WE associate fog with sinister occurrences, as, for example, when Sherlock Holmes finds the body of a missing baronet behind a crate of Persian carpets on the London docks. Meteorologically, however, fog is merely a cloud at ground level but a delicate balance must exist between the various elements for it to form.
Air always contains a little moisture in the form of water vapour. The amount of moisture it can comfortably accommodate, however, is crucially dependent on its temperature: the colder the air, the less moisture it can hold, and vice versa.
If the temperature of a volume of air falls sufficiently, sooner or later it will reach a point where it must rid itself of some moisture, and the excess condenses into tiny drops of water. The result is dew or hoar frost, cloud or fog, depending on the circumstances.
Fog of the kind that has recently been much in evidence, occurring near the centre of an anticyclone, is known as "radiation fog". It depends for its existence on the fact that during clear, starry nights any solar heat absorbed by our planet during the day is reradiated into space.
The air itself is more or less immune from radiation; it takes its temperature from the surface with which it is in contact, so it is the gradual cooling of the ground on dear nights that provides the initial stimulus for fog. On a cloudy night, by contrast, the cloud acts like a blanket over the earth to prevent any dramatic fall in temperature.
The strength of the wind, however, is crucial to what happens next. In the simplest case, with no wind at all, only air in immediate contact with the ground is cooled to the point of condensation, and the exuded moisture takes the form of dew - or white hoar frost if the temperature is low enough.
If, on the other hand there is a strong breeze, the resulting turbulence continually replaces air cooled by contact with the ground with warmer air from higher up: the loss of heat is spread over a considerable depth of atmosphere, and the chances are that nowhere will the temperature fall low enough for condensation to occur.
The intermediate case, however - a slight but noticeable movement of the air - results in fog. The slight turbulence in these circumstances allows cooling to be spread through a layer anything from, say, 10 to 100 feet in depth; a layer which is sufficiently shallow for the temperature of the entire volume of air within it to be lowered to the point where condensation into droplets must take place.