Fog, a cloud that occurs at the earth's surface. It usually consists of tiny water droplets suspended in the air, but in very cold conditions it may consist of ice particles. Mist is similar to fog, but is less dense and generally contains larger water droplets, which fall to the ground. Smog is a term given to fog combined with smoke and other pollutants; it typically occurs over large cities.

Fog greatly reduces visibility and is therefore a hazard to transportation, especially by ship, airplane, or automobile. The use of radar on ships and airplanes has lessened, but not eliminated, the danger posed by fog. Ships and lighthouses often use foghorns to indicate their location in a fog. A number of methods, including the use of jet-engine exhaust to evaporate the droplets of water that make up a fog, have been employed to disperse fog at airports, but these methods generally are uneconomical.

Fog, like other clouds, forms when the water vapor in the air condenses. The amount of water vapor that air can hold depends on the air's temperature—as air is cooled, the amount of water vapor it can contain decreases. At a temperature called the dew point, the air becomes saturated with water vapor and further cooling of the air will cause some of the water vapor to condense. The water vapor condenses on microscopic particles of dust or other material in the atmosphere. The dew-point temperature depends on the amount of water vapor in the air; the greater the amount of water vapor the air contains, the higher is the temperature at which the air will become saturated when cooled.

Kinds of Fog
Radiation Fog

occurs during long, clear nights, typically in autumn, when the land warmed by the sun during the day radiates heat into space. Under relatively calm and humid conditions, the air in contact with the ground can be cooled below its dew point, causing some of the water vapor in the air to condense into fog. This fog tends to develop in valleys and hollows. Radiation fog that extends only a short distance above the ground is called ground fog and is usually dispersed by the morning sun.

Advection Fog

is caused by the movement of a mass of air across the earth's surface. Such a fog may occur at any time of day and sometimes covers a very large area. Most advection fog develops when warm, moist air rides in over a cold surface—land, ice, or water. A large volume of air is cooled below its dew point, forming a thick blanket of fog that may persist for days, drifting with the wind.

Advection fog often develops along seacoasts where warm air comes in contact with cold ocean currents. Noted foggy regions include parts of North and South America's Pacific coast and the coasts of the northeastern United States, the Atlantic Provinces of Canada, and northwestern Europe.

Sometimes advection fog is formed when a mass of air is forced across rising terrain, such as the slope of a mountain. As the air reaches greater elevations, its pressure decreases. The drop in pressure produces a phenomenon called adiabatic cooling, which may sufficiently cool the air to cause its water vapor to begin to condense. Fog that develops in this manner is called upslope fog.

Evaporation Fog

occurs when the amount of moisture in the air is increased beyond the saturation point by the evaporation of water that is warmer than the air. This process is similar to the condensation of steam rising from a teakettle. Evaporation fog sometimes forms in irregular columns rising from the surface of water; such fog is commonly called steam fog.

Ice Fog

In severely cold weather, the water vapor in air that has become saturated can form into ice crystals instead of droplets of water. Fog containing ice crystals is relatively rare and generally occurs only in polar regions. In the northwestern United States, ice fog is known as pogonip.