a large dark low cloud
"The sky turned an oppressive gray as the thick stratus clouds rolled in, promising rain by afternoon."
A principal, low-level cloud type in the form of a gray layer with a rather uniform base, usually not associated with precipitation, and capable of producing corona phenomena and a weak, uniform luminance; abbreviated St.
"The morning sky was covered by a thick blanket of stratus clouds that created a soft, even glow without any rain falling from them."