FAA Private Pilot Helicopter (PRH)WeatherEasy
A helicopter pilot is planning an early-morning flight from an airport located in a valley near a river. The night was clear, calm, and the air was moist. Which condition is most conducive to the formation of radiation fog at this location?
- AClear skies, light winds, and high relative humidity overnight
- BA warm front moving through the area after sunset
- CRapidly falling barometric pressure through the night
- DStrong winds mixing the air near the surface
Show answer & explanationAnswer & explanation
Correct answer: A. Clear skies, light winds, and high relative humidity overnight
Radiation fog forms on clear nights with light or no wind, when the ground cools rapidly by radiation and cools the air above it to its dew point. Moist air near rivers and in low-lying valleys enhances this cooling effect.
Why the other options are wrong
- B. Frontal passage brings different weather processes, not radiation cooling fog.
- C. Falling pressure is associated with approaching weather systems, not radiation fog.
- D. Wind mixes the air and prevents the shallow cooling layer needed for radiation fog.
Radiation Fog
Fog that forms over land on clear, calm nights when the ground radiates heat rapidly, cooling the air near the surface to its dew point.
- Requires clear skies for maximum radiational cooling
- Requires light or calm winds to prevent mixing
- Common in low-lying areas and valleys with moist air
- Usually dissipates after sunrise as the ground warms
Memory trick: Radiation fog = 'Rests on the ground on a Clear, Calm night'