MPSC 2023 Prelims Paper-I · Question 30 of 100

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.

• (1)After reaching to the Himalayan ranges the south-west monsoons retreats — 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.
• (2)The apparent movement of the Sun — 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.
• (4)The occurrence of variability in the atmospheric pressure developed over the Indian sub-continent — 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.

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.

The monsoon is the most heavily examined single topic in the Indian geography syllabus, and MPSC tends to ask about its mechanism rather than its statistics, which makes the direction of movement worth holding precisely. The advance runs from the south and south-west towards the north-west, and the retreat runs from the north-west towards the south-east; candidates who remember only that the retreat is 'the opposite' often reverse it under pressure. This question also illustrates a construction the commission uses repeatedly in its 'because' items. All four options here are drawn from the monsoon chapter and none contains an outright falsehood; they are separated instead by how well each fits the question actually asked. One is true but mistimed, one is true but belongs to astronomy rather than meteorology, one is true but so general that it explains nothing, and one names the system, its location and its direction of movement. Reading the stem for the exact demand — an atmospheric condition, responsible for a retreat that proceeds in a particular order — is what makes the fourth distinguishable from the other three.

• The south-west monsoon blows because a trough of low pressure develops over north-western India and the northern plains in summer, reversing the pressure gradient and drawing in moist air from the Indian Ocean in an Arabian Sea branch and a Bay of Bengal branch.
• The monsoon begins to withdraw in September, when the summer low over north-western India weakens and fills and high pressure establishes itself there; the high expands south-eastwards and the monsoon withdraws in the same direction.
• The retreat is thus the reverse of the advance: the monsoon reaches the north-west last and leaves it first, while the extreme south of the peninsula is the last part of the country it leaves, some three months later.
• October and November, the season of the retreating monsoon, are marked by clear skies with high temperature and humidity, popularly called the October heat, and are the main season for tropical cyclones over the Bay of Bengal.
• The north-east monsoon of the cold season is dry over most of India because it blows out from the land, but the branch that crosses the Bay of Bengal gives the Coromandel coast of Tamil Nadu its principal rainfall in the closing months of the year.

• Reversing the direction of the withdrawal; the monsoon advances towards the north-west and retreats away from it towards the south-east, and the order of withdrawal is what the question turns on
• Accepting a statement that names the astronomical cause of a seasonal change when the stem asks for the atmospheric condition through which that cause acts
• Overlooking a mismatch of timing between a proposed cause and the effect it is supposed to explain
• Choosing an option so general that it would fit every weather event in India, on the ground that nothing in it is untrue

Monsoon questions appear in every MPSC paper in some form, and they cluster into a few types: the mechanism of onset and withdrawal, the normal dates and the direction of advance, the two branches and the areas each serves, the reasons for regional variations in rainfall such as the rain shadow east of the Sahyadri, and the special case of Tamil Nadu's winter rain. The commission favours 'because' constructions for the mechanism questions and fills the option set with statements taken from the same chapter, so elimination on grounds of falsity rarely works; the discrimination has to come from timing, from direction and from whether the option answers the precise question asked. For Maharashtra papers, expect the monsoon's behaviour over the state — the Konkan's heavy rain, the rain shadow of the Ghats and the uncertainty of the plateau's rainfall — to be linked to the same mechanism.

No directly related past PYQ was found.

• practice — not a real PYQ

  The withdrawal of the south-west monsoon from the Indian subcontinent normally proceeds in which of the following directions ?

  • (a)From the north-west towards the south-east
  • (b)From the south-west towards the north-east
  • (c)From the north-east towards the south-west
  • (d)Uniformly from the whole country at the same time

  Answer(a) From the north-west towards the south-east — the summer low fills first over north-western India and high pressure builds there, then expands across the plains and the peninsula, and the monsoon withdraws in the same order. The advance runs the other way, reaching Kerala first and the north-west last, so the two movements are mirror images and the extreme south is the first region the monsoon reaches and the last it leaves.

• practice — not a real PYQ

  The Coromandel coast of Tamil Nadu receives the greater part of its annual rainfall during which of the following periods ?

  • (a)June to September, from the south-west monsoon
  • (b)October to December, from the retreating and north-east monsoon
  • (c)March to May, from pre-monsoon thunderstorms
  • (d)January and February, from western disturbances

  Answer(b) October to December, from the retreating and north-east monsoon — the coast lies in the rain shadow of the Western Ghats during the south-west monsoon, but in the cold season the north-easterly flow crosses the Bay of Bengal, gathers moisture and strikes the coast directly. Western disturbances bring winter rain to north-western India rather than to the peninsula, and pre-monsoon thunderstorms contribute only a small share of the annual total.