Radiation Fog

Radiation fog forms when the ground loses heat rapidly through terrestrial radiation on clear nights, cooling the air above it to the dew point. It is a surface phenomenon — typically shallow (a few hundred feet thick), forming in valleys and low-lying areas, and dissipating within a few hours of sunrise as solar heating warms the surface. Reported in METARs as FG.

Formation Requirements

  • Clear sky (clouds act as a blanket — with overcast, the ground cannot radiate heat effectively)
  • Light winds (calm to about 5 knots — enough to mix moisture through a shallow layer but not enough to destroy the inversion)
  • High relative humidity (close to dew point at the surface)
  • Long nights (autumn through early spring are prime season in Canada)

Aviation Characteristics

Radiation fog typically forms in low-lying areas — valleys, river basins, and coastal lowlands — because cold air is denser and drains downhill. The surface fog may be thick at airport level while a few hundred feet above the visibility is clear. This creates a deceptive scenario for pilots: the aerodrome is IFR while a short climb would be VMC.

Dissipation

Radiation fog dissipates fairly predictably: as the sun rises, the ground warms, the inversion erodes from below, and the fog lifts to form stratus, then dissipates. Forecast: radiation fog that forms before midnight often dissipates by 0900–1000 local time. Fog that forms near sunrise may persist longer. Wind increasing above 10 knots will also disperse radiation fog.

Radiation Fog vs. Advection Fog

  • Radiation fog: Forms at night with calm winds over cooled ground; typically thin; dissipates in the morning; localized to valleys
  • Advection fog: Forms when warm moist air moves over a cold surface; can occur any time; persists until wind shifts; may be thick and widespread

On the TC Written Exam

Radiation fog is a guaranteed exam topic. Know the formation requirements (clear sky, light wind, high RH, long night) and the dissipation mechanism (solar heating). Know it forms in valleys, not on elevated terrain.

Related Terms

See also: Advection Fog, Visibility, Ceiling.