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

Table of contents
1 Cause
2 Hazards
3 External link

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.

External link