During winter nights, when the temperature drops sharply, what happens to the water-vapour-holding capacity of air?

Correct answer: It decreases greatly, so the air may become saturated

Correct answer: B. Warm air can contain more water vapour than cold air because the saturation amount depends strongly on temperature. When air cools during a winter night, its capacity to hold water vapour decreases. If the actual amount of vapour remains high enough, the relative humidity rises until the air reaches saturation. Further cooling may cause some vapour to condense as dew, fog or cloud droplets, depending on the surroundings. A is wrong because cooling reduces, rather than increases, the saturation capacity. C is wrong because the capacity changes with temperature. D is wrong because ordinary cooling does not turn the whole atmosphere into a solid. Important distinction: the capacity decreases; the existing vapour does not automatically vanish. Memory cue: cold air holds less vapour, so cooling can bring air to saturation.