FAA Private Pilot Airplane (PAR)Weather Theory and ServicesMedium
A pilot is planning a night flight and is concerned about radiation fog forming at the destination airport. Which combination of conditions is most favorable for radiation fog formation?
- AOvercast skies, strong winds, and low relative humidity
- BUnstable air with scattered thunderstorms and gusty winds
- CClear skies, light or calm wind, and high relative humidity
- DWarm moist air moving over a colder surface with strong winds
Show answer & explanationAnswer & explanation
Correct answer: C. Clear skies, light or calm wind, and high relative humidity
Radiation fog forms on clear nights with light winds when the ground cools rapidly by radiation, cooling the adjacent air to its dew point. High humidity ensures the air is close to saturation, and calm winds prevent mixing that would disperse the cooling near the surface.
Why the other options are wrong
- A. Clouds trap heat and wind mixes the air, both preventing radiation fog.
- B. Thunderstorms and gusty winds are associated with convective activity, not radiation fog.
- D. This describes advection fog, which requires wind to move warm air over a cold surface.
Radiation Fog
Fog that forms over land on clear, calm nights as the ground cools rapidly, cooling the adjacent moist air to its dew point.
- Requires clear skies for radiational cooling
- Needs light or calm wind (too much wind mixes air and prevents fog)
- Typically dissipates after sunrise as the ground warms
- Usually shallow and confined to low-lying areas
Memory trick: Clear + Calm + Cool = Radiation fog at dawn