Updated: Dec 31, 2020

Fog is just a cloud on the surface. Fog can cause a variety of issues for pilots including icing and IFR conditions. Fog is classified by the manner in which it forms.

Here's what you need to know.


Forms on clear nights with relatively little to no wind present. Most commonly found in low-lying areas like mountain valleys.

Formation is caused when the ground cools rapidly due to terrestrial radiation and the surrounding air temperature reaches its dew point.


Forms when a layer of warm, moist air moves over a cold surface.

Unlike Radiation Fog, Advection Fog needs wind speeds up to 15 knots in order to form. Winds above 15 knots will lift the fog into a low stratus cloud.

Most commonly found in coastal areas.


Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range. This type of fog also requires wind for formation.


Forms when cold, dry air moves over a warmer body of water. As the water evaporates, it rises and resembles smoke or steam.

Most commonly found over bodies of water during the coldest seasons of the year.


Occurs in cold weather when the temperature is much below freezing and water vapor forms directly into ice crystals.

Conditions favorable for its formation include outside air temperatures of -25F or colder.

Author - Nate Hodell

CFI/CFII/MEI/ATP - Creator of wifiCFI - Owner of Axiom Aviation Flight School.

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