fog(redirected from Precipitation fog)
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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.
a cloud of fine water droplets or ice crystals, or both, in the lowest atmospheric layer. Fogs reduce horizontal visibility to 1 km or less and sometimes reach a height of several hundred m.
Fog is formed as a result of the condensation or sublimation of water vapor on liquid- or solid-aerosol particles contained in the air; such aerosol particles are known as condensation or sublimation nuclei. Fog consisting of water droplets is observed mainly at air temperatures above -20°C, but it may occur even at temperatures below - 40°C. Ice fogs predominate at temperatures below - 20°C. Visibility in fog depends on the size of the particles forming the fog and on the water content of the fog, that is, the amount of condensed water per unit volume. The radius of fog droplets ranges from 1 to 60 micrometers (μm). Most such droplets have a radius of 5–15 μm at air temperatures above zero and of 2–5 μm at temperatures below zero. The water content of fog usually does not exceed 0.05–0.1 g/m3, but in individual dense fogs it may reach 1–1.5 g/m3. The number of droplets per cm3varies from 50–100 in thin fogs to 500–600 in dense fogs. In very dense fogs visibility may be reduced to a few meters.
The most general classification of fogs is based on physical origin; in this scheme cooling and evaporation types of fogs are distinguished. The first type occurs when air is cooled to a temperature below the dew point. During such cooling the water vapor in the air becomes saturated and partially condenses. Evaporation-type fog is formed when, in addition to the water vapor already there, water vapor from a warmer evaporating surface penetrates cold air; as a result, this water vapor also becomes saturated. Fog produced by cooling is the more common of the two types.
On the basis of synoptic formation conditions, fogs are classified as air mass and frontal. Air-mass fogs are formed in homogeneous air masses; the occurrence of frontal fogs is associated with atmospheric fronts. Air-mass fogs are more common. In most cases air-mass fogs are produced by cooling; such fogs are further classified as radiation and advection fogs.
Radiation fogs are formed over land when the temperature drops owing to radiational cooling of the earth’s surface and, consequently, of the air. Such fogs occur mainly in anticyclones and most often during clear nights in the presence of a light breeze. As a rule, radiation fogs disperse rapidly after sunrise; in cold weather, however, they may persist during the day in stable anticyclones, sometimes for several days at a time.
Advection fogs are formed by the cooling of warm, moist air as it moves over a colder land or water surface. The density of advection fogs depends on the temperature difference between the air and the underlying surface, as well as on the moisture content of the air. Advection fogs may develop over both land and sea and may cover a vast area, sometimes of the order of several tens or even hundreds of thousands of km2. Such fogs usually occur during cloudy weather and most often in the warm regions of cyclones. Advection fogs are more stable than radiation fogs and frequently do not disperse during the day. Some advection fogs are produced by evaporation and occur when cold air moves over warm water. Fogs of this type are common, for example, in the Arctic, where they develop when air from the ice cap encounters an open sea surface.
Frontal fogs are formed near atmospheric fronts and move with them. The air becomes saturated with water vapor as a result of the evaporation of precipitation that falls in a frontal zone. Atmospheric pressure is observed to fall ahead of a weather front; such a fall in pressure causes a slight adiabatic decrease in air temperature and plays a certain role in the intensification of prefrontal fogs. Fogs occur more often in populated areas than at a distance from them, because urban air has more hygroscopic condensation nuclei, for example, combustion products.
Since fogs hinder the normal operation of all types of transport, fog prediction is very important to the national economy.
Fog is produced artificially in, for example, scientific research, the chemical industry, and heat engineering.
REFERENCESZverev, A. S. Tumany i ikh predskazanie. Leningrad, 1954.
Khrgian, A. Kh. Fizikaatmosfery. Moscow, 1969.
S. P. KHROMOV
What does it mean when you dream about fog?
Fog represents a sense of being lost and confused—not knowing where one is going. Fog can also symbolize the realm of the unconscious, which one may be exploring or attempting to navigate in a dream. Finally, a fog can obscure things, for good or bad.
ii. As it relates to aerial photography, a darkening of negatives or prints because of a deposit of silver that does not form a part of the image. Fog tends to increase density and decrease the contrast. It can be caused by exposure to unwanted light, to air during development, to impure chemicals, etc.