From SKYbrary Wiki
The term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in colour from dark gray to nearly white. Stratus clouds may produce a light drizzle or snow. A "cloudy day" usually features a sky filled with stratus clouds obscuring the disk of the sun. These clouds are essentially above-ground fog formed either through the lifting of morning fog or when cold air moves at low altitudes over a region.