Fog forms through three main processes: radiation fog, advection fog, and upslope fog. Radiation fog occurs at night in clear, humid conditions as the ground cools. Advection fog forms when warm, moist air moves over colder surfaces. Upslope fog develops as humid air rises up sloping land, cooling adiabatically. Each type has distinguishing characteristics in terms of formation, thickness, duration and location.

1. Types of Fog
Forms of Condensation and
Precipitation

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.

6. Radiation Fog

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.

9. Advection Fog

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.