Why will little dew form even if a surface cools strongly when the air is very dry?

Correct answer: Because water vapour availability is low

The correct answer is A. Dew requires two basic conditions: a surface must cool sufficiently, usually to its dew point, and enough water vapour must be present in the nearby air. Very dry air contains little water vapour, so even strong surface cooling may produce only a small amount of condensation. The wording of C is logically true in a limited sense—condensation is not absolutely impossible in dry air—but it does not explain why little dew forms and is therefore not the best answer. B is wrong because dry air has low, not high, relative humidity unless it is cooled substantially. D is wrong because dew can form in many regions, not only in the Himalayas. Memory cue: “cooling is necessary, but moisture supplies the material for dew.”