The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Clouds
Fronts
Visibility and Fog
Aircraft Icing
Thunderstorms
Altimetry
Weather Reports and Forecasts
Practice Exam

Radiation Fog

After sunset, heat begins to radiate away from the Earth and the layer of air in contact with the surface is cooled through conduction. When humidity is high enough, this lower layer of air will cool to its dew point temperature and fog will form. As sea temperatures stay fairly constant, radiation fog only occurs over the land.

Three Conditions for Radiation Fog to Occur

Clear Sky
Cloud cover ‘traps’ in heat, reducing the Earth’s ability to cool through radiation. Relatively clear skies are needed for the surface to cool to its dew point temperature.
Light Wind
A light (5-7 knot) wind creates mixing of the air at low levels, which puts a lot more air in contact with the cold surface. It is when this thicker layer of air is cooled to dew point temperature that radiation fog forms.

On nights with nil wind, this mixing does not occur and only an extremely thin layer of air in contact with the surface will reach dew point temperature. This creates plenty of dew or frost on the ground, but no fog. Strong winds create too much mixing and usually lead to low stratus rather than fog.
Humid Air
The higher the humidity, the less cooling required for dew point temperature to be reached (remember: a higher relative humidity means a higher dew point temperature). Low humidity means the air may not be cooled enough to reach its very cold dew point temperature.

The fine balance needed between these 3 factors means radiation fog is somewhat unpredictable.

In the world of meteorology, fog occurs when the visibility is less than 1000m due to water suspended in the air. When visibility is obscured by water droplets but is greater than 1000m, it is called mist.

How Radiation Fog Evaporates

When the air temperature is again raised above the dew point temperature, the fog will evaporate. This happens through:

Solar Heating
After the sun rises, the surface beings to heat again. Once the temperature rises above its dew point, the fog will evaporate. It can take several hours for the fog to dissipate and often a layer of low stratiform cloud will develop.
Increasing Winds
If the wind increases to 8 knots or more, turbulence and mixing of the air increases. This mixes the cool air in contact with the ground with the warmer air at altitude, leading to an overall increase in temperature. With enough mixing, the temperature rises above the dew point and fog evaporates.
Note: After sunrise, the fog often thickens first before evaporating. The increased mixing from solar heating initially brings more air into contact with the surface, cooling more air and creating a thicker layer of fog.