Why is fog often seen in river valleys after night?

Correct answer: Cold dense air and moisture collect in valleys

Correct answer: C. During a clear night, valley slopes and nearby ground lose heat by radiation. The air touching them cools, becomes denser, and flows downward into the lower valley. Rivers and damp ground supply moisture. Because the valley is relatively enclosed, cold moist air can collect there; its temperature may reach the dew point, producing tiny suspended droplets and fog. This situation can be associated with a temperature inversion, in which colder air remains below relatively warmer air. A is wrong because river valleys are often moist, not always dry. B is wrong because the relevant air becomes colder and denser, not very warm. D is wrong because sunlight does not arrive earlier in valleys. Memory cue: valleys collect cold air and moisture.