The winter rains caused by Western disturbance in North Western Plain of India gradually decreases from

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2019 Q48 — Geography PYQ with Answer

State PCS 2019 Q48 — Geography PYQ with Answer