Radiation Fog

This kind of fog forms when the sky is clear and the wind speed no more than Force 1. Typically, it forms at night and dissipates during the day. In mid-winter, however, particularly in latitudes where the sun is low in the sky (e.g. north-west Europe), it may linger all day.

When the sky is clear at night, land surfaces radiate heat to space and therefore cool. Sea and lake surfaces do not, however, cool by more than a small amount overnight (much less than 1°C). If the air in contact with a surface is cooled to its dew-point temperature, small water droplets form (condensation). If there is no wind, droplets of dew form on, for example, grass. If there is a very gentle breeze, the tiny water droplets are stirred upwards to form a shallow layer of radiation fog, as in the picture of fog at Cardiff above.

This type of fog does not form over the sea because the temperature of the sea's surface stays much the same day and night. If the dew-point temperature lies below minus 0.5°C, hoar-frost forms instead of dew. It does not form between 0°C and minus 0.5°C because latent heat is released when condensation occurs and this heat is sufficient to melt the tiny ice crystals that make up hoar frost as they form. During the day, the sun's rays heat the ground beneath the fog. Most of the rays are actually reflected from the top of the fog but some reach the surface, otherwise it would not be daylight in the fog! The ground is gradually heated until the dew-point temperature is exceeded. The fog then dissipates, often very quickly.