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For other meanings, see Fog (disambiguation)

Image:Fog-towerbridge.700px.jpg Image:Sugarloaf Mountain in Rio Brazil - with cloud layer.JPG Image:Fog&Sunny.png

Fog is a cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog. Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not.



All types of fog form when the relative humidity reaches 100% and the air temperature drops below the dewpoint, pushing it lower by forcing the water vapor to condense. Fog can form suddenly, and can dissipate just a rapidly, depending on what side of the dewpoint the temperature is on.

Fog reduces visibility. Although some forms of transport can penetrate fog using radar, road vehicles have to travel slowly and use more lights. Localised fog is especially dangerous, as drivers can be caught by surprise. Fog is particularly hazardous at airports, where some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.


Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred:

Radiation fog is formed by the cooling of land after sunset by thermal (infrared) radiation in calm conditions with clear sky. The cool ground then produces condensation in the nearby air by heat conduction. In perfect calm the fog layer can be less than a metre deep but turbulence can promote a thicker layer. Radiation fog is common in autumn and usually does not last long past sunrise. Image:Img20040611 0140 ch engelberger aa.jpg Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is sometimes used to refer to radiation fog.

Image:Golden Gate.jpg Advection fog occurs when moist air passes over cool ground by advection (wind) and is cooled. This form is most common at sea when tropical air encounters cooler higher-latitude waters. It is also extremely common as a warm front passes over an area with significant snowpack.

Steam fog, also called evaporation fog, is the most localized form and is created by cold air passing over much warmer water or moist land. Water vapor quickly enters the atmosphere by evaporation and condensation occurs once the dewpoint has been reached, thus creating a wispy steam. Steam fog is most common in polar regions, Image:Morningfog.jpgand around deeper and larger lakes in late autumn and early winter. It is closely related to lake-effect snow and lake-effect rain, and often causes freezing fog, or sometimes hoar frost.

Salt fog (or salt-fog) is characteristic of coastal atmospheres; the water droplets of this form of fog, formed by evaporated seawater, carry in solution microscopic particles of salt. It is best known as the airborne cause of corrosion of objects by salt with which coastal inhabitants have to contend. This problem is a major reason why engineering reliability laboratories offer salt-fog or airborne salt corrosion simulation tests.

Precipitation fog (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor. The water vapor cools and at the dewpoint it condenses and fog forms.

Upslope fog forms when winds blow air up a slope (called orographic lift), adiabatical cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.

Valley fog forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into Image:Mount Lushan - fog.JPG the valley, with warmer air passing over the mountains above. It is essentially radiation fog confined by local topography, and can last for several days in calm conditions. In California's Central Valley, Valley fog is often referred to as Tule fog.

Image:Fog-laser.jpg Ice fog is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally this requires temperatures well below the freezing point, making it common only in and near the Arctic and Antarctic regions. Extremely small amounts of this falling from the sky form a type of precipitation called ice crystals, often reported in Barrow, Alaska.

Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white rime ice. This is very common on mountaintops which are exposed to low cloud. It is equivalent to freezing rain, and essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type.

Artificial fog is artificially generated fog that is usually created by vaporizing a water and glycol-based or glycerine-based fluid. The fluid is injected into a heated block, and evaporates quickly. The resulting pressure forces the vapor out of the exit. Upon coming into contact with cool outside air the vapor forms a fog.

See also


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