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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.
Raindrops are formed when the cloud droplets grow big enough to
fall out of the clouds. Most of the rain that falls in the winter,
and even a lot of it that falls in the summer, is from melting of
snowflakes as they fall through warmer air. Rainfall is measured
by the depth of water that falls on a level surface without soaking
in. Rainfall is measured with a rain gauge. A basic rain gauge is
nothing more than a cylindrical container, like a metal can, with
a flat bottom. The only problem is to get an accurate measurement
of the depth of water that has fallen. Accurate rain gauges are
arranged so that the water that falls into the container is funneled
into a much narrow container inside. That way, the height of the
water is magnified, and is easier to read.
If you live in a part of the United States where it snows in winter,
you can easily measure the snow depth with a ruler. The best time
to make the measurement is right after the snow stops falling. The
measurement can be tricky, because wind can blow snow from one place
to another. The best place to measure snow depth is on level ground
far away from buildings and trees.
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