Nimbostratus pannus

In weather lore and popular culture, pannus clouds—often called "scud"—are associated with gloomy, stormy weather. Their ragged, dark appearance beneath rain clouds evokes imagery of shreds or tattered fabric, symbolizing unsettled skies and advancing precipitation in art and storytelling.

• Pannus clouds form rapidly and dissipate within minutes to hours, reflecting changes in precipitation intensity and turbulence.
• They are globally distributed wherever nimbostratus precipitation occurs, especially common in mid-latitude cyclones and frontal systems.
• Despite their dramatic appearance, pannus clouds themselves do not produce precipitation.
• They are often referred to as "scud" in North America and the UK, a term used more broadly for ragged low clouds beneath rain clouds.
• In satellite imagery, pannus is generally indistinguishable from the main nimbostratus cloud due to its low altitude and lack of thermal contrast.

🌦️ Nimbostratus pannus forms as precipitation from the overlying nimbostratus cloud descends through lower layers of moist, turbulent air. Mechanical turbulence and localized updrafts in this layer cause condensation of water vapor into irregular, ragged cloud fragments. These fragments continuously evolve, merging and separating in response to shifting winds and precipitation intensity, resulting in the characteristic shredded appearance beneath the main cloud deck.

🗺️ Nimbostratus pannus occurs at low altitudes, typically from the surface up to about 2,000 meters, always beneath the parent nimbostratus cloud. It is found worldwide wherever nimbostratus clouds produce precipitation, most commonly in mid-latitude cyclones and frontal weather systems.

⚠️ While pannus clouds themselves do not produce precipitation, their presence signals ongoing rain or snow and turbulent low-level air. This can lead to reduced visibility, low cloud ceilings, and obscured terrain, posing hazards for aviation, especially during approach and landing phases. Observers should be cautious of the associated gloomy and turbulent weather conditions.

🔍 Nimbostratus pannus is primarily identified through visual ground observation, noted for its ragged, fast-moving fragments beneath nimbostratus. Ceilometers and cloud base recorders can detect pannus as a distinct low cloud layer. Satellite sensors generally cannot resolve pannus separately due to its low altitude and lack of thermal contrast.

🛰️ In satellite imagery, nimbostratus pannus is not distinctly visible from the main nimbostratus cloud deck because of its low altitude and minimal thermal contrast. It may contribute to a diffuse, textured appearance in visible-light images but lacks unique infrared signatures, making it challenging to isolate in remote sensing data.