Mist formation

by Harry Geurts


Clouds found on the earth’s surface which obscure visibility to less than 1000 metres are called fog. Ground fog or low lying fog refers to fog located below eye level. Fog is formed when vapour-laden air cools down or when cool air and warm air mix together. The names given to sorts of fog in meteorology reveal the conditions under which a given sort is formed.

Radiation fog is formed by surface radiation during periods of clear weather as the ground temperature decreases after sundown. The cooler and heavier air flows down an incline towards ditches, where it combines with moister air and creates fog.

Radiation fog can form above snow around sundown when temperatures are very low. It can be very dense and reduce local visibility to less than 10 metres. When fog and wet roads begin to freeze, road conditions become slippery. Fog can also form above snow when the thaw sets in, as the warmer air flows over the cold snow. The fog is formed by the movement of warmer air, so that conditions are also often windy.

At sea, fog forms when cool air flows over relatively warm seawater or when warm air comes into contact with the cold sea. In the spring and early summer, warm air coming from Southern Europe can cause a mass of dense fog (a fog bank) to form above the cold North Sea. If the offshore wind is not strong enough, during the afternoon wind will blow the fog inland from the coast. This type of sea fog which suddenly moves in from the sea is called sea smoke.

Frontal fog forms when rain falls from warm air relatively high up in the atmosphere through cooler air on the ground. The rain is warmer as it falls through the cold air, causing fog to form. Rain can also cause saturation fog when the sun comes out after a storm and there is not much wind. In the bright sunlight we can see the vapour rise up off streets and roofs.

Visibility is measured with a device called a transmissometer: a lamp that projects a narrow beam of light on to a photoelectric cell detector. When it is foggy, the light will disperse among the water droplets so that less light is emitted to the photoelectric cell a short distance away. Less light means denser fog and more limited visibility. Transmissometers can be found at airports, among other places, where they are used to measure visibility on runways. On motorways, a specially adapted version of these visibility detectors is used to give fog alerts. Electrical warning signs alert motorists and display a new maximum speed limit.


Check here the actual visibility in meters (Netherlands only)


Transmissometer at the Royal Netherlands Meteorological Institute KNMI (photo Laura d'Ors)

Transmissometer at the Royal Netherlands Meteorological Institute KNMI (photo Laura d'Ors)


Excerpt from ‘The weather explained’, KNMI / Ed. Elmar

It does not come as a surprise that Harry Geurts ended up in meteorology considering his education (part mathematics, part physical geography). Since 1989, Geurts has been the press officer of the Royal Dutch Meteorological Institute KNMI. Geurts has written numerous brochures, articles for a variety of magazines, books, and developed an interactive weather guide. He contributes news and background information to the website of the Royal Dutch Meteorological Institute. Since 1990, he has written the Netherlands Broadcasting Foundation’s teletext page ‘The weather explained’, a layman’s guide to weather report terminology, meteorological terms and themes such as climate change and the ozone layer.
www.knmi.nl