Radiation fog is a land based fog in its development. Clear skies, a high relative humidity, very low wind speeds, and a relatively long period during which the air can cool, are the most suitable conditions for its formation.
The clear sky condition allows the maximum loss of long wave radiation from the surface during the night. Surface temperatures decrease rapidly, and the air immediately above is cooled through conduction aided by turbulence. Once the air is cooled below its dew-point temperature, condensation occurs and radiation fog is produced. Since the length of the cooling period is critical, radiation fog is more common during the autumn and winter in mid and high latitudes, e.g. in the British Isles. The cooling of the air by the surface results in a progressive decrease in the environmental lapse rate and the atmosphere eventually becomes stable. A ground level inversion may well develop.
Although conditions favourable for the formation of radiation fog may exist over the sea, the diurnal ranges of both sea surface and air temperatures are too small for the air to be cooled below its dew-point temperature, and therefore no condensation occurs. Radiation fog will affect visibility at sea if it drifts over estuaries and coastal waters as a result of light offshore winds. The air will then be heated by the warmer water over which it passes and, as its temperature increases, the fog droplets will gradually evaporate.
Radiation fog may disperse as a result of an increase in land surface temperature during the day, since the surface heats the air immediately above, and lowers its relative humidity. An increase in wind speed can also cause dispersal since it overturns the air. The RH is thus decreased as a result of mixing with unsaturated air. As the increase in land surface temperature is small during the winter day, fog may persist on land and continue to affect estuaries and coastal waters.
In tropical regions, radiation fog is comparatively rare at sea level, but may be experienced over river estuaries during the early hours of the morning. The fog develops during the night over adjacent river banks, where the air has a high relative humidity due to the presence of open water. Similar conditions also apply in swamp and marsh areas.
