Fog is a relatively rare event in Tucson because the air is so
dry so much of the time. To produce fog you
first need to increase the relative humidity (RH) to 100%.
You can do this either by cooling the air (radiation fog is an
example) or adding moisture to and saturating the air (evaporation
or steam fog). Both will increase the ratio in the RH
formula above.
Probably the most common type of fog in Tucson is radiation
fog. The ground cools during the night by emitting IR
radiation (left figure below). The ground cools most rapidly
and gets coldest when the skies are free of clouds and the air is
dry (except for a thin layer next to the ground.
Air in contact with the ground cools and radiation fog can form
(right figure above). Because the fog cloud is colder,
denser, and heavier than the air right above, this is a stable
situation. The fog clouds "hugs" the ground.
A photograph of a very thin layer of radiation fog at left (source).
The
fog
layer
at right was photographed in Tanzania (source).
The
air
was
probably fairly moist and only a little cooling overnight raised
the relative humidity to 100% and produced fog.
Radiation fog is sometimes called valley fog.
This is because the cold dense foggy air will move downhill and
fill low lying areas. Because the fog reflects
sunlight, it is often difficult for the sun to warm the air and
dissipate thick clouds of valley fog.
Valley fog (source).
Steam fog or evaporation fog (also sometimes known as mixing
fog) is commonly observed on cold mornings over the relatively
warm water in a swimming pool.
In the sketch above, water evaporating from the pool
saturates the cold air above. Because the fog cloud is
warmer than the cold surrounding air, the fog clouds float
upward. The photograph at right (source)
was an Earth Science Picture of
the Day.
When you "see your breath" on a cold day
you're seeing mixing fog (source
of the right image above). Warm moist air from your mouth
mixes with the colder air outside. The mixture is saturated
and a fog cloud forms (the next time this happens to you watch and
see if the warm fog cloud doesn't float upward).
You might remember the following two reactions from
earlier in the semester when we were talking about photosynthesis
and combustion
Combustion is in principle just the same reaction in
reverse. Note that water vapor is one of the by products of
combustion.
Combustion sometimes adds enough water vapor to the air to
saturate the air. Clouds form in that case. Here are a
couple of examples
There is sometimes enough water vapor in automobile exhaust to
saturate the air and form a cloud. The exhaust from a car
may, of course, also be smoke from burning oil or something like
that. The same is true of the exhaust from aircraft jet
engines, water vapor in the exhaust sometimes forms contrails.
Exhaust from a natural gas fueled furnace or hot water heat also
contains water vapor. Most of the time you won't see the
exhaust gases, but when the relative humidity is high you can see
a cloud coming from one of the vents pipes on the house
roof. People will sometimes mistake this for smoke and will
call the fire department.
