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A Stratus cloud, coming from the Latin word meaning "spread out" 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 (abbreviated St) is used to describe flat, featureless clouds of low altitude (below 6500-8000 feet or 2400 meters) varying in colour from dark gray to nearly white. These clouds are essentially fog that is above ground level and are formed either through the lifting of morning fog or when cold air moves at low altitudes over a region. These clouds do not usually bring precipitation, although if sufficiently low in altitude to become fog, drizzle or mist may result.

Stratus formations that are accompanied by precipitation are known as nimbostratus. Stratus formations at higher altitudes include altostratus and cirrostratus.