FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesMedium
Under which combination of conditions is radiation fog most likely to form?
- AStrong winds and an overcast sky throughout the night
- BStrong daytime heating over a large body of water
- CA clear night sky, light or calm winds, and high relative humidity over land
- DWarm, moist air moving horizontally over a much colder land or water surface
Show answer & explanationAnswer & explanation
Correct answer: C. A clear night sky, light or calm winds, and high relative humidity over land
Radiation fog forms on clear nights with light winds when the ground cools rapidly through radiational cooling, cooling the adjacent air to its dew point. High humidity provides ample moisture, and light winds allow cooling to concentrate near the surface without mixing it away.
Why the other options are wrong
- A. Overcast skies inhibit radiational cooling, and strong winds mix and disperse cooling air, preventing fog formation.
- B. Daytime heating over water does not produce radiation fog, which requires nighttime radiational cooling over land.
- D. This describes advection fog, which forms from horizontal movement of warm moist air over a colder surface.
Radiation Fog
Radiation fog forms at night over land when the ground cools rapidly under clear skies and light winds, cooling the surface air to its dew point.
- Requires clear skies for radiational cooling
- Needs light or calm wind (too much wind mixes the layer)
- Forms over land, typically dissipates after sunrise
- Common in valleys where cold air settles
Memory trick: Radiation fog: clear night, calm air, cool ground — nature's blanket settles in.