Fog

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For other meanings, see Fog (disambiguation)

Fog is a cloud in contact with the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges.

Characteristics

Most types of fog form when the relative humidity reaches 100% at ground-level. Fog can form suddenly, and can dissipate just as rapidly, depending on what side of the dewpoint the temperature is on.

An important exception to the general rule is sea fog. This is due to the peculiar effect of salt. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there. Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon.

Fog occasionally produces precipitation in the form of drizzle. Drizzle occurs when the humidity of fog attains 100% and the minute cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.

Fog reduces visibility. Although most sea vessels 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.

Types

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. 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.

Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snowpack. It's most common at sea when tropical air encounters cooler higher-latitude waters, or areas of upwelling, such as along the California coast. The advection of fog along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coastward, an occurrence most typical in the spring or late fall. During the summer months, a low pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoon, produces a south to southeasterly flow which can drive the offshore marine layer up the coastline, a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it may instead break up the marine layer and any fog it may contain.

and 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.

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.

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.

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.

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Characteristics

Types

See also

External links