Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Jet streams discovered during World War II are high altitude easterly winds.
Reason (R) :

Question

Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Jet streams discovered during World War II are high altitude easterly winds.
Reason (R) : Jet streams flow with a speed of 300 – 500 km/hour.
Select the correct answer from the codes given below :

UnlockIAS · Accepted Answer

Correct — B, (A) is false, but (R) is true. The Assertion fails on a single word: direction. The jet streams that aviation ran into during the Second World War — the polar-front jet and the subtropical jet — are WESTERLY winds. They blow from west to east, in narrow, fast, meandering ribbons of air near the tropopause, roughly 9 to 12 kilometres up. That is how they were discovered in the first place: American B-29 crews flying eastward towards Japan at high altitude in 1944–45 found themselves pushed along by ferocious tailwinds and, flying back, almost stalled against them, and the same westerly current had already been noticed by the Japanese meteorologist Wasaburo Oishi in the 1920s from pilot balloons released near Mount Fuji, and named Strahlstromung by Heinrich Seilkopf in 1939. Calling them 'easterly' inverts the fact, so the Assertion is false. The Reason survives. A speed of 300 to 500 km per hour is the band Indian geography textbooks give for jet-stream velocity, especially in winter when the temperature contrast that drives the flow is steepest; observed maxima are around 400 km per hour, and in summer the jets slow considerably. Because the figure is the one standard sources carry and is of the right order, the Reason is treated as true. With (A) false and (R) true, the answer is (b). Note also that the Reason could never have explained the Assertion in any case: a speed tells you nothing about a direction. Even had the Assertion been worded as 'westerly', the correct code would then have been (c) — both true, but R not the explanation of A — never (a). One point of provenance, because it causes real confusion. The printed Series-D booklet reads 'high altitude easterly winds', and the Hindi column of the same booklet prints purvi, meaning easterly, which confirms it. Some widely circulated typed reproductions of this paper print 'westerly' instead; on that wording the commission's key of (b) would make no sense. The booklet is the authority, the key is coherent with it, and this card is written to the booklet.

Why the other options are wrong — (a) Wrong twice over. First, the Assertion is false — the jet streams identified during the Second World War are westerly, flowing from west to east, not easterly. Second, even if the Assertion had been true, the Reason could not explain it: the speed at which a wind travels has no bearing on the compass direction from which it comes. In assertion–reason items, always test the logical link separately from the two facts.  (c) This is the trap for anyone reading a version of the paper that prints 'westerly'. On the booklet's actual wording the Assertion is false, so the option collapses at the first test. It is worth noticing why (c) is so tempting: the Reason genuinely is a free-standing true statement that does not explain the Assertion, so if you accept the Assertion at face value this is exactly where you land.  (d) Reverses both judgements. The Assertion is the false half — jet streams are westerly — while the Reason is the true half. On speed, 300 to 500 km per hour is the range standard Indian texts quote for jet streams, with the higher values in winter; the number in the Reason is not the planted error.

Concept: Jet streams are narrow bands of very fast wind in the upper troposphere, close to the tropopause, formed where a sharp horizontal temperature gradient exists between air masses. Because the pressure gradient produced by that temperature contrast strengthens with height and is then deflected by the Coriolis force, the resulting flow runs almost along the temperature boundary — which in mid-latitudes means from west to east. Both hemispheres have them. The two classic ones are the polar-front jet, over the boundary between polar and mid-latitude air at roughly 60 degrees latitude, and the subtropical jet at about 25 to 30 degrees, both westerly. Over India the subtropical westerly jet is the winter feature: it shifts south of the Himalaya, splits around the Tibetan Plateau, and steers the western disturbances that bring winter rain to the north-west. In summer it withdraws north of the mountains, and its place is taken by the tropical easterly jet.

State PCS 2024 Q60 — Geography PYQ with Answer

State PCS 2024 Q60 — Geography PYQ with Answer