Clouds

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

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There's nothing magic about identifying clouds. If they look like flat sheets, they are flat sheets. If they look low, they are low. Many things in science are not what they seem. Clouds, on the other hand, are pretty much what they seem.

Clouds are classified as sheetlike (stratiform) or vertical, and stratiform clouds are classified by level. Of course clouds can occur at any level.

Clouds are further subdivided by degree of stratiform or vertical development. Stratiform clouds at mid- to upper levels have the suffix -stratus, those with some vertical structure have the suffix -cumulus.

Low Clouds

Fog

	Fog is merely a cloud at ground level

Stratus

	Low sheetlike clouds are called stratus.

Nimbostratus

Very low-hanging rain clouds are sometimes termed nimbostratus.

Stratocumulus

	Low sheet-like clouds with some vertical structure are stratocumulus.
	Stratocumulus from above.

Intermediate Clouds

Altostratus

Featureless blah skies like this are usually altostratus. The term "overcast" usually means altostratus if the weather is otherwise fair.

Altocumulus

	High overcast with a hint of puffy structure is altocumulus

High Clouds

Cirrus

	Wispy, fibrous-looking high ice clouds are cirrus.

Cirrostratus

	Cirrostratus is cirrus that is thick and continuous enough to form a solid sheet. It is usually thinner and less opaque than altostratus. Often they produce haloes around the sun and moon.
	Below: cirrostratus can give otherwise sunny days a cold, grim, and cheerless look.
	Left: A cirrostratus sheet with a sharp edge.

Cirrocumulus

	Cirrocumulus are high clouds with just a bit of puffy structure. They often form the rhythmic pattern nicknamed "mackerel sky" (like fish scales) by sailors.

Vertical Clouds

Cumulus

	These highly photogenic clouds are identifiable by their vertical structure.
	Fair-weather cumulus, with high cirrus above them.
	A very turbulent but non-violent sky. In autumn, the cool air near the ground is often much warmer than very cold air above, so that convection results in cumulus formation. It looks dramatic but seldom is associated with bad weather
	Hot air plus water vapor plus solar heat absorbed by dark smoke combine to trigger the formation of a cumulus cloud over a large forest fire. Sometimes called pyrocumulus.
	Cumulus clouds from above.

Cumulonimbus

	The most dramatic of clouds, distinguished by great vertical development and a tendency to flare out into an "anvil" at the top.
	The anvil head is high enough to form cirrus. This anvil head is from a storm that is already dissipating. Often the anvil head will be visible after the cumulonimbus itself has disintegrated.

Special Clouds

Contrails

Contrails (short for "condensation trails") are artificial clouds produced by aircraft. It's no accident that contrails tend to form when there are lots of cirrus clouds. Jet engines alone do not produce enough water vapor to result in long-lasting contrails.

Long lasting contrails form when the upper air is supersaturated in water vapor, so that condensation of jet exhaust triggers the condensation of yet more water vapor from the air. These conditions tend to occur ahead of fronts, when cirrus clouds are likely to be seen. In turn, contrails are significant contributors to cirrus cloud formation. People of a certain mind-set call these "chemtrails" and believe the planes are spraying chemicals. They are: water.

Lenticular

	Graceful lenticular (lens-shaped) clouds are formed when air rises smoothly, condenses in a lens-shaped region, then sinks and warms again.
Above and right: lenticular clouds are often associated with air flow over mountains.
	Lenticular clouds at sunset.
	Lenticular clouds can form at any level. They can be stratus, altostratus or cirrostratus. They often herald an approaching front. They need not be associated with mountains but can form whenever there are undulations in the air flow.

Mammatus

	Cold air sinking out of clouds can produce hanging bulges called mammatus.

Extremely High Clouds

Extremely high clouds are best seen at dusk, when they remain illuminated long after lower clouds are dark. These clouds are something of a mystery, since they are rare. Some are natural, others can be high-altitude aircraft or rocket contrails.

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Created 8 February, 1997
Last Update 23 January, 2001

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