UPPSC 2019 Prelims GS Paper-I · Question 48 of 150

Correct — B, West to East. Western disturbances are extra-tropical cyclonic storms that form over the Mediterranean, Black Sea and Caspian region, are picked up by the subtropical westerly jet stream — which in winter shifts south of the Himalaya — and travel eastward into India across Iran, Afghanistan and Pakistan. They therefore arrive over the north-western plain already loaded and leave it progressively drained: each system rains itself out as it moves on, and it is also cut off from its Mediterranean moisture source the further east it goes. The result is the classic winter-rainfall gradient — Punjab, Haryana and western Uttar Pradesh get the most, eastern Uttar Pradesh and Bihar get progressively less, and by Bengal the winter rain from this source is negligible. The rain decreases in the direction of travel, that is from west to east.

• (a)East to West — This is the monsoon pattern, not the winter pattern, and that is exactly why it is the favourite wrong answer. During the south-west monsoon the moisture arrives from the Bay of Bengal and rainfall over the Ganga plain falls off from east to west — about 200 cm around the Bengal-Bihar side down to under 60 cm in western Rajasthan. In winter the source lies in the opposite direction, so the gradient reverses.
• (c)North to South — Winter rain from western disturbances does thin out southwards as you move away from the Himalayan foothills, but that is a secondary effect of the storms tracking along the mountain front, not the gradient the question is about. The question fixes attention on the north-western plain and on how the disturbances themselves weaken, and that weakening runs along their eastward track.
• (d)South to North — The reverse of even the secondary gradient. Winter precipitation increases, not decreases, as you go north towards the western Himalaya, where western disturbances deliver heavy snowfall to Jammu and Kashmir, Himachal Pradesh and Uttarakhand.

A western disturbance is a mid-latitude (extra-tropical) low-pressure system, not a tropical cyclone. It is born over the Mediterranean-Caspian belt, is steered eastward by the subtropical westerly jet stream that migrates south of the Himalaya once the winter cold sets in, and picks up extra moisture over the Caspian and the Persian Gulf on the way. Entering India from the north-west, it produces light to moderate rain over the plains and snow over the western Himalaya. Roughly four to five such systems cross northern India in a winter month; between them they also drive cold waves, fog and, in a bad year, hailstorms.

Reason from the source of moisture, always. Whichever direction the moisture comes from is the direction from which rainfall decreases as you move away from it — Bay of Bengal in the monsoon, so rain falls off eastward-to-westward; Mediterranean in winter, so it falls off westward-to-eastward. For Uttar Pradesh this is a lived fact rather than a textbook one: the west of the state gets useful winter showers, called mahawat, while eastern UP gets far less of them.

• Western disturbances originate over the Mediterranean, Black Sea and Caspian region and are steered into India by the subtropical westerly jet stream.
• They are extra-tropical cyclonic systems — unlike the monsoon, they draw energy from temperature contrasts rather than from warm tropical seas.
• Winter rainfall in the north-western plain is greatest in Punjab, Haryana and western Uttar Pradesh and falls away eastward towards Bihar and Bengal.
• The same systems bring the western Himalaya its winter snow, which feeds the Indus, Jhelum, Chenab, Ravi, Beas, Sutlej and Yamuna in the following summer.
• This winter rain — mahawat in the local idiom — is disproportionately valuable to the rabi crop, above all to wheat, gram and mustard.

• Mixing up the two rainfall gradients of the Ganga plain: monsoon rain decreases east to west, winter rain decreases west to east.
• Calling a western disturbance a cyclone of the Arabian Sea. It is an extra-tropical Mediterranean system that arrives overland, and it has nothing to do with tropical cyclogenesis.
• Assuming winter rain is trivial because the totals are small. A few centimetres in December-January can decide the wheat yield across Punjab, Haryana and western Uttar Pradesh.

UPPSC asks it directly — the cause of winter rain in north-west India, or the direction in which it varies; UPSC prefers to wrap the same fact in a statement pair about westerlies, jet streams or the origin of winter rainfall.

• practice — not a real PYQ

  Western disturbances are steered into north-western India mainly by which of the following?

  • (a)The subtropical westerly jet stream
  • (b)The tropical easterly jet stream
  • (c)The south-west monsoon current
  • (d)The retreating north-east monsoon

  Answer(a) The subtropical westerly jet stream — in winter it shifts south of the Himalaya and carries Mediterranean-origin depressions eastward into India.

• practice — not a real PYQ

  The winter showers brought by western disturbances over north-western India are agriculturally most valuable for which crop?

  • (a)Rice
  • (b)Jute
  • (c)Wheat
  • (d)Cotton

  Answer(c) Wheat — the rabi crop is standing in the fields in December and January, and this rain (mahawat) arrives exactly when wheat, gram and mustard need it.