Complex sentence: 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.
Split into simpler sentences: 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. It is lighter in color than nimbostratus and darker than high cirrostratus.