Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds formed by rising thermals. The term stratus describes flat, hazy, featureless clouds at low altitudes varying in color from dark gray to nearly white. The word stratus is derived from the prefix Strato- meaning 'layer'. Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes. Some call these clouds "high fog" for their fog-like form.
Formation
Stratus clouds form when weak vertical currents lift a layer of air off the ground and it depressurizes, following the lapse rate. This causes the relative humidity to increase due to the adiabatic cooling. This occurs in environments where atmospheric stability is abundant.
Description
Stratus clouds look like featureless gray to white sheets of cloud. or as a striated sheet. They are sometimes similar to altostratus and are distinguishable from the latter because the Sun or Moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent. Cirrostratus come in two species, fibratus and nebulosus. Elevations that have temperatures below freezing cause water droplets to freeze and turn into ice crystals. The height of the cloud typically reaches between . They form when warm, moist air rises, cools down, and condenses into water droplets or ice crystals at a low altitude, which forms the dark gray cloud layer. They are often a sign of a warm or occluded front approaching.
Stratocumulus clouds
thumb|Stratocumulus cloud
A stratocumulus cloud is another type of a cumuliform or stratiform cloud. Like stratus clouds, they form at low levels;