Mountain and Valley Breezes
These are similar to land and sea breezes and blow in mountainous regions. During day, the mountain slope is heated more than the valley often. The air from the valley blows up the slope in the form of valley breeze. After sun-set, the pattern in reversed. Rapid loss of heat through terrestrial radiation along the mountain slopes results in sliding of cold dense air from higher elevations to valleys. s is called mountain breeze.