CDS (II) 2021 General Knowledge · Question 65 of 120

Correct — A, Punjab. The south-west monsoon reaches India in two arms, and Punjab is where both of them arrive. The Bay of Bengal arm enters Bengal from the south and south-east, is turned west by the Himalaya, and, in NCERT's words, 'moves westward along the Ganga plains reaching as far as the Punjab plains'. The Arabian Sea arm splits into three, and the third of those strikes Saurashtra and Kachchh, crosses west Rajasthan along the Aravalli with very little rain to give, and then, again in NCERT's words, 'in Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.' Punjab, in other words, is the meeting ground — which is why it is the answer even though it is far from the wettest of the four States listed.

• (b)Maharashtra — Maharashtra is watered by the Arabian Sea arm alone. That arm climbs the Western Ghats and drops 250 to 400 centimetres on the windward slope, then descends and dries, which is why the Deccan east of the Ghats is a rain-shadow belt.
• (c)Meghalaya — Meghalaya is the Bay of Bengal arm's work alone. The sub-branch that strikes the Garo and Khasi hills gives Mawsynram the highest average annual rainfall in the world, but no Arabian Sea air reaches it.
• (d)Tamil Nadu — Tamil Nadu is the State that misses the south-west monsoon twice over — its coast runs parallel to the Bay of Bengal arm and it lies in the rain shadow of the Arabian Sea arm. Its rain comes later, in October and November, from the retreating monsoon and its cyclonic depressions.

The south-west monsoon reaches the subcontinent as two arms because the peninsula splits the incoming flow: one arm over the Arabian Sea and one over the Bay of Bengal. The Arabian Sea arm divides again into three — one blocked by the Western Ghats, one entering north of Mumbai and travelling inland along the Narmada and Tapi valleys, and one striking Saurashtra and Kachchh. The Bay arm is deflected by the Arakan hills into Bengal, then splits under the pull of the low pressure over north-west India, one part running west up the Ganga plains and the other up the Brahmaputra valley.

The instinct here is to pick the wettest State on the list, and that instinct picks Meghalaya and gets it wrong. The question is not about how much rain a State gets but about how many arms deliver it. Work along the path of each arm and the answer follows: the Bay arm ends its westward run in Punjab, and the driest of the three Arabian Sea streams ends its north-eastward run in the same place. That is also why the western Himalaya get rain from a combined flow rather than from either arm alone.

• The south-west monsoon approaches India in two arms, one over the Arabian Sea and one over the Bay of Bengal.
• The Bay of Bengal arm runs west along the Ganga plains as far as the Punjab plains, while another part of it goes up the Brahmaputra valley.
• The third stream of the Arabian Sea arm crosses Saurashtra, Kachchh and west Rajasthan and joins the Bay arm over Punjab and Haryana.
• Mawsynram, on the crest of the Khasi hills, receives the highest average annual rainfall in the world from the Bay arm.
• Tamil Nadu stays dry in the south-west monsoon because its coast lies parallel to the Bay arm and in the rain shadow of the Arabian Sea arm.

• Choosing the wettest State on the list instead of the one reached by both arms.
• Assuming Tamil Nadu gets heavy south-west monsoon rain because it is in the deep south.
• Treating the Arabian Sea arm as a single stream; it divides into three, with very different outcomes.

As a State-to-arm identification, as a statements item on the two branches, or as a question on why a named region stays dry in a particular season.

• practice — not a real PYQ

  The Tamil Nadu coast receives little rain during the south-west monsoon mainly because

  • (a)it lies too far south for the monsoon to reach
  • (b)it runs parallel to the Bay of Bengal arm and lies in the rain shadow of the Arabian Sea arm
  • (c)the Eastern Ghats are too high for moist air to cross
  • (d)the monsoon withdraws before reaching it

  Answer(b) — the coast is aligned with the Bay arm rather than across it, and the Western Ghats have already wrung out the Arabian Sea arm.

• practice — not a real PYQ

  Which one of the following places receives the highest average annual rainfall in India?

  • (a)Agumbe
  • (b)Mawsynram
  • (c)Amboli
  • (d)Pasighat

  Answer(b) Mawsynram — on the crest of the Khasi hills, where a sub-branch of the Bay of Bengal arm is forced to rise.