Why can fog remain longer in a valley during the morning?

Correct answer: Cold, moist air collects in the valley and mixes less

Correct answer: C. At night, the ground loses heat and the air touching it becomes colder. Cold air is denser than warm air, so it tends to flow downhill and collect in low-lying valleys. If this air contains enough moisture, cooling can bring it to saturation, and tiny droplets remain suspended as fog. Valleys may also be sheltered from strong winds, so vertical and horizontal mixing is weak. After sunrise, sunlight gradually warms the ground and the trapped air, allowing evaporation and mixing; the fog then disappears. A is wrong because valleys do not always contain warm air; they often collect cold air at night. B is wrong because a valley does not destroy water vapour. D is wrong because air pressure is never zero simply because land is low. Memory cue: cold air sinks and pools in valleys, so valley fog may persist until heating and mixing begin.