UPPSC 2024 Prelims GS Paper-I · Question 27 of 150

Correct — A, both (A) and (R) are true but (R) is not the correct explanation of (A). Take the two statements separately first. (A) is true: relative humidity is not a quantity of water at all but a ratio — the water vapour actually present in a parcel of air divided by the maximum that parcel could hold at its current temperature, written as a percentage. The maximum, the saturation capacity, climbs steeply with temperature (the Clausius–Clapeyron relation: roughly 7 per cent more capacity for every 1 °C of warming). So if you heat air without adding or removing a single gram of moisture, the numerator stays put while the denominator grows, and the relative humidity falls. That is why relative humidity is at its highest in the coldest hour of the day, just before sunrise, and at its lowest in the warm early afternoon, even though the actual vapour content hardly changes across the day, and it is why a heated room in winter feels dry. (R) is also true, but it is a statement about a different quantity: absolute humidity is the mass of water vapour per unit volume of air, usually grams per cubic metre, and evaporation is precisely the process that moves liquid water into the air as vapour — more evaporation into a given body of air means more vapour per cubic metre, so absolute humidity rises. What (R) cannot do is explain (A). The two statements describe different causes acting on different variables: (A) is what happens when the temperature changes at a fixed moisture content, (R) is what happens when the moisture content itself changes. Worse for the 'explanation' reading, they push relative humidity in opposite directions — adding vapour by evaporation raises the numerator of the ratio and therefore raises relative humidity. If (R) were the mechanism behind (A), relative humidity would go up with warming, not down. Hence (a).

• (b)(A) is false, but (R) is true. — (A) is a standard and correct statement of how relative humidity behaves. Because relative humidity is measured against the air's saturation capacity, and that capacity rises sharply with temperature, warming air whose moisture content is unchanged must show a lower relative humidity.
• (c)Both (A) and (R) are true and (R) is the correct explanation of (A). — This is the trap, and it is tempting because warming does in fact speed up evaporation. But follow that chain through: more evaporation puts more water vapour into the air, which raises the numerator of the relative-humidity ratio and pushes relative humidity up. The fall described in (A) comes from the denominator — the rising saturation capacity — not from anything happening to the vapour supply. A reason that would produce the opposite effect cannot be the explanation.
• (d)(A) is true, but (R) is false. — (R) is true. Absolute humidity is the actual mass of water vapour held in a unit volume of air, and evaporation is the process that supplies that vapour, so a body of air receiving more evaporation does become absolutely more humid. The fault in (R) is relevance, not truth.

Atmospheric moisture is measured in two quite different ways, and Assertion–Reason questions in this area almost always turn on the difference. Absolute humidity and specific humidity are quantities — how much water vapour is actually there (grams per cubic metre of air, or grams per kilogram of air). Relative humidity is a ratio — how close the air is to saturation, that is, to the point at which it can hold no more vapour and condensation begins. Because the saturating capacity of air is fixed almost entirely by its temperature, relative humidity changes whenever either the vapour content changes or the temperature changes, while absolute humidity responds only to vapour being added (evaporation) or removed (condensation, precipitation).

The way to attack an Assertion–Reason item is to grade the two statements independently before you look at the codes at all, and only then ask a third question: is the mechanism named in (R) the mechanism that produces (A)? Here both statements survive the first test, so the real work is the third question — and the cleanest test is directional. Assertion (A) says relative humidity goes DOWN. Reason (R), if you let it act, adds vapour and pushes relative humidity UP. A reason that drives the variable the wrong way is not an explanation, so the code has to be 'both true, R not the explanation'. Note also that UPPSC has scrambled the familiar code order in this paper: what UPSC normally prints as option (b) sits here at option (a), and what UPSC prints as (a) sits here at (c). Read the four codes before you shade a bubble.

• Relative humidity = (water vapour actually present ÷ the maximum the air can hold at that temperature) × 100; absolute humidity = mass of water vapour per unit volume of air, usually grams per cubic metre.
• The saturation vapour pressure of air rises steeply with temperature (Clausius–Clapeyron), by roughly 7 per cent per 1 °C, so warming air without adding moisture lowers its relative humidity and cooling it raises the relative humidity.
• Relative humidity therefore peaks in the coldest part of the day, shortly before sunrise, and reaches its daily minimum in the warm early afternoon, even when the actual vapour content of the air barely moves.
• Specific humidity (grams of vapour per kilogram of air) and the mixing ratio do not change when a parcel merely expands or is compressed; absolute humidity, being measured per unit volume, does change as the parcel expands, which is one more reason it is a separate idea from relative humidity.
• Air cooled to its dew-point temperature reaches 100 per cent relative humidity, and further cooling condenses the excess vapour as dew, frost, fog, mist or cloud.

• Assuming (R) must explain (A) because warming speeds up evaporation — that chain would RAISE relative humidity, not lower it, so it points the wrong way.
• Mixing up the three humidity measures: absolute humidity (g/m³), specific humidity (g/kg) and relative humidity (%). Only the last is a ratio, and only the last changes when you merely change the temperature.
• Reading 'warm air holds more moisture' as 'warm air IS more humid'. It is a statement about capacity, not about content — a hot desert afternoon has huge capacity and very little vapour, hence a very low relative humidity.

Both commissions run this area as Assertion–Reason, and the usual design is exactly this one — two individually true statements about different mechanisms, testing whether you can separate 'true' from 'explains'. UPSC asked almost the mirror image in 2003 (moisture varies with latitude, because capacity varies with temperature) and marked it 'R IS the correct explanation'; UPPSC also scrambles the four code options away from UPSC's standard order, so the letters must be read every time.

• practice — not a real PYQ

  On a day when the actual water-vapour content of the air stays nearly unchanged, the relative humidity is normally highest:

  • (a)shortly before sunrise
  • (b)around midday
  • (c)in the mid-afternoon
  • (d)shortly after sunset

  Answer(a) shortly before sunrise — relative humidity is inversely related to temperature at a fixed vapour content, so it peaks in the coldest hour of the day and bottoms out in the warm early afternoon.

• practice — not a real PYQ

  Two parcels of air contain exactly the same mass of water vapour per cubic metre. Parcel X is at 15 °C and parcel Y at 30 °C. Which one of the following is correct?

  • (a)Parcel X has the higher relative humidity
  • (b)Parcel Y has the higher relative humidity
  • (c)Both parcels have the same relative humidity
  • (d)Relative humidity does not depend on the temperature of the air

  Answer(a) Parcel X has the higher relative humidity — the two parcels have the same absolute humidity, but the warmer parcel Y can hold far more vapour, so the same content is a smaller fraction of its capacity.