How can one determine the relative humidity on a particular day at a given place?
- (a)If temperature is given
- (b)If absolute humidity is given
- (c)If both temperature and absolute humidity are given
- (d)If percentage of absolute humidity is given
Correct — C, if both temperature and absolute humidity are given. Relative humidity is a ratio: it compares how much water vapour the air actually holds with the most it could hold at that temperature. The numerator is what absolute humidity gives you — the actual mass of water vapour in a given volume of air. The denominator depends on temperature alone, because warm air can hold far more vapour than cold air before it saturates. So you need both numbers. Absolute humidity by itself tells you the vapour present but not how close that is to saturation; temperature by itself tells you the capacity but not what is actually there. Give a student both and the relative humidity follows at once.
- (a)If temperature is given — Temperature fixes only the maximum vapour the air can hold. Without knowing how much vapour is actually present, you cannot form the ratio.
- (b)If absolute humidity is given — Absolute humidity gives the vapour actually present, but the same amount of vapour is a high relative humidity on a cold morning and a low one on a hot afternoon. You still need the temperature.
- (d)If percentage of absolute humidity is given — There is no such standard quantity as a percentage of absolute humidity; absolute humidity is an amount, usually in grams per cubic metre, not a percentage. The option is a decoy built from familiar words.
Absolute humidity is the mass of water vapour per unit volume of air, typically grams per cubic metre. Relative humidity is the ratio of the water vapour actually present to the saturation amount at the same temperature, expressed as a percentage. Saturation vapour pressure rises steeply with temperature, so relative humidity changes as air warms or cools even when no water is added or removed.
The way into this question is to write the definition as a fraction and then ask what each option supplies. The top of the fraction is the vapour present, which is absolute humidity. The bottom is the saturation capacity, which is set by temperature. An option that gives you only one of the two leaves the fraction unfinished. This also explains a familiar daily observation the exam likes to reuse — relative humidity is usually highest just before dawn and lowest in the afternoon, not because the amount of vapour changed much but because the temperature did.
- Relative humidity is the ratio of the partial pressure of water vapour to the saturation vapour pressure at the same temperature, written as a percentage.
- Saturation vapour pressure rises with temperature, so warm air can hold much more vapour before condensing.
- Colder air can contain less vapour, so water tends to condense out at lower temperatures — which is why dew and fog form on cool nights.
- Absolute humidity is measured as a mass per unit volume, so it is an amount rather than a percentage.
Both parts are needed, which is why neither temperature alone nor absolute humidity alone is enough.
- Treating absolute humidity and relative humidity as the same measurement in different units.
- Assuming high relative humidity means a lot of water vapour — cold saturated air holds very little.
- Being drawn to option (d) because 'percentage' appears in it, although no such quantity is standard.
Asked as a reasoning item — you are given a definition in disguise and must work out which pair of inputs completes the ratio.
Which of the following statements regarding humidity is/are correct? 1. It is a ratio between the actual amount of water vapour present in the atmosphere and the maximum amount it can hold at a given temperature. 2. When the relative humidity is high, more water evaporates from the skin. 3. Higher the air temperature lower the relative humidity of the air. Select the answer using the code given below:
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(c) 1 and 3 only
Same definition, asked from the other side. Statement 1 there spells out the ratio this CDS item wants you to reconstruct, and statement 3 there is the temperature dependence that makes absolute humidity alone insufficient. Note that the NDA 2024 papers are still awaiting the official UPSC key, so treat that answer as our own solving rather than as settled.
Which one of the following is not a form of condensation?
- (a) Dew
- (b) Fog
- (c) Frost
- (d) Sleet
Answer(d) Sleet
The consequence of the ratio this item defines. When cooling air pushes relative humidity to 100 per cent the vapour condenses out as dew, fog or frost; sleet is precipitation that has already condensed and then frozen. Knowing which processes are condensation is what makes the temperature side of relative humidity intuitive.
- practice — not a real PYQ
If a parcel of air is warmed while no water vapour is added or removed, its relative humidity will
- (a)rise, because warm air holds more vapour
- (b)fall, because the saturation capacity increases
- (c)stay exactly the same
- (d)become zero
Answer(b) fall, because the saturation capacity increases — the same vapour is now a smaller fraction of what the air could hold.
- practice — not a real PYQ
Absolute humidity is usually expressed in
- (a)per cent
- (b)grams of water vapour per cubic metre of air
- (c)degrees Celsius
- (d)millibars of atmospheric pressure
Answer(b) grams of water vapour per cubic metre of air — it is an amount, not a ratio.