Clouds

Clouds are formed when air containing water vapor is cooled below a critical temperature called the dew point and the resulting moisture condenses into droplets on microscopic dust particles (condensation nuclei) in the atmosphere. There are three general layers of clouds classified as high-level clouds, mid-level clouds and low-level clouds.

High-level clouds
High-level clouds are called cirrus or have the prefix "cirro" added to their names. High-level clouds form above 6,000 meters (approximately 20,000 feet) (NASA, 20081) and since the temperatures are so low at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance but can show a magnificent array of colors when the sun is low on the horizon. There are three forms of high-level clouds: cirrus, cirrostratus, and cirrocumulus. Contrails are also categorized as a form of high-level cloud.
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(Image embedded from WW2010 on 3 June 2009)

Mid-level clouds
The two forms of mid-level clouds have the prefix "alto" added to their names . The bases of mid-level clouds typically appear between 2,000 to 6,000 meters (6,500 to 20,000 feet) (NASA, 20081). Because of their lower altitudes, they are composed primarily of water droplets. However, they can also be composed of ice crystals when temperatures are cold enough. Middle clouds of the layer type are referred to as altostratus, and heap type clouds as altocumulus.
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(Image embedded from WW2010 on 3 June 2009)

Low-level clouds
Low clouds are mostly composed of water droplets since their bases generally lie below 2,000 meters (6,500 feet) (NASA, 20081). However, when temperatures are cold enough, these clouds may also contain ice particles and snow. There are various forms of low-level clouds, including stratus, nimbostratus, cumulus, stratocumulus, cumulonimbus, and fog.
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(Image embedded from NASA on 3 June 2009)

References
1. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (2008). On-line cloud chart. Retrieved from National Aeronautics and Space Administration on 20 March 2009.

Want to know more?

AviationKnowledge - Further information on clouds
AviationKnowledge also offers more information for high-level clouds, mid-level clouds, and low-level clouds.
Virtualskies (NASA) - Aviation weather
This VirtualSkies page offers a full tutorial on aviation weather, including cloud types and cloud formation.
Wikipedia - Clouds
This Wikipedia page offers more detailed information about cloud formation and cloud types. You can also consult a list of cloud types.
Wikipedia - Dew point
This Wikipedia page offers further information about the dew point for clouds.

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