Stratus cloud

Formation and occurrence

Stratus develops when a shallow layer of moist air is cooled or gently lifted so that condensation occurs throughout a broad horizontal layer rather than in discrete towering elements. Typical triggers include nocturnal radiational cooling, gentle overrunning of warm air above a cooler surface, and the formation of a marine layer over cool sea surfaces. These clouds are especially common in stable air masses and along some coastlines. Regional and marine behavior are discussed in many field guides; see cloud type guide for practical notes.

Variants and precipitation

Pure stratus usually produces little or no precipitation, though it commonly generates drizzle, mist, or light snow in cold conditions. When precipitation is steadier or more persistent from a widespread, low layer, the cloud type is classified as nimbostratus (nimbus = rain). Photographs and comparisons that illustrate drizzling stratus and nimbostratus can be found in image collections such as image gallery. Observers should note the transition from fine drizzle to steadier rain as a sign of changing cloud type.

Visibility, light and impacts

Because stratus forms a low ceiling, it commonly reduces visibility and diffuses daylight, often producing dim, flat lighting conditions. Daily temperature ranges may be smaller under persistent stratus, and mornings can remain cool until the layer breaks up. For coastal areas, the presence of stratus or a persistent marine deck influences local fog, visibility, and near-surface temperature profiles; see discussions in marine and coastal guides.

Importance for aviation and forecasting

Low stratus layers can create instrument meteorological conditions for aviation by producing low ceilings and reduced visibility. Forecasting attention focuses on layer depth, coverage, and whether the cloud will produce drizzle or thicken into nimbostratus. Guidance and technical classification materials are available from meteorological resources such as weather resources and classification manuals at detailed cloud classifications.

Distinguishing stratus from other clouds

• Stratus vs stratocumulus: stratus is more uniform and lacks the rounded rolls or cellular pattern of stratocumulus.
• Stratus vs altostratus: altostratus is a mid-level, thinner layer that often allows the sun to be seen as a bright spot; stratus is lower and typically opaque.
• Fog and ground-level stratus: when the base reaches the surface it is called fog; elevation and visibility measurements help differentiate them.