Fog has sometimes had a sinister reputation. Medieval fogs were dreaded as the embodiment of an unhealthy dampness, as catalysts for rheumatic aches and pains, and as evil vectors for every kind of ague and fever.
Worst of all was "freezing fog", a fog with the special characteristics which develop when the air temperature is below zero.
The strange behaviour of freezing fog is due to the fact that, contrary to what most of us might think, ice does not always form at 0C. The freezing point of water is not fixed at all and varies widely depending on the circumstances, with a maximum value of 0C. Water will never freeze when the temperature is above zero but it may well remain liquid far into the zone of negative temperature values.
The tiny water droplets which make up fog are very pure in composition, and it is the absence of impurities which allows them to remain in the liquid state even at very low sub-zero temperatures.
It has been estimated that only one droplet in a million is frozen at temperatures between 0C and 10C and only a few in 10,000 turn to ice at 30C.
Such drops, still in liquid form when the temperature is negative, are described as supercooled. The temperature has to fall to 40C before all the drops of water suspended in the air solidify.
The supercooled state, however, is very unstable. A supercooled water droplet quickly freezes given the opportunity, such as when it comes into contact with a solid object. Freezing fog, therefore, is hazardous for road traffic, as a coating of ice may form on the windscreens of cars or on roads.
The most spectacular consequence of freezing fog, however, is the formation of rime, a white, feathery, crystalline, opaque deposit which occurs when the supercooled droplets, blowing along in a breeze, hit a vertical surface and are transformed immediately to ice. This can be seen building up on the windward side of obstacles such as clothes lines, shrubs or garden fences, sometimes at a rate of up to half an inch a day.
Overhead electricity wires are particularly susceptible and the build-up over a lengthy period may be sufficient to cause them to collapse.
Fog itself is harmless to health. It is likely that its sinister reputation arose from the prevalence of tuberculosis in times past and the fact that breathing harsh, cold air, whether containing little particles of ice or not, probably exacerbated existing lung conditions.