Fog is a cloud in contact with the ground.
Fog differs from other clouds only in that fog touches the surface of the Earth.
The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges.
The foggiest place in the world is the Grand Banks off the island of Newfoundland, Canada.
Fog is frequent here as the Grand Banks is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south.
The foggiest land areas in the world are Point Reyes, California and Argentia, Newfoundland, both with over 200 foggy days a year.
Most types of fog form when the relative humidity reaches 100% at ground-level.
Fog can form suddenly, and can dissipate just as rapidly, depending what side of the dewpoint the temperature is on.