hourly observations of surrounding stations, especially
those southeastward. If one of these stations starts
reporting stratus, the chances of stratus formation at
your station are high.
FOR FRONTAL FOG
Frontal fogs are of three types: prefrontal (warm
front), postiontal (cold front), and frontal passage.
Prefrontal (warm front) fogs occur in stable
continental polar (cP) air masses when precipitating
warm air overrides the colder air. The rain raises the
dewpoint in the cP air mass sufficiently for fog
formation. Generally, the wind speeds are light, and the
area most conducive to the formation of this type of fog
is one between a nearby secondary low and the primary
The northeastern area of the
United States is probably the most prevalent region for
this type of fog. Prefrontal fog is also of importance
along the Gulf and Atlantic coastal plains, the Midwest,
and in the valleys of the Appalachians.
A rule of thumb for forecasting ceiling during
prefrontal fog is as follows: If the gradient winds are
greater than 25 knots, the ceiling will usually remain 300
feet or higher during the night.
As with the prefrontal fog, postfrontal (cold front)
fogs are caused by falling precipitation. Fogs of this
type are common when cold fronts with east-west
orientations have become quasi-stationary and the
continental polar air behind the front is stable. This type
of fog is common in the Midwest. Fog, or stratiform
clouds, may be prevalent for considerable distances
behind cold fronts if the cold fronts produce
Frontal Passage Fog
During the passage of a front, fog may form
temporarily if the winds accompanying the front are
very light and the two air masses are near saturation.
Also, temporary fog may form if the air is suddenly
cooled over moist ground with the passage of a
precipitating cold front. In low latitudes, fog may form
in the summer if the surface is cooled sufficiently by
evaporation of rain that fell during a frontal passage,
provided that the moisture addition to the air and the
cooling are great enough to allow for fog formation.
CONDITIONS FAVORABLE FOR SEA FOG
Sea fogs are advection fogs that form in warm moist
air cooled to saturation as it moves over colder water.
The colder water may occur as a well-defined current,
or as gradual latitudinal cooling. The dewpoint and the
temperature undergo a gradual change as the air mass
moves over colder and colder water. The surface air
temperature falls steadily, and tends to approach the
water temperature. The dewpoint also tends to
approach the water temperature, but at a slower rate. If
the dewpoint of the air mass is initially higher than the
coldest water to be crossed, and if the cooling process
continues sufficiently long, the temperature of the air
ultimately falls to the dewpoint, and fog results.
However, if the initial dewpoint is less than the coldest
water temperature, the formation of fog is unlikely.
Generally, in northward moving air masses or in air
masses that have previously traversed a warm ocean
current, the dewpoint of the air is initially higher than
the cold water temperature to the north, and fog will
form, provided sufficient fetch occurs.
The rate of temperature decrease is largely
dependent on the speed at which the air mass moves
across the sea surface, which, in turn, is dependent both
on the spacing of the isotherms and the velocity of the
air normal to them.
The dissipation of sea fog requires a change in air
mass (a cold front). A movement of sea fog to a warmer
land area leads to rapid dissipation. Upon heating, the
fog first lifts, forming a stratus deck; then, with further
heating, this cloud deck breaks up into a stratocumulus
layer, and eventually into convective type clouds or
evaporates entirely. An increase in wind velocity can
lift sea fog, forming a stratus deck, especially if the
air/sea temperature differential is small. Over very cold
water, dense sea fog may persist even with high winds.
CONDITIONS FAVORABLE FOR ICE FOG
When the air temperature is below about -25°F,
water vapor in the air that condenses into droplets is
quickly converted into ice crystals. A suspension of ice
crystals based at the surface is called ice fog. Ice fog
occurs mostly in the Arctic regions, and is mainly an
artificial fog produced by human activities, It occurs
locally over settlements and airfields where
hydrocarbon fuels are burnedthe burning of
hydrocarbon fuels produces water vapor.