fog, cloud of small water droplets that is near ground level and sufficiently dense to reduce horizontal visibility to less than 1,000 metres (3,281 feet). The word fog also may refer to clouds of smoke particles, ice particles, or mixtures of these components. Under similar conditions, but with visibility greater than 1,000 metres, the phenomenon is termed a mist or haze, depending on whether the obscurity is caused by water drops or solid particles.
Fog is formed by the condensation of water vapour on condensation nuclei that are always present in natural air. This results as soon as the relative humidity of the air exceeds saturation by a fraction of 1 percent. In highly polluted air the nuclei may grow sufficiently to cause fog at humidities of 95 percent or less. Growth of the drops may be helped by the absorption of certain soluble gases, notably sulfur dioxide to form dilute sulfuric acid. The relative humidity of the air can be increased by three processes: cooling of the air by adiabatic expansion; mixing two humid airstreams having different temperatures; and direct cooling of the air by radiation.
The first process, adiabatic expansion, is responsible for the formation of clouds and plays a part in the formation of upslope fogs that are formed by the forced ascent of humid air up the sides of hills and mountains.
The mixing process is manifest when air that has been in contact with a wet ground or water surface having a different temperature from that of the air above is mixed with this air.
The most stable fogs occur when the surface is colder than the air above; that is, in the presence of a temperature inversion. Fogs also can occur when cold air moves over a warm, wet surface and becomes saturated by the evaporation of moisture from the underlying surface. Convection currents, however, tend to carry the fog upward as it forms, and it appears to rise as steam or smoke from the wet surface. This is the explanation of steam fogs that are produced when cold Arctic air moves over lakes, streams, inlets of the sea, or newly formed openings in the pack ice; hence, the term Arctic sea smoke.
One of the most critical situations for frost is encountered in the citrus fruit regions. There the few killing frosts likely each winter are combated by a variety of methods, including heating the groves with special burners using oil or a petroleum-derived solid fuel, mixing the air with large fans mounted above the trees, or casting a fine water spray over and on the trees to keep the temperature at or very near the nondamaging level of 32 °F (0 °C)...have a bless thursday everyone and goodnight..thanks for upvoting my blog..