Which fog forms when air near the surface is cooled by contact with the ground, often overnight in valleys?

Which fog forms when air near the surface is cooled by contact with the ground, often overnight in valleys?

• A. Frontal fog
• B. Radiation fog
• C. Ice fog
• D. Advection fog

Answer: (B)

Fog forms when the ground radiates heat away at night, cooling the air in direct contact with the surface to its dew point. Calm, clear nights enhance this radiational cooling, and in valleys the cooled air tends to collect in low-lying areas, giving rise to fog that often lingers overnight. This specific mechanism—surface cooling from radiational heat loss—is what creates radiation fog. Frontal fog develops at a weather front when saturation occurs due to air lifting or mixing near the boundary, not primarily from ground cooling. Ice fog happens in extremely cold conditions when water vapor changes directly to ice crystals, which isn’t about surface radiational cooling. Advection fog forms when moist air moves horizontally over a cooler surface, cooling to the dew point due to movement rather than ground contact.

Fog forms when the ground radiates heat away at night, cooling the air in direct contact with the surface to its dew point. Calm, clear nights enhance this radiational cooling, and in valleys the cooled air tends to collect in low-lying areas, giving rise to fog that often lingers overnight. This specific mechanism—surface cooling from radiational heat loss—is what creates radiation fog.

Frontal fog develops at a weather front when saturation occurs due to air lifting or mixing near the boundary, not primarily from ground cooling. Ice fog happens in extremely cold conditions when water vapor changes directly to ice crystals, which isn’t about surface radiational cooling. Advection fog forms when moist air moves horizontally over a cooler surface, cooling to the dew point due to movement rather than ground contact.