Fog is caused when water vapour in the atmosphere condenses into water droplets. Essentially fog is cloud at ground level. According to the International Meteorological Code, fog is a visibility of less than 1 km, and in the UK a thick fog represents a visibility of less than 200 m. (220 yards).
There are two main situations in which fog can be formed. One causes advection fog and the other causes radiation fog.
When a warm, moist mass of air passes onto a cooler land or sea surface, the lower layers of the atmosphere will be suddenly cooled. If they cool below dew point, condensation will occur and advection fog will be the result.
The best example of an advection fog is found on the Grand Banks off the coast of Newfoundland, where the cool Labrador Current moves down to meet the North Atlantic Drift coming up from the south.
Radiation fog can be caused on a still, cool night when heat is radiated up from the earth’s surface, cooling the lower layers of the atmosphere. Again, if the temperature cools below dew point, condensation will cause fog to form. This sort of fog is common in settled weather conditions, and particularly in land hollows or valleys where the coldest, densest air will collect after nightfall.
Particularly dense fog will occur when there is a dense amount of water vapour in the atmosphere as the cooling takes place. It will also occur in situations where there are other particulates (tiny specks) of material within the atmosphere. Each of these will form a nucleus around which condensation can take place. So a thick fog can be caused in areas of dense smoke and fog, or any other particulate material mixed within a fog.
The classic example of a smoke and fog mix was the smog which used to form in cities like London, PIttsburgh and other industrial areas. This was during the time before Clean Air Acts, when a lot of coal was burned both in domestic fires and by industry. Soot in the smog formed the nuclei for the condensation and sulphur dioxide caused the acrid smell.
Smog is a development of radiation fog and forms in lowland areas such as the valleys where many industrial towns are situated.
The types of photochemical smogs which occur in places like Los Angeles do not produce low visibility. They are caused by a chemical reaction between sunlight and nitrogen Dioxide and other hydrocarbons.