Fog is a cloud that is on the surface. It occurs when the temperature of the air near the ground is cooled to the air's dew point. Fog poses a significant hazard to aviation in that it reduces visibility to a level that makes takeoff and landing difficult to impossible. To be classified as fog for the purposes of a METAR, the visibility must be reduced to less than 5/8 statute mile.


Fog types are named according to their formation mechanism. By being aware of the types of fog and how they form, a pilot has the ability to anticipate foggy conditions and plan for alternative courses of action if the fog occurs.

Type Description
Radiation Fog Produced over a land area when radiational cooling reduces the air temperature to the dew point, it is generally a nighttime occurence. Factors that favor radiation fog are:
(1) shallow surface layer of moist air,
(2) clear skies, and
(3) light surface winds.
Advection Fog Produced when moist air moves over a cold surface, it is most common along coastal areas. Advection fog deepens as wind speed increases up to 15 knots. Above that, the fog typically lifts into a layer of low stratus clouds.
Upslope Fog Produced when moist, stable air is pushed up sloping terrain, it is most common in mountainous areas. Upslope fog is often quite dense and extends to high altitudes.
Precipitation-Induced (Frontal) Fog Produced when evaporation of precipitation cools the air to the dew point, it is most commonly associated with warm fronts. This fog may extend over large areas.
Steam Fog Produced when very cold air moves across warm water, it is commonly observed over lakes and ponds on cold mornings. When the cold air moves over the water, enough moisture may evaporate to cause the air to reach saturation.


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