Why may fog be more common in a valley in the morning?

Correct answer: Cold moist air can collect in the valley

Correct answer: A. At night, the ground loses heat, especially when the sky is clear. The air touching the ground cools, and colder air is denser, so it can flow downhill and collect in a valley. If this air contains enough moisture and cools to its dew point, water vapour condenses into many tiny suspended droplets. These droplets make fog, which is especially noticeable in the morning before sunlight warms the valley and evaporation or mixing reduces it. A is correct because it describes both important conditions: moist air and pooling of cold air. B is wrong because valleys do contain air; the air may simply be relatively still or trapped. C is wrong because a valley is not always hot, and morning cooling is often important. D is wrong because a valley does not destroy water vapour. Memory cue: valley fog = moist air plus cold-air pooling.