The Altostratus reference article from the English Wikipedia on 24-Apr-2004
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Altostratus

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Altostratus is a cloud belonging to a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. Compare also with lower altitude stratus clouds.

    
    
    
    
    
Photo of Altostratus
Where found: 2,000-5,000 m
Asscociated Weather: Wide areas of rain and snow.

Cause

Altostratus is caused by a large air mass that is lifted then condensed, usually by an incoming frontal system.

Hazards

Altostratus can cause ice accretion on aircraft.

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