For condensation to occur, how should the air temperature generally be?

Correct answer: It should fall

Direct answer: B, the temperature should fall. Condensation is the change of water vapour into tiny liquid-water droplets. When moist air cools, it can hold less water vapour at that temperature. If cooling continues to the dew point or beyond, excess vapour changes into droplets on particles in the air or on a cool surface. The reasoning is: moist air cools; its vapour-holding capacity decreases; vapour becomes liquid; condensation occurs. A is wrong because warming generally increases the capacity of air to hold vapour and does not by itself cause condensation. C is not the general condition asked for; constant temperature does not normally initiate cooling-related condensation. D is wrong because very dry air contains little vapour available to condense. Condensation can form clouds, fog or dew when suitable moisture and cooling are present. Memory cue: evaporation is helped by heating, while condensation is commonly helped by cooling. The word “generally” matters because actual condensation also requires sufficient moisture and suitable surfaces or particles.