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 reduces visibility. Road vehicles have to travel slowly and use more lights. Localised fog is especially dangerous, as drivers can be caught by surprise.
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