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Fog is cloud in contact with the ground. It can form in a number of ways, depending on how the cooling that caused condensation occurred.

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Radiation fog is formed by the cooling of land after sunset by radiation in calm conditions with clear sky. The cool ground then produces condensation in the nearby air by 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.

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 water.

Steam fog is the most localized form and is created by cold air passing over much warmer water. The air is quickly saturated by evaporation and the condensation thus created is seen as wispy steam. Steam fog is most common in polar regions.

Precipitation fog (or Frontal fog) forms as precipitation falls into drier air below the cloud, the liquid drops evaporates into water vapour. The water vapour cools and increases the moisture content of the air. As the air saturates below the cloud fog forms.

Upslope fog forms when winds blow air up a slope, adiabatically cooling it as it rises.

Valley fog forms in mountain valleys during winter, it is essentially radiation fog confined by local topology. It can last for several days in calm conditions.

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