Which two factors combine to form advection fog on a coast?

Correct answer: Moist maritime air and cold coastal surface

Direct answer: A, moist maritime air and a cold coastal surface. Advection means the horizontal movement of air. In coastal advection fog, moist air moves from over the sea across a colder land or sea surface. Contact with the cold surface cools the lower part of the moving air. If its temperature reaches the dew point, the air becomes saturated and some water vapour condenses into tiny droplets. These droplets remain suspended and reduce visibility. A correctly supplies both necessary ideas: a source of moisture and cooling by a colder surface. B is wrong because dry air lacks sufficient water vapour and hot sand generally warms rather than cools the air. C is wrong because complete dryness prevents condensation. D is wrong because zero water vapour cannot produce droplets, while strong sunshine usually promotes warming and evaporation. Memory cue: advection fog needs air transported sideways over a colder surface; remember moisture plus horizontal movement plus cooling.