Advection fog forms when warmer air moves over cold ground or water surface.

• A. Warmer air moving over cold ground or water surface
• B. Cooler air moving over warmer surfaces
• C. Warm air rising over cool air
• D. Cold air moving over warm water

Answer: (A)

Advection fog happens when warm, moist air moves horizontally over a surface that is cooler than the air. The contact cools the air to its dew point near the surface, causing water vapor to condense and form fog. So the correct description is warmer air moving over a cold ground or water surface.

If cooler air moves over warmer surfaces, the air tends to warm rather than cool to the dew point, reducing the chance of surface fog. Warm air rising over cool air describes convection and cloud formation higher up, not a surface fog layer. Cold air moving over warm water can produce fog, but through a different mechanism (evaporation/steam fog), not the horizontal advection of warm air over a cooler surface.