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Extra - November 30, 2011
What is fog?
How does it form?

By Philip Eden

The persistent fog which affected much of eastern England between Sunday and Tuesday 22-24 November was about par for the course in late-autumn, and the serious disruption to flights at Heathrow airport simply shows how close to capacity this airport is, not just at holiday time, but throughout the year. During foggy episodes, Air Traffic Control at Heathrow is required by law to apply Low Visibility Procedures, resulting in a reduction in the number of aircraft movements. Clearly, if the airport is close to capacity, a large number of flights - and therefore people - will be affected.

The official definition of fog is "an obscuration of the lowest layer of the atmosphere by a suspension of water droplets". Thus the layer of air above the ground needs to have very high relative humidity even before fog can form. We know from the laws of physics that warm air is able to hold more moisture, in the form of invisible water vapour, than cool air can, so if the temperature of the air-mass begins to fall, the moisture has nowhere to go, so it condenses into tiny water droplets, resulting in fog. Scientists will tell you that you also need "condensation nuclei" onto which the droplets collect; these may be microscopic particles of dust, pollution (carbon particles), salt or pollen.

The official demarcation between fog and mist is a horizontal visibility of one kilometre; anything above that, up to about 10 km, is mist, and anything below is fog. However, in popular parlance, the boundary is probably somewhat lower - say, around 500 metres. Officially, thick fog has a visibility of 200 metres or less, and dense fog 50 metres or less.

There are various types of fog, characterised by their method of formation. Radiation fog occurs on clear, calm nights when cool air collects in valleys and hollows; advection fog forms when a warm and moist airstream flows over much colder, often snow-covered, ground; upslope fog forms over hills and moors when the air is forced to rise over the higher ground, cools as it does so, resulting in condensation of water droplets; and sea fog develops in warm, moist air travelling over a relatively cold sea. Haar, sea-roke, and sea-fret are familiar terms along North Sea coasts when a sea-fog affects the coastline.

By Philip Eden

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