Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Relative humidity decreases with increasing air temperature.
Reason (R) : Absolute humidit

Question

Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : Relative humidity decreases with increasing air temperature.
Reason (R) : Absolute humidity increases with increasing evaporation.
Select the correct answer from the codes given below :

UnlockIAS · Accepted Answer

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).

Why the other options are wrong — (b) (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) 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) (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.

Concept: 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).

State PCS 2024 Q27 — Geography PYQ with Answer

State PCS 2024 Q27 — Geography PYQ with Answer