Question

Which of the following are the main reasons for the origin of the southwest monsoon in India?
I) Development of low pressure in northwest India.
II) Deflection of trade winds.
III) The arrival of a cyclonic stream from the Pacific Ocean.
IV) Heating of the Arabian Sea and the Bay of Bengal.

A) I, II and III
B) I and II
C) II and III
D) III and IV

Answer 1

Hint: The climate of India is classified as Monsoon type and is characterised by seasonal reversal of winds and a definite rainy season. India receives the majority of its precipitation from the southwest monsoon in the season of advancing monsoon from June to September.

Complete answer:
According to the Thermal Theory by Sir Edmund Halley, the southwest monsoon in India originates from the great difference of temperature between the heated land surface of India and the surrounding oceans that exists during the summer months. The general idea is that the warm air over the land in north-west India rises to create a vacuum effect, therefore, producing a low-pressure zone and damp air from the sea flows in to take its place, thus resulting in the strong south-west winds.

In summer, due to the apparent movement of the sun, the InterTropical Convergence Zone(ITCZ)- Region of Earth near the equator where the trade winds of Northern and Southern Hemisphere come together- shifts near Tibet causing the heating of the Tibetan Plateau. Winds from the Mascarene Islands head in a north-westerly direction towards the east coast of Africa and experience the Coriolis effect near the equator. Coriolis effect being a pseudo force that exists because of the Earth’s rotational movement deflects these winds eastwards causing them to blow from south-west to north-east direction. These winds enter into India in two branches:-
I) Arabian sea Branch
II) Bay of Bengal branch

Only statements I and II are correct and thus, the correct answer is Option B- I and II.

Note: The vast size of the Indian subcontinent helps in developing the thermal gradient necessary to attract the vapour laden air from the sea. Very high and extensive mountain systems of the Himalayas in the north pose a formidable physical barrier between tropical and polar air masses. These hurdles induce the precipitation of water from moisture-laden air. As the air climbs up the Himalayas, their moisture retention capacity decreases due to the cooling effect and this results in high rainfall.