Fog is a cloud which has come into contact with the ground. It is usually the result of condensation of water vapor in the air at the earth's surface. But on mountains a fog may merely be the result of clouds being formed in the free air and blown on to the slopes.
Fog is made up of water droplets, or sometimes ice crystals. It varies from a light haze or mist to the thick fog experienced in cities. The densest kind - also called smog - may be caused as much by smoke as by water.
During the Second World War, many attempts were made to dispose of fog artificially, so that runways for aircraft could be kept clear. A practical method developed in Britain was the burning of oil in jets arranged around an airfield. The water drops evaporated and an area up to 100 feet or more above the runway was cleared in a few minutes. But it was an expensive remedy and was not adopted for general use.
Chemical methods, such as "seeding" the fog with dry ice, silver iodide or calcium chloride spray to make the water droplets fall as rain, have been investigated since 1955.