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 :
- (a)Both (A) and (R) are true and (R) is the correct explanation of (A).
- (b)(A) is false, but (R) is true.
- (c)Both (A) and (R) are true, but (R) is not the correct explanation of (A).
- (d)(A) is true, but (R) is false.
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.
- (a)Both (A) and (R) are true and (R) is the correct explanation of (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)Both (A) and (R) are true, but (R) is not the correct explanation of (A). — 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)(A) is true, but (R) is false. — 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.
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.
The point being tested is that 'jet stream', used without qualification and tied to the wartime discovery, means a WESTERLY flow. Easterly jets are real, but they are separate, individually named features — and knowing that is what lets you see why the examiner planted the word 'easterly'. The tropical easterly jet blows from east to west in the upper troposphere over peninsular India during the summer monsoon, driven by the reversed temperature gradient that develops when the Tibetan Plateau heats up and becomes warmer aloft than the equatorial region to its south; its establishment is one of the markers of monsoon onset. The African easterly jet is a mid-level easterly current over the Sahel, and the Somali Jet is a low-level cross-equatorial current that feeds the Indian monsoon. None of these is what was found over the Pacific in the Second World War. The disciplined way to answer any assertion–reason item is the three-step one this question rewards: judge the Assertion alone, judge the Reason alone, and only then ask whether the Reason causes the Assertion. Here step one settles it, and steps two and three merely confirm which of the four codes to pick.
- Jet streams are narrow, fast, meandering currents of air near the tropopause, roughly 9 to 12 kilometres up; the polar-front jet and the subtropical jet are both WESTERLY, flowing from west to east, and they occur in both hemispheres, not only the north.
- They were recognised during the Second World War when high-flying aircraft, especially American B-29s over the Pacific, met powerful westerly winds; the current had already been observed by Wasaburo Oishi with pilot balloons near Mount Fuji in the 1920s, and Heinrich Seilkopf coined the German term Strahlstromung in 1939.
- Indian geography texts give jet-stream speeds of about 300 to 500 km per hour, strongest in winter when the temperature contrast driving them is greatest; observed maxima are around 400 km per hour, and summer speeds are much lower.
- Easterly jets exist as separate named features: the tropical easterly jet blows east to west in the upper troposphere over peninsular India during the summer monsoon, tied to heating of the Tibetan Plateau; the African easterly jet crosses the Sahel; the Somali Jet is a low-level cross-equatorial current.
- In winter the subtropical westerly jet lies south of the Himalaya and steers western disturbances into north-west India; in summer it withdraws north of the mountains, and its retreat and the appearance of the tropical easterly jet accompany the onset of the south-west monsoon.
The Assertion inverts the direction of the wartime jets, so it is false; the Reason is a true but unrelated statement about speed. Hence code (b), (A) false and (R) true.
- Assuming any wind called a 'jet' is easterly because the tropical easterly jet is the one most drilled in monsoon chapters. The unqualified 'jet stream' of the wartime discovery is westerly; easterly jets are separate, named features.
- Marking (a) whenever both statements look true. Speed cannot explain direction, so even a true Assertion here would give (c), never (a). Always test the causal link on its own.
- Believing jet streams occur only in the Northern Hemisphere. Both hemispheres have polar-front and subtropical jets; UPSC tested exactly this misconception in 2020.
UPPSC likes upper-air circulation as an assertion–reason item where one word — a direction, a hemisphere, a season — is quietly inverted; UPSC asks it as multi-statement questions on where jets occur and how they interact with cyclones and the monsoon. Either way, learn direction, latitude, altitude and season together, not just the name.
Consider the following statements: 1. Jet streams occur in the Northern Hemisphere only. 2. Only some cyclones develop an eye. 3. The temperature inside the eye of a cyclone is nearly 10°C lesser than that of the surroundings. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 and 3 only
- (c) 2 only
- (d) 1 and 3 only
Answer(c) 2 only
UPSC planting the same kind of error in the same topic — there it is the hemisphere ('Northern Hemisphere only') that is quietly wrong, here it is the direction ('easterly'). Both reward a student who has stored jet streams with all their attributes attached: both hemispheres, westerly, near the tropopause.
Westerlies in southern hemisphere are stronger and persistent than in northern hemisphere. Why? 1. Southern hemisphere has less landmass as compared to northern hemisphere. 2. Coriolis force is higher in southern hemisphere as compared to northern hemisphere Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(a) 1 only
The same westerly belt, one layer lower. The mid-latitude westerlies at the surface and the jet streams aloft belong to one circulation system, so a student who is clear that mid-latitude flow runs west to east will never accept 'easterly' in this Assertion.
Which one of the following causes is responsible for rainfall during winters in north-western part of India ?
- (a) Retreating Monsoon
- (b) Cyclonic depression
- (c) Western disturbances
- (d) South-West Monsoon
Answer(c) Western disturbances
The practical consequence of the westerly jet in the Indian syllabus. Western disturbances are extratropical storms carried in from the Mediterranean by the subtropical WESTERLY jet after it shifts south of the Himalaya in winter — the very direction this Assertion gets wrong.
The winter rains caused by Western disturbance in North Western Plain of India gradually decreases from
- (a) East to West
- (b) West to East
- (c) North to South
- (d) South to North
Answer(b) West to East
Follows the same westerly flow across the map — the disturbances enter from the west and weaken as they travel east, which is only intelligible once you know the steering current runs west to east. A direct rebuttal of the 'easterly' Assertion in this question.
- practice — not a real PYQ
The tropical easterly jet stream over the Indian subcontinent is associated with which one of the following?
- (a)The winter season, when it steers western disturbances into north-west India
- (b)The summer monsoon season, its development being linked to intense heating of the Tibetan Plateau
- (c)The retreating monsoon, when it forms over the Bay of Bengal
- (d)The pre-monsoon period, when it causes Nor'westers in eastern India
Answer(b) The summer monsoon season, its development being linked to intense heating of the Tibetan Plateau — the tropical easterly jet appears in the upper troposphere over peninsular India in summer and its establishment accompanies monsoon onset. Western disturbances in winter are steered by the subtropical WESTERLY jet, not by an easterly one.
- practice — not a real PYQ
Consider the following statements about jet streams: 1. They are found in both the Northern and the Southern Hemisphere. 2. The polar-front and subtropical jet streams blow from west to east. 3. They are located close to the tropopause. Which of the statements given above are correct?
- (a)1 and 2 only
- (b)2 and 3 only
- (c)1 and 3 only
- (d)1, 2 and 3
Answer(d) 1, 2 and 3 — jet streams occur in both hemispheres, the polar-front and subtropical jets are westerly, and they lie near the tropopause at roughly 9 to 12 kilometres. All three statements are correct.