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 similar cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is different from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.
The foggiest place in the world is the Grand Banks off the island of Newfoundland, Canada. Fog is common 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.