Why is dew formation more likely in a valley at night?

Correct answer: Because cool, dense air collects in valleys and may bring surfaces to the dew point.

Direct answer: A is correct. Dew forms when water vapour changes into tiny liquid droplets on a surface that has cooled to or below the dew point. At night, the ground commonly loses heat by radiation. Valley slopes and nearby surfaces can therefore become cold, while cool, dense air drains downward and collects in the valley. If this air contains enough moisture, its temperature may reach the dew point; the vapour then condenses on grass, leaves, or other surfaces as dew. B is wrong because sunlight is absent at night and cannot provide extra heating. C is wrong because valleys can contain moist air. D is wrong because lower altitude does not make condensation impossible; local cooling is the important factor. Dew is different from fog because dew forms on surfaces, while fog consists of suspended droplets in air. Memory cue: clear night plus cold-air pooling plus moisture favours dew.