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.