fog, aggregation of water droplets or ice crystals immediately above the surface of the earth (i.e., a cloud near the ground). A light or thin fog is usually called a mist. Fog may occur when the moisture content of the air is increased beyond the saturation point. For example, fog usually results from the evaporation of warm water into cold air, which occurs when cold air streams over a warm water surface (steam fog) or when a warm rain falls through a layer of cold air near the ground (frontal fog). Fog also occurs when the air is cooled below a critical temperature called the dew point. Fog may be caused by radiation of heat from the ground during a windless, cloudless cool night (radiation fog); by the flow of warm air over a cold land or water surface (advection fog); or by air ascending a slope and cooling by expansion (upslope fog). In all cases condensation of the excess moisture takes place on the microscopic dust particles (condensation nuclei) in the atmosphere. Fog commonly found in valleys and depressions in the morning, especially during autumn, is of the radiation type, which because of its shallow nature is dissipated by the sun's heat as the day progresses. The extensive fog banks frequently occurring along the coasts of Newfoundland and Labrador are of the advective type and, being generally quite deep, often persist for days at a time, hindering shipping and aviation activity. In arid areas where fog is common, water may be harvested from the fog by using so-called fog nets or fog catchers.
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