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©
Michael Collier
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These stratocumulus clouds over Monument
Valley in Arizona indicate a stable air mass.
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Clouds are formed when moist air rises upward. As the air rises, it becomes
colder. Eventually the air can't hold all of the water vapor in it, and
some of the water vapor condenses to form tiny water droplets. When moist
air is cooled at the ground, fog is formed in the same way.
Clouds form at a wide range of altitudes, from near the ground to very
high in the atmosphere. The appearance of clouds varies a lot, depending
on the motions of the air as the clouds are formed. Other important things
to observe about clouds are the percentage of the sky they cover, where
they are located in the sky, how much of the sky they cover, and their
direction of movement. A good way to find their direction of movement
is to stand under a tree branch or an overhang on a building and watch
the clouds move relative to that stationary object.
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