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Early morning fog obscures the surface of this lake in Carrollton, Georgia, but the sky remains clear.

Fog is 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 clouds in that fog touches the surface of the Earth, while clouds do not. It can form in a number of ways, depending on how the cooling that caused the condensation occurred:

Evening fog obscures 's  from passers by.
Evening fog obscures London's Tower Bridge from passers by.
  • Precipitation fog (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapour. The water vapour cools and at the dewpoint it condenses and fog forms.
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Fog early in the morning
  • 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 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.
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Ice fog in Winnipeg condenses on a -40°C January day from nearly any moisture at all, from cars, homes, or industry.
  • Freezing fog is when liquid fog droplets freeze to surfaces, forming white rime ice. This is very common on mountaintops which are exposed to low clouds. It is equivalent to freezing rain, and essentially the same as the ice which forms inside a freezer which is not of the "frostless" or "frost-free" type.

All types of fog form when the relative humidity reaches 100%, and the air temperature tries to drop below the dewpoint, pushing it lower by forcing the water vapor to condense.

Fog reduces visibility. Some vehicles have radar etc., cars have to drive slower and use more lights. Especially dangerous is when fog is very localized, and the driver is caught by surprise. Fog is particularly baneful for airport operators, some of whom have attempted to develop methods (such as using heaters or salt particles) to aid fog disperal. These methods enjoy some success at temperatures below freezing.


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