Also found in: Dictionary, Thesaurus, Medical, Legal, Acronyms, Idioms, Wikipedia.


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.


To fog a room or building is to use a fog machine during a blower door test, in order to reveal the locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely nontoxic.



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.


Zverev, A. S. Tumany i ikh predskazanie. Leningrad, 1954.
Khrgian, A. Kh. Fizikaatmosfery. Moscow, 1969.


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.


(graphic arts)
A dark, hazy deposit or veil of uniform density over all or parts of a piece of film or paper; can be caused by light other than that forming the image, lens flare, aged materials, or chemical impurities.
Water droplets or, rarely, ice crystals suspended in the air in sufficient concentration to reduce visibility appreciably.


i. The meteorological condition in which the horizontal visibility at the earth's surface is less than 1000 m because of suspended small droplets or ice crystals in the air. Relative humidity in this case is generally 100% but not less than 95%. Fog is formed by the cooling of the air by contact and mixing or, occasionally, through saturation of the air by increasing water content. Fogs are classified according to their effect on visibility: in dense fog, visibility is less than 50 m; in thick fog, visibility is between 50 and 200 m; in fog, visibility is between 200 and 500 m; in moderate fog, visibility is between 500 m and 1000 m; and in thin fog, visibility is between 1000 and 1500 m.
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.


1. a mass of droplets of condensed water vapour suspended in the air, often greatly reducing visibility, corresponding to a cloud but at a lower level
2. a cloud of any substance in the atmosphere reducing visibility
3. Photog a blurred or discoloured area on a developed negative, print, or transparency caused by the action of extraneous light, incorrect development, etc.
4. a colloid or suspension consisting of liquid particles dispersed in a gas


a. a second growth of grass after the first mowing
b. grass left to grow long in winter


When driving, fog is an obstacle or a hindrance that makes things obscure. In a dream it may represent those things that are creating confusion and prevent the dreamer from seeing clearly. The setting of the dream may also give you clues about the area of life that it is referring to. Is the fog over water or land; are you in familiar or unfamiliar surroundings? Does the dream represent the journey into the unconscious, where things are obscure and mysterious, or does it symbolize a part of daily life that is unclear, ambivalent, and challenging? Either way, if the fog lifts before the dream ends, it suggests that awareness and clarity are forthcoming. However, if the fog in the dream was oppressive and frightening, take some time to reflect on what causes such feelings in daily life.
References in classic literature ?
Suddenly a very little counsel with a terrific bass voice arises, fully inflated, in the back settlements of the fog, and says, "Will your lordship allow me?
Leaving this address (delivered like a sepulchral message) ringing in the rafters of the roof, the very little counsel drops, and the fog knows him no more.
When the sun had entirely emerged from the fog, and fields and mist were aglow with dazzling light- as if he had only awaited this to begin the action- he drew the glove from his shapely white hand, made a sign with it to the marshals, and ordered the action to begin.
The lot of towheads was troubles we was going to get into with quarrelsome people and all kinds of mean folks, but if we minded our business and didn't talk back and aggravate them, we would pull through and get out of the fog and into the big clear river, which was the free States, and wouldn't have no more trouble.
The fog seemed to break away as though split by a wedge, and the bow of a steamboat emerged, trailing fog-wreaths on either side like seaweed on the snout of Leviathan.
It was evident to us that all pursuit was in vain until the fog had lifted.
The host had gone below to the cellar, and had brought up bottles of ruby, straw-coloured, and golden drinks, which had ripened long ago in lands where no fogs are, and had since lain slumbering in the shade.
The order was instantly obeyed, and the fog was stirred by the explosion of fifty muskets.
Still fog, which the sunrise cannot pierce, I know there is sunrise because I am a sailor, why else I know not.
At some time or other the fog thinned a little; we did not know when, for we were facing the empty universe and the thinness could not show; but at last Harris happened to look around, and there stood a huge, dim, spectral hotel where the precipice had been.
As suddenly as I had pitched into space, with equal suddenness did I emerge from the fog, out of which I shot like a projectile from a cannon into clear daylight.
was not the liveliest object even in Saint Mary Axe--which is not a very lively spot--with a sobbing gaslight in the counting- house window, and a burglarious stream of fog creeping in to strangle it through the keyhole of the main door.