Fog

Fog is officially defined as visibility of less than 1,000 metres. This limit is appropriate for aviation purposes, but for the general public and motorists an upper limit of 200 metres is more realistic and severe disruption to transport occurs when the visibility falls below 50 metres. Useful labels for these three categories are aviation fog, thick or motoring fog and dense fog. Visibility reduction is caused by tiny water droplets suspended in the air. The thickest fogs tend to occur in industrial areas where there are many pollution particles acting as nuclei for the water droplets. This is no longer the case in most of Europe following Clean Air Regulations and also as a result in the reduction of heavy industry.

Away from coastal regions, the most common type of fog is 'radiation fog'. It forms overnight when the ground loses heat by radiation, and cools. The ground, in turn, cools the nearby air to saturation point, thus forming fog. Often the fog remains patchy and is confined to low lying ground, but sometimes it becomes more dense and widespread through the night. Ideal conditions for the formation of this type of fog are light winds, clear skies and long nights. Consequently, the months of November, December and January are most prone to foggy conditions, particularly the inland areas of England and the lowlands of Scotland during high pressure conditions.

Freezing fog is composed of supercooled water droplets (i.e. ones which remain liquid even though the temperature is below freezing point). One of the characteristics of freezing fog is that rime (composed of feathery ice crystals) is deposited on the windward side of vertical surfaces such as street lights, road signs, fence posts, telephone and power cables, pylons and transmitting masts.

Once the sun comes up, fog tends to disperse because it is 'burnt off' by the incoming solar radiation, some of which penetrates the fog and begins to heat the ground. This then heats the layer of air immediately above, causing the minute fog droplets to evaporate. This improves the visibility and, if the fog is thin enough, it soon clears. Thicker fogs sometimes lift into low cloud before they clear. An area of fog will also contract as the solar radiation raises the temperature of ground at the edge more quickly than under the fog itself. However, in winter, when solar radiation is low, fogs can be very persistent if they become widespread. In such cases, clearance is most often the result of an increase in wind.

Sea fog plagues some coastal regions of the UK, which forms when moist air is cooled to saturation point by travelling over a cooler sea. The wind may then take the fog into coastal regions. This type of fog tends to occur in spring and summer, and particularly affects south-western and North Sea coasts. It is not cleared by solar radiation since the sea-surface temperature changes little, even on a sunny day. Sea fog is cleared by the advection of drier air into it.