FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesEasy
A pilot is briefed that radiation fog is likely to form at the destination airport overnight. Which combination of conditions most favors this type of fog?
- ARapid vertical lifting of unstable air with high moisture content
- BStrong winds, overcast skies, and high humidity
- CClear skies, light winds, and high relative humidity near the surface
- DWarm moist air advecting over a cold surface with moderate winds
Show answer & explanationAnswer & explanation
Correct answer: C. Clear skies, light winds, and high relative humidity near the surface
Radiation fog forms on clear nights with light or calm winds when the ground cools rapidly by radiational heat loss, cooling the air near the surface to its dew point. High relative humidity is required to reach saturation.
Why the other options are wrong
- A. This describes conditions for cloud formation via lifting, not radiation fog.
- B. Strong winds mix the air and prevent the cooling needed for radiation fog.
- D. This describes advection fog, not radiation fog.
Radiation Fog
Fog that forms over land on clear, calm nights due to radiational cooling of the surface, which cools the adjacent air to saturation.
- Requires clear skies for radiational cooling
- Requires light or calm wind (light wind can help mix a thin fog layer)
- Typically dissipates after sunrise as the ground warms
Memory trick: Clear night, calm air, ground gets cold — radiation fog takes hold