Fog is defined as a cloud having a base of less that 50 feet above the surface.
For or low clouds are likely to develop when the temperature/dewpoint spread is
close and decreasing. The different types of fog are detailed below.
The ground is heated by the sun during the daytime. At night,
however, it radiates heat upward. If the skies are overcast, this radiated
heat is trapped. The clouds act like a blanket, in a way. If the skies
are clear, though, heat will be radiated out into space. The ground will cool
and air touching the ground will be cooled by contact. If there is no wind,
the ground is able to cool the air below its dewpoint, resulting in radiation fog.
Radiation fog forms on clear nights with calm wind. It is common in
In the morning, the sun heats the ground, which heats the air. As a result,
radiation fog usually burns
off within a few hours of sunrise. Any wind will
hasten its disappearance.
Advection fog is common is coastal
areas and requires wind to form.
When warm moist air moves over a colder surface and is cooled below its dewpoint,
advection fog forms. Commonly, air moves from over a body of water over
a colder land mass. Advection fog will build and intensify with winds up to
15 knots and usually lifts to form a low overcast layer of clouds.
As air pressure decreases, air cools. If wind is blowing against upsloping terrain, such as a mountain range, the air is forced upward. As
it moves upward, pressure decreases and the air cools. If it cools to its
dewpoint, upslope fog forms.
Upslope fog requires wind to form and may last for several days at a time.
If cold dry air moves over warmer water, the water will evaporate
into the dry air, then immediately condense, forming steam fog. Steam fog
occurs under cold conditions. It commonly occurs when a light breeze blows
colder air out to sea.
Low level turbulence can occur and aircraft structural icing can become
hazardous in steam fog.