Definition
A type of fog that forms over land on clear, calm nights when the ground rapidly loses heat by radiating it into the sky. The cooled ground chills the air directly above it until that air reaches its dew point, causing water vapor to condense into a shallow layer of fog near the surface. Radiation fog typically forms in low-lying areas, dissipates after sunrise as the ground warms, and does not form over water.
Plain English
Fog that appears overnight when the ground cools quickly under a clear sky, chilling the air just above it until moisture condenses. It usually sits low to the ground and burns off in the morning sun.
Context Anchor
Seen in weather briefings, preflight planning, and early-morning departure decisions, especially after clear, calm nights.
Derivation
‘Radiation’ here refers to the way the ground loses heat — by radiating infrared energy out into space — not to anything nuclear. It comes from the Latin ‘radiatus,’ meaning ‘to emit rays.’ Knowing this helps because the fog is named after the cooling process that creates it.
Why Pilots Care
It can reduce surface visibility to near zero, delaying departures or forcing use of alternate airports until the sun warms the ground.
Grounding Statement
Picture a clear, still night over a grass field. The ground gives up its warmth to the sky, the air just above it cools to its dew point, and a shallow white layer of fog settles in by dawn.
Intuition Check
Radiation does not mean nuclear radiation here. It means the ground is losing heat by sending energy away, which cools the air near the surface.
Example Sentence 1
The pilot delayed the 6 a.m. departure after the AWOS reported radiation fog reducing visibility to a quarter mile.
Example Sentence 2
Radiation fog often forms over the runway at rural airports on autumn nights with clear skies and light winds.