If the surface is cold at night but the air above is dry and well mixed, why can both dew and fog remain limited?

Correct answer: Because dry, well-mixed air weakens both moisture supply and a stable surface layer

Direct answer: A. Cooling is important for dew and fog, but cooling alone is not enough. Water vapour must also be available, and the air near the surface often needs to remain relatively stable so that a cool, moist layer can develop. Dry air supplies little moisture for condensation. Strong mixing can bring drier air from above downward and carry the cool, moist air away from the surface, preventing a concentrated near-surface layer. Therefore dew on the surface and fog in the air may both remain weak even though the surface is cold. B is wrong because a cold surface does not automatically produce heavy rain; rain needs suitable cloud growth and precipitation processes. C is wrong because both dew and fog come from water vapour. D is scientifically meaningless: mixing does not create icy mountains. Memory cue: condensation needs cooling plus moisture; fog also benefits from near-surface stability.