A stratus cloud (St) is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide (these are termed cumulus clouds). More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color 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. Some call these clouds "High fog" for the fog like cloud. For instance, this cloud in particular may mean not much activity, but rain might fall.

A stratus cloud can form from stratocumulus spreading out under an inversion indicating a continuation of prolonged cloudy weather with drizzle for several hours and then an improvement as it breaks into stratocumulus. Stratus clouds can persist for days in anticyclone conditions. It is common for a stratus to form on a weak warm front, rather than the usual nimbostratus. If it is seen after rain it should clear due to warm front succession, but if this does not occur the stratus must be part of the warm sector of a frontal system; heavy rain may start again with the arrival of a cold front after several hours of stratus.