The climatic phenomena retreating monsoon is the result of which of the following atmospheric condition in India ?

Question

UnlockIAS · Accepted Answer

Correct — option (3). The south-west monsoon exists because of a pressure gradient. Through the hot weather season the north-western part of India and the plains beyond it are intensely heated, a trough of low pressure develops over them, and air is drawn in from the higher pressure over the Indian Ocean, arriving as the moisture-laden south-west monsoon. Everything about the season depends on that low being maintained. The retreat, therefore, is not a separate phenomenon requiring a separate cause; it is what happens when the low is dismantled. From about the beginning of September the overhead sun has moved south, the intense heating of the north-west slackens, the trough there weakens and fills, and high pressure begins to establish itself over that corner of the country. Since the pressure gradient that sustained the inflow has gone, the monsoon current withdraws from the area where the high has formed. The high then expands, and the crucial point for this question is the direction in which it expands: outward from the north-west towards the south-east, across the plains, then the central highlands, then the peninsula. The monsoon withdraws in exactly the same order, which is why the retreat is described as progressive and why it takes some three months to clear the country, leaving the extreme south last. That is the reverse of the advance, in which the monsoon reached the south first and the north-west last. Option (3) names all of this: the pressure system responsible, high pressure; the place it begins, the north-west; and the direction in which it spreads, towards the south-east. The direction is what makes the option an explanation rather than a description, because it accounts not only for the fact that the monsoon leaves but for the order in which the different parts of India lose it.

Why the other options are wrong — (a) It is true that the advancing monsoon reaches the Himalaya, and true that the range matters — it walls the monsoon inside the subcontinent and forces the Bay of Bengal branch to turn westward up the Ganga plain instead of passing into Central Asia. But arriving at a mountain barrier is not a reason to withdraw, and the timing disproves the option by itself. The monsoon covers the whole country including the Himalayan foothills by about the middle of July, and the rains then continue through the whole of August; if contact with the Himalaya caused the retreat, the retreat would begin in July. It does not begin until September, when the pressure field changes. This option is a good example of a statement that is factually unobjectionable in every clause and still fails as an explanation, and the way to catch it is to check whether the proposed cause and the observed effect are even close together in time.  (b) The apparent southward movement of the sun is the ultimate driver of the whole seasonal cycle, so this option is not wrong so much as too far upstream, and the stem's own wording rules it out. The question asks which atmospheric condition is responsible, and the movement of the sun is an astronomical fact rather than a condition of the atmosphere; it acts on India only through a chain of consequences — reduced insolation, cooling of the land in the north-west, weakening of the thermal low, building of high pressure — and it is the last link of that chain that actually pushes the monsoon out. The timing again shows the gap: the sun begins its southward journey after the June solstice, but the monsoon is at its strongest through July and August and does not withdraw for another two months. Naming the cause of the cause and stopping there leaves the question unanswered.  (d) This option is true in the sense that nothing false can be found in it, and useless for the same reason. Every change in Indian weather could be described as variability in atmospheric pressure over the subcontinent — the onset of the monsoon, a break in the rains, a western disturbance in January, a pre-monsoon thunderstorm. A statement that fits every case distinguishes none of them, and an explanation has to do more than assert that something changed. What the keyed option supplies and this one omits is the specific content: which kind of pressure system is involved, where it forms, and which way it moves. The direction in particular is indispensable, because the retreating monsoon is defined by its progression from the north-west to the south-east, and no statement that leaves out the direction can account for the order in which the rains end.

Concept: The Indian monsoon is a seasonal reversal of the pressure gradient and therefore of the winds. In summer the land heats far faster than the surrounding seas, a trough of low pressure forms over north-western India and extends across the northern plains, and the inter-tropical convergence zone shifts north with the overhead sun. Air flows in from the high pressure over the southern Indian Ocean, crosses the equator, is deflected to the right and arrives on the subcontinent as the south-west monsoon, in two branches, one from the Arabian Sea and one from the Bay of Bengal. Onset over Kerala is normally around the first of June and the current covers the country by about the middle of July. In winter the arrangement is reversed: the land cools faster than the sea, high pressure sits over the northern subcontinent and offshore winds blow out towards the ocean, which is the north-east monsoon and is dry over most of India because it originates over land. The retreating monsoon season of October and November is the transition between the two, and its mechanism is simply the replacement of the summer low by winter high pressure, beginning in the north-west and spreading south-eastwards. The season has a character of its own: clear skies, high temperatures and oppressive humidity, the so-called October heat, and it is the principal season for tropical cyclones forming over the Bay of Bengal and striking the eastern coast.

State PCS 2023 Q30 — Geography PYQ with Answer

State PCS 2023 Q30 — Geography PYQ with Answer