Altostratus is a middle altitude cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-gray sheet or layer, lighter in color than nimbostratus and darker than high cirrostratus.
The sun can be seen through thin altostratus, but thicker layers can be quite opaque.
Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud. It can produce light precipitation, often in the form of virga. If the precipitation increases in persistence and intensity, the altostratus cloud may thicken into nimbostratus.