FAA Aircraft Dispatcher (ADX)MeteorologyEasy
A dispatcher is reviewing overnight conditions for a departure airport. Which combination of conditions is most favorable for the formation of radiation fog?
- AClear skies, light winds, and high relative humidity at night
- BWarm moist air moving over a cold surface during the day
- CStrong winds, overcast skies, and low humidity
- DRapidly rising unstable air with showers
Show answer & explanationAnswer & explanation
Correct answer: A. Clear skies, light winds, and high relative humidity at night
Radiation fog forms on clear nights with light winds when the ground cools rapidly by radiational cooling, chilling the moist air near the surface to its dew point. High humidity and calm to light wind conditions maximize this cooling effect.
Why the other options are wrong
- B. This describes advection fog, which forms from warm moist air over a cooler surface, not radiational cooling.
- C. Strong winds mix the air and prevent the shallow surface cooling layer needed for radiation fog.
- D. This describes conditions for convective activity, not fog formation.
Radiation Fog
Fog that forms over land on clear nights with light winds due to radiational cooling of the surface, which cools the adjacent moist air to saturation.
- Requires clear skies for maximum radiational cooling
- Requires light or calm winds (too much wind prevents fog, but some mixing helps spread it)
- Typically forms in valleys and low-lying areas, dissipates after sunrise
Memory trick: 'Radiation fog needs a clear, calm, cool night' to let heat radiate away.