Fog is actually a cloud, but it is a low cloud that lies close to the ground or sea. It is formed when warm currents of air hit against cold air resting above land or water.
What happens is that the sudden cooling causes moisture in the warmer air to condense, or form tiny droplets of water. Fog occurs only when the air is cooled below what is called its dew point.
Fogs lasting weeks and weeks cover much of Newfoundland, Canada, for more than 120 days every year!