Radiation fog is caused by radiation of the earth’s heat at night, and the conductive cooling of the air in contact with the ground to below dew point.
If there is a light wind, then fog will form, and in calm conditions the result will be the formation of dew.
Conditions necessary for radiation fog to form:
• Clear sky – to increase the rate of terrestrial radiation.
• High relative humidity – so that a little cooling will be enough to cause saturation and condensation.
• Light wind – of 2 – 8 kt to mix the layers of air causing turbulence so that droplets will be kept in suspension and so that warmer air from above can be brought into contact with the cold ground to thicken the fog.
A natural result of the radiative cooling at the surface will be an inversion above the fog layer (usually the friction layer).
Times of occurrence:
• Predominantly in autumn and winter.
• Night and early morning. The lowest temperatures are early morning. Additionally, the first insolation provides thermal turbulence and light winds. The latest time at which radiation fog can form is about 30 minutes after sunrise.
• Over land – not over sea because there is little DV of temperature.
• Firstly in valleys because of the katabatic effect.
• In anticyclones, ridges and cols.
• By insolation causing convection which will lift the fog. It will also help to evaporate the lower layers.
• By a strong wind lifting the fog to form stratus cloud.
Note: In the UK, radiation fog usually clears by 1000 – 1100 hours but may persist all day in
valleys and in winter.