ALTOSTRATUS

n. medium-altitude cloud formation consisting of a gray horizontal layer with a relatively flat bottom

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. You can usually see the sun shining through them, and frequently cover the whole sky. Compare also with lower altitude stratus clouds.
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Noun
1. a stratus cloud at an intermediate altitude of 2 or 3 miles
(synonym) altostratus cloud
(hypernym) stratus, stratus cloud

adj. alto cumulus

altostratus