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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) 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 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.
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. Warm moist air from your mouth mixes
with the colder air outside. The mixture is saturated and a fog
cloud forms.
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.
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 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.
Exhaust from a natural gas fueled furnace or hot water heat 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.
