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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 the condensation occurred:

Picture from the
NOAA Photo Library

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 vapour 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.