NDA (II) 2017 General Ability Test · Question 100 of 150

Correct — C, South-west Monsoon. From June to September the subcontinent heats up and a deep low-pressure trough forms over north-western India, while the Indian Ocean to the south stays relatively cool and at higher pressure. Air is drawn in from the sea, and after crossing the equator it is deflected by the earth's rotation so that it arrives from the south-west. Its two branches, the Arabian Sea branch and the Bay of Bengal branch, sweep moisture across almost the whole country. These four months deliver the overwhelming share of India's annual rainfall, which is why the agricultural year, the reservoirs and much of the economy are keyed to them.

• (a)Western Disturbances — These are extratropical low-pressure systems that travel eastwards from the Mediterranean and Caspian region and reach north-western India in winter. Their rain and snow are important out of all proportion to their volume, because they water the standing rabi wheat crop and feed the Himalayan snowpack, but the total they bring is small beside the summer monsoon.
• (b)North-east Monsoon — In winter the wind reverses and blows from the land out to sea, so it is dry over most of India. It picks up moisture crossing the Bay of Bengal and rains on the Coromandel coast, which is why Tamil Nadu gets much of its rain in October to December, but that is a regional pattern and not the national maximum.
• (d)Retreating Monsoon — The retreating monsoon is the withdrawal phase of October and November, when the low-pressure trough weakens and the winds turn. It is associated with the transition and with tropical cyclones over the Bay of Bengal, and it overlaps with the north-east monsoon rain of the south-east coast; the amounts are far smaller than those of the advancing south-west monsoon.

A monsoon is a seasonal reversal of wind direction, and with it a reversal of the rainfall regime. The Indian monsoon is driven by the differential heating of the huge Asian landmass and the surrounding oceans, and is modified by the shifting of the inter-tropical convergence zone, by the Tibetan plateau's heating and by the tropical easterly jet. Relief decides where the rain actually falls — the Western Ghats and the north-eastern hills stand across the moisture-bearing winds and receive very heavy rain, while the leeward Deccan and western Rajasthan stay dry.

The four options are worth holding as one system rather than four separate facts, because each belongs to a different season and a different region. The south-west monsoon of June to September gives the national maximum. The retreating monsoon of October and November marks the turn. The north-east monsoon of October to December waters the Coromandel coast. The western disturbances of December to February bring winter rain to the north-west. Read that way, the question is simply asking which of the four covers the most ground for the longest time.

• The south-west monsoon runs from about June to September and brings the great bulk of India's annual rainfall.
• It arrives in two branches, the Arabian Sea branch and the Bay of Bengal branch.
• Its winds come from the south-west because the south-east trades, on crossing the equator, are deflected to the right.
• The north-east monsoon of October to December is the main rainy season of the Coromandel coast, including Tamil Nadu.
• Western disturbances bring winter rain and snow to north-western India and are important for the rabi wheat crop.

• Choosing the north-east monsoon because Tamil Nadu depends on it; that is a regional maximum, not a national one.
• Treating the retreating monsoon as a separate rain-bearing system rather than as the withdrawal phase.
• Overrating western disturbances because of their importance to the wheat crop; their volume is small.

NDA sets monsoon questions in almost every paper, on the mechanism, on which system waters which region, or on the seasonal reversal that defines a monsoon climate.

• practice — not a real PYQ

  Tamil Nadu receives most of its rainfall from the

  • (a)South-west monsoon
  • (b)North-east monsoon
  • (c)Western disturbances
  • (d)Local convectional storms

  Answer(b) North-east monsoon — the winter winds pick up moisture over the Bay of Bengal and rain on the Coromandel coast.

• practice — not a real PYQ

  The south-west monsoon winds reach India from the south-west because

  • (a)the Himalayas deflect them
  • (b)the south-east trades are deflected to the right after crossing the equator
  • (c)the Western Ghats block the easterlies
  • (d)the earth is nearest the sun in June

  Answer(b) the south-east trades are deflected to the right after crossing the equator — the Coriolis effect turns them into south-westerlies.