Fog is a body of microscopically small drops of water condensed from the air. This body of water, suspended near the surface of the earth, reduces horizontal visibility to 0.6 mile or less. As the droplets become larger, fogs merge gradually into a drizzle.
Fog is usually formed when a body of moist warm air comes in contact with cold land or sea. The cold causes the invisible water vapor to condense and become visible as a mass of tiny particles of water.
In a sea fog warm air is carried over cold water. For example, off Newfoundland fogs form when the air from the south blows over the cold waters of the Labrador Current. Such a fog is called an advection fog, one produced by the movement of moist air over a cold surface. An evaporation fog, or steam fog, is produced when cold air moves over warm water; moisture may be evaporated from the water and added to the cold air to the point of saturation.
A thick fog lying close to the ground, formed when moist surface air is cooled at night, is one type of radiation fog. The other type of radiation fog is a high-inversion fog, in the form of a low stratus cloud. In an upslope fog, cooling is due to the ascent of moist air moving against a mountainside.
During the day fog is usually evaporated as the sun warms the cool air and allows it to hold more water.
Advection, radiation, and upslope fogs are all produced chiefly by the cooling of air to its dew point. Such fogs are classified as air-mass fogs. Another main classification of fogs, called frontal fogs, includes fogs produced when vapor from falling rain raises the dew point while other factors lower the temperature.
