Which phenomenon forms as surface clouds due to nocturnal cooling at or near the ground surface?

• A. Advection fog
• B. Steam fog
• C. Radiation fog
• D. Valley fog

Answer: (C)

Radiation fog forms when nocturnal cooling of the Earth's surface chills the air at ground level to its dew point, causing water vapor to condense into tiny droplets and create a surface cloud. This usually happens on clear, calm nights when heat radiates away from the ground and there’s enough humidity. Because the fog is produced by radiational cooling rather than horizontal movement of air, it stays near the surface and often dissipates after sunrise as the ground heats up. This differs from advection fog, which forms when moist air moves horizontally over a cooler surface; steam fog, which occurs when cold air moves over warm water and condensation forms above the surface; and valley fog, which develops in low-lying areas as cold air pools. In the described situation, the phenomenon is radiation fog.