Stratus is a low, uniform gray cloud layer that can produce drizzle.
The word stratus comes from Latin for “layer,” reflecting the cloud’s broad, sheet-like appearance. Stratus often forms when moist air near the ground is lifted gently or cooled to saturation. It can cover much of the sky without the towering structure associated with thunderstorms.
The cloud base is commonly low enough to create gloomy conditions and reduced visibility. Light precipitation from stratus is usually drizzle, although thicker associated cloud layers can produce more persistent rain or snow. Fog is essentially stratus at or very near the ground, so the distinction depends on whether the cloud base is above the observer.
Stratus differs from stratocumulus, which has more visible lumps or rolls and usually larger breaks. It also differs from nimbostratus, a thicker precipitation cloud that can bring widespread, steadier rain or snow.