Question

origin of monsoon in points

Answer 1

Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation,but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is sometimes incorrectly used for locally heavy but short-term rains,although these rains meet the dictionary definition of monsoon.

Answer 2

(i) Vast size of the Indian subcontinent and adjacent seas

(ii) Very high and extensive mountain systems of the Himalayas in the north, extending in an east-west direction, thus posing a formidable physical barrier between tropical and polar air masses.

The second factor is of great meteorological significance.

The high mountain chains of the Himalayas which border the subcontinent on three sides work as both a break and motor at the same time. During the winter season, they prevent the penetration of the cold polar air masses from Siberia into the subcontinent, while in summer, the Himalayas do not allow the equatorial maritime air masses to cross the Himalayas and force them to curve round the north-west. The mighty Himalayas produce hydro-dynamic effects that determine the type of precipitation in India.

According to the thermal concept, during the period following the Spring Equinox (March 23), the sun starts its apparent northward shift. As a result, the areas lying north of the equator (tropics and sub-tropics)- get a progressive high incidence of solar radiation.

The effect of this phenomenon ‘ on the Indian subcontinent is seen in the form of intense heating of the vast northern plains and the adjoining highlands. As a result, a massive low pressure trough is formed extending from the Punjab plains in the north-west to the Bengal delta in the east.

This low pressure zone attracts wind regimes from the adjoining areas, from short distances in the beginning. But as the level of solar incidence reaches its peak during May-June, the pressure gradient between this low pressure trough and the adjoining seas is so great that it attracts winds from as far as the south of the equator. Accompanying this process and helping this pull of wind regimes is the development of some high pressure centres—in the Indian Ocean, Arabian Sea and over Australia (it being the winter season in Australia).

The wind patterns which are prevalent south of the equator are actually the south-east trade winds which blow from the south-east towards the north-west. These winds, attracted by the low pressure trough over the Indian subcontinent, while moving north of the equator, turn in a clockwise direction (or towards the right), following Farrel’s law. This shift in direction is brought about by the earth’s rotation. Now, the originally south-east trade winds become south-west monsoons blowing towards the north-east.

At this juncture, the Inter- Tropical Convergence Zone (ITCZ) also shifts northwards. The ITCZ is the hypothetical line where the north-east trade winds from the northern hemisphere and the south-east trades from the southern hemisphere meet. The south-west wands now approaching the Indian peninsula have to travel a long distance over the Indian Ocean.

During their long journey, these winds pick up large amounts of moisture and by the time they reach India they are oversaturated. Here, they are known as the south-west monsoons which get divided into the Arabian Sea branch and the Bay of Bengal branch because of the shape of peninsular India. There moisture-laden winds cause heavy rainfall on the windward sides. 

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