Radiation fog develops overnight when clear skies and light winds allow the temperature to fall quickly. The layer of air closest to the surface generally cools more than the layer above. If this happens, its water vapour can condense in minute droplets that form the fog.
Radiation fog over a lake in the Algonquin National Park, Ontario, Canada
In this example, radiation fog has developed preferentially over a lake due to the enhanced humidity caused by the body of water.
In this case, and the one below, the photographs were taken in the summer, when the lake water temperature was still quite cold compared to the surrounding land.
In both pictures the fog thins with altitude due to the presence of warmer air. In the second picture, the fog has cleared from the near shoreline, suggesting a slight drift of air from the shore towards the lake.
Radiation fog on a early summer morning over a lake in the Algonquin National Park, Ontario, Canada
In Britain, radiation fog is more common during the winter half of the year. It typically occurs on low lying land, especially in river valleys.
It is typical of anticyclonic periods when high pressure affects the country and winds are light. Sometimes the fog is very thick and fails to burn off in the day.
At night, away from the street lighting of urban areas, it presents a serious hazard to motoring, hence the need for fog lights. Fog is actually much more common in rural areas as they get cooler at night than nearby urban areas, which benefit(?) from the heat island effect. Any moisture on the surface of urban areas is also quicking removed by the draining system, so the humidity of the air tends to be lower.
Radiation fog, near Chamonix, French Alps
Radiation fog actually looks a lot more photogenic, when viewed from above and where the valleys are deep, e.g. in the Alps!
This scene is typical of alpine winters. During air on the mountain slopes becomes colder and denser than air at the same altitude over the valleys. As a result it moves downhill into the valleys, where it continues to cool until it reaches its dew point and fog forms. The downslope movement of cold dense air is described as katabatic flow.
If the temperature in the fog is below freezing then a layer of rhime is usually deposited on trees and vegetation creating some spectacular effects.
Photo montage of images of rhime from the French Alps and Wolstanton