The Ultimate Guide to Clouds
Clouds are an essential part of
the Earth's water cycle. Their appearance can change as they
take different forms and create weather phenomena.
Clouds are formed when water on
the Earth's surface evaporates. The resulting water vapor
rises and meets cool air in the troposphere (the lowest level of the
Earth's atmosphere). The cool air causes the vapor to condense
and form water droplets on condensation nuclei - which are small
particles of salt, dust, pollen, and other debris - floating in the
The sun plays a part in how the
human eye sees the color of clouds. Waves of every color in
the rainbow make up the sun's white light. The sky appears
blue because the human eye sees the blue light that reflects off
nitrogen and oxygen molecules in the air, while clouds appear white
because they are made up of water or ice, which do not reflect light
waves. When the sun's light passes through clouds, the water molecules
scatter the light's rainbow of colors in all directions, making clouds
appear white. If a cloud becomes so thick with water droplets
that the sun's light cannot penetrate it, the cloud will appear dark to
someone standing underneath it. If someone were in an
airplane, looking down on that cloud, it would still appear white.
Stratus, cumulus, and cirrus are
the three general types of clouds, and they can be identified by their
appearance and height in the atmosphere.
Stratus clouds are the lowest
clouds in the atmosphere. They may stay on the ground as fog
or go up to 9,800 feet and spread across the sky like a
blanket. Nimbostratus clouds are the thick, dark blanket of
clouds that are responsible for overcast skies and constant
Cumulus clouds have their base at
a low level, between 1,600 to 9,800 feet, but rise vertically into the
atmosphere. These are tall white, puffy clouds. The
cumulonimbus is the tallest of all clouds. It may look like a
tower or mountain, and rise up to over 60,000 feet.
Altostratus and altocumulus are
middle base level clouds. They are found between 9,800 to
23,000 feet. The altostratus clouds give the sky a gray
appearance, but it is difficult to distinguish any
details. The altocumulus clouds may often look like a sheet of
puffy, white ripples.
Cirrus clouds form in the highest
levels of the troposphere at over 23,000 feet. These clouds
are composed of ice crystals and have a wispy
appearance. Sometimes a combination of cloud types will
form. Cirrostratus clouds are ice clouds that blanket the sky,
but have distinguishing wispy or fibrous strands. Cirrocumulus
clouds are high, ice clouds that resemble light ripples.
There are other cloud phenomena
that form under unique circumstances. When water droplets become too
dense and heavy for the cloud to support them, they drop in the form of
precipitation (like rain or snow). When there is low humidity
and high temperatures, the water may evaporate before the precipitation
hits the ground. This creates virga, the fibrous strands that
hang under a rain cloud.
Funnel clouds and waterspouts have
the potential to become destructive. A funnel cloud is a
funnel-shaped, cumuliform cloud caught in a rotating column of
wind. If the funnel cloud touches the ground, it becomes a
tornado. If it touches down on water, it becomes a "water
tornado", called a waterspout, which can pose a serious risk to boats
and marine life.
Some cloud phenomena are optical
illusions. Crepuscular rays are often seen as bright shafts of
sunlight, coming from a central point in the sky. They usually
appear at dusk or dawn, when the sunlight breaks through thin areas of
a cloud, but is shadowed in other areas. A glowing,
ring-shaped, halo cloud can be created when different air temperatures
combine and the cloud reflects a setting sun.
People may admire a cloud's
changing shape, or watch for dark clouds as predictors of rain, but
clouds in all their forms are key elements of our weather.
For more information about clouds,
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