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clouds
CLOUDS
Everyone has heard the
expressions "cloud nine" or "walking in the clouds." Although
clouds look like delicious whipped cream or cotton candy, they are actually
masses of condensed water vapor.
Clouds usually form when
cool, moist air rises through the atmosphere. As the earth's surface warms,
water evaporates until the air becomes saturated. As one goes higher in the
atmosphere, pressure decreases while air expands and cools. Water droplets condense
and become visible as clouds.
During storms, dense clouds
form. Water droplets grow heavy and eventually descend as rain. In time, this
moisture will evaporate from the ground and reform in the atmosphere as clouds.
This process is called the water cycle.
There are many different
types of clouds. One common type is cumulus, a billowy cloud heaped high on
a flat base. Cirrus clouds are very high clouds that are wispy or streaked.
In the winter, low level stratus clouds are predominant with fog. Nimbus clouds
are dark sheet-like clouds that bring rain. Cumulonimbus are large anvil shaped
clouds which produce thunder and lightning. Lenticular clouds are smooth, stationary
mid level clouds that develop over mountains.
Clouds develop in many
different ways besides the evaporation of water into the atmosphere. When air
passes over a cooler surface, such as the dewy ground at night, the wind may
stir some of the coolness upward, creating fog. Artificial clouds form when
damp air masses of different temperatures mix. Examples include the steam made
by expelled breath in cold weather and the exhaust trails produced by high-flying
aircraft. A rapid local reduction of pressure, along with the expansion and
cooling of the air lead to the violent rotating motions found in funnel clouds
and tornadoes.
When clouds reach saturation,
they precipitate or produce rain. Under cold conditions, clouds produce snow
but not directly. Condensing water droplets at below freezing temperatures are
said to be supercooled. Occasionally a very small number of these will freeze.
However, ice crystals absorb more water vapor so that they quickly grow, shed
splinters, and multiply. Eventually, the water droplets completely evaporate
and the cloud becomes a pure ice cloud.
Many factors cause clouds
to come in many shapes and sizes. As a cumulonimbus hits the bottom of the stratosphere,
a ceiling effect occurs; the top of the cloud flattens against this invisible
surface and stretch into an oval. Persistent moist winds over mountainous terrain
can produce smooth elongated clouds that stand in one location, known as lenticular
clouds.
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