In which situation is advection fog most likely to form?

• A. An air mass moving inland from the coast in winter
• B. Cold air moving from inland to the coast on a windy day
• C. Fog formed by radiational cooling in a valley
• D. Steam fog forming in a warm, humid morning

Answer: (A)

Advection fog forms when a warm, moist air mass moves horizontally over a surface that is cooler than the air, causing the air to cool to its dew point and condense into fog. In winter, an air mass moving inland from the coast brings relatively warm, moist sea air over land that is cooler than the air above it. As this air travels over the cooler surface, it is cooled to its dew point, and the moisture condenses into fog along the inland path from the coast.

Other scenarios don’t fit this mechanism: fog formed by radiational cooling happens when the ground loses heat overnight and the air near the ground cools enough to condense, usually in valleys and on calm, clear nights; steam fog forms when cold air moves over warm water and moisture condenses in the air; and a cold, inland air mass moving to the coast, especially if it’s not moist, won’t produce advection fog.