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
FAA exams you will be expected to know about five types
of fog. The are easy to remember if you use this memory
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
Hint: Think of the name - this type of fog results
Advection: Results from warm moist air moving over a
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.