Why can radiation fog persist longer in valleys?

Correct answer: Cold dense air collects below and can create an inversion

Correct answer: A. Radiation fog commonly develops after surface cooling, especially during clear and calm conditions. In a valley, the air touching the cold slopes and valley floor loses heat, becomes denser, and flows downward. Cold air can collect in the lowest part of the valley. If warmer air remains above this cold layer, a temperature inversion forms: temperature increases with height instead of decreasing normally. The inversion suppresses vertical mixing, so the cold, moist, foggy air remains trapped for a longer time. A is correct because it explains both valley cold-air pooling and the inversion that helps fog persist. B is wrong because valleys can contain water vapour from soil, vegetation, water bodies, and nearby air. C is absurd; gravity is essential to the downward movement of dense air. D is astronomically false and unrelated to fog. Memory cue: valley fog means cold air pools below and mixing is blocked above.