stratus

noun

1. (uncountable, usually)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.

   “[page 99] Stratus. […] A widely extended, continuous, horizontal sheet, increasing from below. […] [page 357] When a stratus covers the ground at sun-rise, however, we often see it collect into cumuli upon the evaporation of that part of it which is immediately contiguous to the earth: […]”