Why may less dew form on a cloudy night?

Correct answer: Because clouds reduce radiational cooling of the ground

Correct answer: A. Dew forms when a surface cools sufficiently, usually during the night, and brings the moist air touching it to its dew point. On a clear night, the ground loses heat by outgoing terrestrial radiation more easily, so leaves, grass and other exposed surfaces can cool strongly. Clouds absorb and re-radiate some of this outgoing energy back toward the surface. This reduces the net radiational heat loss and usually keeps the surface warmer than it would be under a clear sky. Therefore, the surface may not reach the dew point, and less liquid dew forms. A is correct because it states the actual process. B is wrong because clouds generally reduce, rather than increase, surface cooling. C is wrong because clouds do not destroy water vapour. D is wrong because frost forms only when the relevant surface reaches freezing conditions; clouds do not always produce frost. Memory cue: clear, calm nights favour dew; cloudy nights usually reduce radiational cooling.