FAA Instrument Rating Airplane (IRA)Weather and Weather ServicesEasy
Which combination of conditions is most favorable for the formation of radiation fog?
- ARapidly rising air with high instability and abundant condensation nuclei
- BCold, dry air passing over warm ocean water
- CStrong winds, overcast skies, and warm moist air moving over a cold surface
- DClear skies, light or calm wind, and high relative humidity near the ground on a long night
Show answer & explanationAnswer & explanation
Correct answer: D. Clear skies, light or calm wind, and high relative humidity near the ground on a long night
Radiation fog forms on clear nights with light or calm winds, allowing the ground to cool rapidly by radiational cooling. If the air near the surface is moist, this cooling brings the air to its dew point, forming fog, typically in low-lying areas such as valleys.
Why the other options are wrong
- A. Fog forms in stable air with little vertical motion, not in rising, unstable air.
- B. This describes conditions for steam fog or advection fog, not radiation fog.
- C. This describes conditions favorable for advection fog, not radiation fog.
Radiation Fog
Radiation fog forms over land on clear, calm nights when the ground cools by radiation, cooling the adjacent moist air to its dew point.
- Requires clear sky for radiational cooling
- Requires light or calm wind (too much wind mixes the air and prevents fog)
- Common in low-lying valleys where cool air settles
- Usually dissipates after sunrise as the ground warms
Memory trick: Radiation fog needs a Radiant clear night and Restful calm air