UPPSC 2025 Prelims GS Paper-I · Question 115 of 150

Correct — A, both (A) and (R) are true, but (R) is not the correct explanation of (A). Read that option text carefully, because this booklet has inverted the code you have drilled: on this paper (a) is the 'both true, R does NOT explain A' slot and (d) is the 'both true and R IS the explanation' slot — the reverse of the usual UPSC arrangement. Now the substance. The Assertion is true, and it is straightforward physics. Turning liquid sweat into vapour costs energy — the latent heat of vaporisation, roughly 2,400 kilojoules for every litre of sweat that actually evaporates — and that energy is drawn out of the skin and the blood flowing beneath it. The body is therefore cooled. This route matters most exactly when the others fail: radiation, conduction and convection all need the surroundings to be cooler than the skin, so once air temperature climbs towards skin temperature, evaporation is very nearly the only way left to shed heat. The Reason is also true. Humid heat is more uncomfortable than dry heat of the same temperature, which is why weather services report a heat index rather than the bare thermometer reading. But the Reason does not explain the Assertion. It is the other way round. Because cooling depends on evaporation, and because evaporation slows when the surrounding air is already close to saturated — the vapour-pressure difference between wet skin and humid air collapses — sweat then drips off instead of evaporating, less heat leaves the body, and the person feels worse. The Assertion is the cause and the Reason is the consequence; the explanatory arrow runs A to R, not R to A. Both true, R does not explain A, and on this paper that is option (a).

• (b)(A) is true, but (R) is false — This is the single most dangerous option on the page, and not because the physics in it is tempting. It is what a candidate marks who reasons about the substance perfectly — both statements true, R does not explain A — and then reaches for the position that answer occupies on an ordinary UPSC paper. On this booklet that position holds a different statement altogether. On the merits the option is plainly wrong: the Reason is true. Humid heat genuinely is more uncomfortable, and the effect is measurable through the heat index and the wet-bulb temperature, which is why humidity, not temperature alone, is the standard measure of heat stress.
• (c)(A) is false, but (R) is true — The Assertion is not false. Evaporative cooling is the dominant route by which the human body sheds heat in hot conditions, precisely because it is the only one that does not require the surroundings to be cooler than the skin. Candidates sometimes reject the Assertion on the ground that sweating alone does not cool you — which is a real point, since sweat that drips off carries away almost nothing — but the Assertion says evaporation of sweat, not secretion of sweat, and evaporation is exactly the step that removes the heat.
• (d)Both (A) and (R) are true and (R) is the correct explanation of (A) — Both statements are true, so this option gets that far, but the explanatory arrow points the wrong way. Discomfort in humid weather does not explain why evaporating sweat cools the body; the latent heat of vaporisation explains that, and it works whatever the humidity. What high humidity does is obstruct the mechanism in the Assertion, and the resulting discomfort is the consequence. Note also that in this booklet (d) is the 'R IS the explanation' slot — the meaning that sits at (a) on a normal UPSC paper — so a candidate applying the remembered code arrives here for the wrong reason.

The human body exchanges heat with its surroundings by four routes: radiation, conduction, convection and evaporation. The first three are driven by a temperature difference and only work while the environment is cooler than the skin, which sits at roughly thirty-three to thirty-five degrees Celsius. Evaporation is different. It works on a vapour-pressure difference rather than a temperature difference, so it keeps working even when the air is hotter than the body. Sweat glands wet the skin, the water absorbs its latent heat of vaporisation from the skin and the blood beneath, and it leaves as vapour taking that heat with it. Crucially, only sweat that evaporates cools; sweat that runs off the body has done nothing. The rate of evaporation depends on how far the air is from saturation, which is why relative humidity, not temperature alone, governs how hot a hot day feels.

Work this item in three steps and the code cannot catch you out. Is A true on its own? Yes — latent heat of vaporisation. Is R true on its own? Yes — humid heat is measurably more oppressive. Does R account for A? No, and the test that settles it is to ask which way the causal arrow runs. If you delete the Reason, the Assertion survives untouched: sweat evaporating would still cool the body in perfectly dry air, indeed better. If you delete the Assertion, the Reason loses its basis entirely, because humidity is uncomfortable only because it obstructs evaporative cooling. A statement that depends on the Assertion cannot be the Assertion's explanation; it is its consequence. Reversed-arrow pairs like this are among the commonest assertion-reason traps, and they are the reason the three-step method is worth more than instinct. Only then look at the printed options — and on this paper, read them, because (a) and (d) have exchanged meanings.

• Evaporating water absorbs its latent heat of vaporisation — about 2,260 kilojoules per kilogram at 100 degrees Celsius and roughly 2,400 kilojoules per kilogram at skin temperature — and that heat is drawn from the skin and the blood beneath it
• Only sweat that evaporates cools the body; sweat that drips off carries away almost no heat
• Radiation, conduction and convection require the surroundings to be cooler than the skin, so once the air approaches skin temperature evaporation becomes very nearly the only route left for shedding heat
• Evaporation is driven by the vapour-pressure difference between wet skin and the surrounding air, so high relative humidity slows it — which is why dry heat is far more bearable than humid heat at the same temperature
• This is why heat stress is measured by wet-bulb temperature and heat-index values rather than by the dry-bulb thermometer reading alone, a sustained wet-bulb temperature near 35 degrees Celsius being regarded as the limit of human tolerance
• The same latent-heat mechanism explains the desert or evaporative cooler and the traditional unglazed earthen pot, which keeps water cool by seepage and evaporation through its porous walls — and both, like sweating, work poorly in humid weather

• The inverted code in this booklet. Here (a) means 'both true, R does NOT explain A' and (d) means 'both true and R IS the explanation'. A candidate who reasons correctly and then applies the remembered UPSC layout marks (b) and loses the mark
• Accepting a reversed explanatory arrow. Test it by deletion: remove the Reason and the Assertion still stands, remove the Assertion and the Reason collapses — so the Assertion explains the Reason, not the other way round
• Confusing sweating with cooling. Secreting sweat costs the body water; it is the evaporation of that sweat, and only that, which removes heat

UPPSC likes everyday-physics assertion-reason pairs of exactly this kind — a familiar sensation paired with a textbook mechanism — and it also asks the underlying physics directly, as in its 2019 questions on the vacuum flask and on sublimation. UPSC prefers to embed the same idea in a climate framing, asking what a rising wet-bulb temperature means for the human body's ability to shed heat.

• practice — not a real PYQ

  Evaporation of sweat cools the human body chiefly because

  • (a)sweat is at a lower temperature than the skin
  • (b)evaporating sweat absorbs its latent heat of vaporisation from the skin
  • (c)sweat reflects incident solar radiation away from the skin
  • (d)sweat increases the thermal conductivity of the skin

  Answer(b) evaporating sweat absorbs its latent heat of vaporisation from the skin — roughly 2,400 kilojoules for every litre that evaporates, which is why sweat that merely drips off does not cool you.

• practice — not a real PYQ

  On a hot day, a high relative humidity makes the heat feel more oppressive mainly because

  • (a)water vapour in the air absorbs more solar radiation
  • (b)the air's temperature rises further as humidity increases
  • (c)evaporation of sweat from the skin slows down
  • (d)the atmospheric pressure falls as humidity increases

  Answer(c) evaporation of sweat from the skin slows down — humid air is close to saturation, so the vapour-pressure difference driving evaporation shrinks and the body sheds less heat.