What are types of fog?

The primary types of fog are radiation fog, advection fog, upslope fog, frontal fog, and steam fog.

Fog is visible water droplets in the air near the ground.  Fog forms when the air temperature and dew point approach the same value.  Fog is basically a cloud with the base at the surface.  From down in a valley, a pilot may see a cloud near a mountain top.  If the pilot was at the top of the mountain, it would be considered fog.

Radiation fog is formed over land by the cooling of the surface.  Radiation fog is generally shallow and burns off rapidly after sunrise.  A strong wind also displaces the fog. Factors favoring the formation of radiation fog are: 1) a shallow surface layer of relatively moist air beneath a dry layer, 2) clear skies, and 3) light surface winds.  Radiation fog is often called ground fog.

Advection fog forms when moist air moves over a cooler surface.  Advection fog is common in coastal areas, but often moves inland.  When at sea, advection fog is referred to as sea fog.  Wind greater than 15 knots lifts the fog into a layer of low stratus or stratocumulus clouds.

Upslope fog forms as moist, stable air is adiabatically cooled as it moves up sloping terrain.  Winds speeds of 5 to 15 knots are most favorable since stronger winds tend to lift the fog into a layer of low stratus clouds.

Frontal fog is associated with frontal zones and frontal passages.  When warm, moist air, is lifted over a front, clouds and precipitation may form.  Frontal fog can become quite dense and continue for an extended period of time.  Frontal fog may extend over large areas, completely suspending air operations.  Frontal fog is most commonly associated with warm fronts but can occur with other fronts as well.

Steam fog forms when cold, dry air moves over warm water.  As the water evaporates, it rises and resembles smoke.  This type of fog is common over bodies of water during the coldest times of the year.  Low-level turbulence and icing are commonly associated with steam fog.


FAA AC 00-6B Aviation Weather pg. 16-1

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