Radiation Fog

At night, the ground loses its heat by radiation. The ground becomes cold and cools the air in contact with it. If this lowers the air temperature below the dew point, water vapour condenses out as droplets. For radiation fog to form, the following requirements have to be met:

  • The clear sky which increases the rate of terrestrial radiation. A light cover of high clouds cover (up to 4/8 cover of cirrus) is said to not affect the rate of terrestrial radiation.
  • High relative humidity so that only a little cooling will be required for the air to reach saturation.
  • A light wind of 2 to 8 kt which mixes the air bringing warmer air from above to the surface to be cooled and thickening the fog.

Radiation fog is most common in autumn and winter when there is a long night giving the land time to cool. It occurs at night and early morning after a prolonged period of cooling. It doesn’t occur over the sea as the sea has insufficient diurnal variation. It forms first in the valleys due to katabatic effect and is common in anticyclones, ridges, and cols where the air remains in contact with the ground for a prolonged period.

Dispersal of the fog can occur by:

  • The increase of insolation during the course of the morning, raising the temperature above the dew point and evaporating the fog away from the base.
  • The increase of thermal turbulence during the morning which lifts the fog to form low stratus.
  • An increase of cloud cover preventing the loss of radiation from the lower atmosphere and raising the temperature of the air above the dew point.
  • Replacement of the air mass with a drier air mass by advection.

Radiation fog 
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