Moist air climbs a hill slope, cools, and forms fog near the surface. What is it called?

Correct answer: Upslope fog

The correct answer is B, upslope fog. First identify the movement of the air: moist air is moving upward along a hill slope. As it rises, the surrounding pressure decreases, the air expands, and it cools adiabatically. If it cools to its dew point, water vapour condenses into tiny droplets near the ground, producing fog. This type is also called hill or mountain fog. A is wrong because advection fog forms when warm, moist air moves horizontally over a colder surface, not mainly because it rises up a slope. C is wrong because frost is ice deposited on a cold surface, usually when the surface is below freezing; it is not fog. D is not the name of this atmospheric process and refers only to dew on grass. Memory cue: “slope plus rising air equals upslope fog.”