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Clouds


Clouds typically form when air rises. The reduction in pressure as air rises causes the air to expand and cool. Rising motion can be associated with convection in unstable air, lifting of air over topography (mountains), or lifting of air by fronts. (Fronts will be discussed in more detail in the next section.) When air rises, the air's temperature cools and may reach its dewpoint temperature, at which point it becomes saturated. Once saturation is reached, condensation occurs and the water vapor in the air will condense into tiny water droplets. As millions of droplets form, a cloud will begin to take shape.






Cloud Classification

Clouds are classified by their height (high, middle, low, or vertically developing), physical appearance and whether they produce precipitation. Here are a few Latin roots that are helpful when identifying cloud types:

"cirro": high, 'curl of hair'
"alto": 'middle'
"stratus": layer, sheet-like, low
"cumulus": heap-like, puffy
"nimbus": clouds producing precipitation

Combinations of these Latin roots are used to describe the most common types of clouds (i.e. a cirrostratus cloud is one that is high and layered).



High Clouds
Cirrus, Cirrostratus, Cirrocumulus

High level clouds form above 20,000 feet (6000 meters). Since they form high in the atmosphere, high clouds are composed of ice crystals, due to the cold temperatures in this part of the atmosphere. High-level clouds are typically thin and white in appearance, but may display an array of colors when the sun is low on the horizon






Mid-Level Clouds
Altostratus, Altocumulus

Mid-level clouds typically have bases between 6,500 to 20,000 feet (2000 to 6000 meters). Since these clouds are located lower in the atmosphere, they are primarily composed of water droplets. In the cold season, they can be composed of ice crystals since the temperatures are cold enough.






Low Clouds
Stratus, Stratocumulus, Nimbostratus

Low clouds typically have bases below 6,500 feet (2000 meters). These clouds are located low in the atmosphere and are mostly composed of water droplets. On occasion, if the temperatures are cold enough, they may contain some ice particles and snow.






Vertically Developing Clouds
Cumulus, Cumulonimbus

Some clouds can span the depth of the troposphere and therefore cannot be classified as high, middle or low. These clouds are classified as vertically developing. Cumulus clouds are characterized by a flat base and can grow to heights exceeding 39,000 feet (12,000 meters). They can contain both liquid droplets and ice particles because they cover a large depth of the troposphere. With the right conditions, these are the clouds that become powerful thunderstorms.




© 2006 Katie R. Roussy, University of Illinois at Urbana-Champaign
Images and photographs courtesy of the National Oceanic and Atmospheric Administration