FAA ACS Knowledge Test Code: PA.I.C.K4k


Fog is one of those topics that every pilot must understand in detail. Even with today's in-flight weather reporting, knowing the conditions that indicate fog at your destination airport is crucial to safety. Fog is simply a cloud that occurs near the ground, but weather reports may be slow in reporting this. Knowing what conditions cause the formation of fog can help you make safety of flight decisions. Plus, this information can appear on just about any aviation test you will ever take.

You should expect fog and low clouds any time that the air temperature is within 5 of the dew point and the temperature/dew point spread is decreasing.

Five Types of Fog

On FAA exams you will be expected to know about five types of fog. The are easy to remember if you use this memory aid:

Fog Usually Seems Present After Rain.

Upslope: Results from the cooling of warm moist air as it is forced up sloping terrain.
Hint: "up sloping terrain"

Steam: Occurs in cold temperatures when cold, dry air passes from land areas over relatively warm bodies of water. Low level turbulence and icing may be associated with steam fog.
Hint: Think of the "steam" you see in your breath when you exhale into cold air.

Precipitation-induced: Occurs when warm rain/drizzle falls through cool air. Usually associated with fronts (beware of associated icing, turbulence, and thunderstorms).
Hint: Think of the name - this type of fog results from precipitation.

Advection: Results from warm moist air moving over a cooler surface.
Hint: "Advection" means horizontal movement of air.
(Convection, a term you've heard associated with thunderstorms, refers to the vertical movement of air.)

Radiation: Usually occurs in conditions of clear skies and calm winds. Small temperature/dew point spread is the cause.
Hint: Imagine that the "radiation" would get blown away with any wind. It appears when there is no wind.


Learn the memory aid (Fog Usually Seems Present After Rain) and the five hints and
you are guaranteed to never miss an FAA fog question. Easy!




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