2. Types of Fog
• Fog is defined as a cloud with its base at or
near the ground.
• Depending upon the conditions, the ground
may become shrouded in radiation
fog, advection fog, or up-slope fog.
3. Types of Fog – Radiation Fog
• Radiation Fog: Results from radiation cooling of
the ground and the surrounding air.
• It occurs at night
• It requires clear skies and fairly high relative
humidity.
• Under these circumstances, the ground and the
air just above it will cool rapidly.
• Because the relative humidity is so high, just a
small amount of cooling will lower the
temperature to the dew point.
4. Types of Fog
• If the air is calm, the fog may be patchy.
• Because the air containing the fog is relatively
cold and dense, it drains downslope where it
is hilly.
• Therefore, radiation fog is thickest in
valleys, while the surrounding hillsides are
clear.
• These types of fog dissipate shortly after
sunrise.
5. Types of Fog
• In the Eastern U.S. radiation may also form
when skies are clear after a rainstorm.
• Air near the surface is close to saturation and
only a small amount of radiation cooling is
required for condensation to occur.
• This type of fog usually occurs around
sunset, making for dangerous driving
conditions.
7. Types of Fog – Advection Fog
• Advection Fog
• When warm moist air is blown over a cold
surface, it becomes chilled by contact and, to
a certain extent, by mixing with the cold air
created by the cold surface below.
• If cooling is sufficient, a blanket of fog, called
an advection fog, will result.
8. Types of Fog
• Therefore, advection fog result from air giving
up heat to the surface below during horizontal
movement.
• A classic example is San Francisco’s Golden
Gate Bridge.
10. Types of Fog – Advection Fog
• A certain amount of turbulence (wind) is
required for proper development of an
advection fog.
• Winds between 6 and 18 mph are associated
with advection fog.
• This turbulence promotes cooling through a
thicker layer of air and carries the fog to
greater heights.
11. Types of Fog – Advection Fog
• As a result, advection fogs are generally thick
(300 to 600 meters deep) and persistent.
• The foggiest location in the U.S. due to
advection fog is Cape
Disappointment, Washington.
• It averages 2552 hours of fog per year. That’s
106 days!
12. Types of Fog – Advection Fog
• These West Coast fogs are produced during
the summer and early autumn
• Warm, moist air from the Pacific Ocean moves
over the cold California Current.
• The fog is then carried on shore by westerly
breezes.
13. Types of Fog – Advection Fog
• Advection fogs are common along the Gulf
and Atlantic coasts.
• Warm, moist air from the Gulf of Mexico and
the Atlantic moves over cold and occasionally
snow-covered surfaces to produce wide-
spread foggy conditions.
• These fogs are frequently and result in
hazardous driving conditions.
14. Types of Fog – Upslope Fog
• Created when relatively humid air moves up a
gradually sloping plain, or even up the steep
slopes of mountains.
• Air expands adiabatically (cooling or warming
that results when air expands or contracts, but
not because heat was added or removed.)
• This is the only type of fog that forms
adiabatically.
• If the dew point is reached, an extensive layer of
fog may form.
15. Types of Fog – Upslope Fog
• In this U.S., this type of fog can occur in
mountainous areas, and also on the Great
Plains.
• Humid Gulf air moves westward from the
Mississippi towards the Rocky Mountains and
gradually glides upslope.
