Fogs occur most often near big bodies of water. The land often cools off much faster than the water. Warm, moist air moving in over the land is cooled quickly. Some of the water vapor in the air changes to drops of water and forms a fog. In cities fog may have so much smoke mixed with it that it is called "smog."
Some cities are famous for their fogs. London is one of them.
Fogs disappear when the ground warms up or when a brisk wind blows them away. They can be driven away by fires. During World War II millions of dollars were spent to keep airfields free of fog.
