In a valley cold air collects at night and humidity is high near the surface which event is likely?

Correct answer: Valley fog

Correct answer: D, valley fog. At night, the ground loses heat by radiation and the air touching the ground cools. Cold air is denser than warm air, so it tends to flow downward and collect in low places such as valleys. If this air contains much water vapour, cooling may bring its temperature to the dew point. At the dew point, the air becomes saturated, and some water vapour condenses into very small suspended droplets. When these droplets form close to the ground, the phenomenon is called fog; because it collects in a valley, it is called valley fog. A is incorrect because a sandstorm needs strong winds and loose dry particles, not saturated cold air. B is incorrect because sea tides are caused mainly by gravitational effects involving the Moon and Sun, not valley cooling. C is incorrect because boiling requires heating, whereas the situation describes night-time cooling. D is correct because cold, moist, nearly saturated air in a low valley is favourable for fog. Memory cue: cold night plus moist air plus a low place often means fog.