Altocumulus undulatus cloud
Altocumulus undulatus clouds
Abbreviation	Ac
Symbol
Genus	Alto- (high) -cumulus (heap)
Species	undulatus (wavy)
Altitude	2,400 - 6,100 m (8,000 - 20,000 ft)
Classification	Family B (Medium-level)
Appearance	undulated
Precipitation Cloud?	No
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The altocumulus undulatus is a mid-level cloud (about 8000 - 20,000 ft or 2400 - 6100 m), usually white or grey with layers or patches containing undulations that resemble "waves" or "ripples" in water. Elements within the cloud (such as the edges of the undulations) are generally darker than those in cirrocumuli and smaller than those in stratocumuli. These clouds may appear both as patches or as covering the sky. The layer of these clouds is generally under about 300 feet (91.44 meters) thick.

As with other altocumuli, the undulatus variety can form in all seasons, announcing an approaching system within the general area (about 100 - 200 mi or 160 - 322 km). They result from wind shear (an abrupt speed or directional shifting in the wind, acutely changing with height). Lines one might see indicate the direction of the shear. In the southern United States, these formations may be the result of tropical or subtropical system outflow in a northern direction. They may also come from a southwestern monsoon making its way across the Western and Plains States.

Altocumulus undulatus clouds at dawn.

See also

• Weather
• Atmosphere
• Cloud types

External links

• American Meteorological Society's Glossary of Meteorology
• Stormeyes, a website for storm watchers (and storm chasers)
• Weather Photography
• Airline Pilots site, an Internet resource for airline pilots
• Uploaded photo and explanation
• Australian Cloud Atlas
• From Atmospheric Science Department at University of Wyoming