Stratus is the cloud type that forms a broad, uniform gray layer across the sky.
Stratus clouds develop when moist air near the surface is lifted gently or cooled until condensation occurs. Instead of growing into tall towers, the cloud spreads horizontally in a relatively flat sheet. It may cover hills and produce misty, gloomy conditions.
Stratus can bring light drizzle, snow grains, or very light snow, but it is not normally associated with the violent showers and lightning produced by cumulonimbus clouds. When a similar layer occurs higher in the atmosphere, it is classified as altostratus or cirrostratus rather than stratus.
The word stratus comes from Latin for spread out or layer. Fog is essentially stratus at ground level, though operational definitions distinguish cloud touching the ground from cloud above it.