Advection fog is produced by which mechanism?

• A. Fog resulting from the movement of warm, humid air over a cold surface.
• B. Fog formed by cooling of air near the surface (radiation fog).
• C. Fog produced by rapid evaporation from a warm surface.
• D. Fog caused by precipitation freezing in the air.

Answer: (A)

Advection fog forms when warm, humid air is transported horizontally by wind over a surface that is cooler than the air above it. As this moist air moves over the cooler surface, it is cooled to its dew point and the water vapor condenses into tiny droplets, creating fog. The defining feature is the horizontal movement of a moist air mass over a cooler surface, which is why this type of fog arises from advection rather than from surface cooling in place or from evaporation alone.

Radiation fog, by contrast, develops from cooling near the ground under low wind and clear skies, not from horizontal transport. Evaporation (or steam) fog comes from vigorous evaporation from a warm surface mixing into cooler air, not from the air being moved horizontally over a cold surface. Freezing or ice fog forms when very cold air causes water vapor to deposit directly as ice, typically without the specific horizontal moisture transport characteristic of advection fog.