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Pannus (from the Greek word meaning "shred"), or scud clouds, is a type of fractus cloud at low height above ground, detached, and of irregular form. It is technically called an accessory cloud because it is always found beneath another cloud, such as nimbostratus or cumulonimbus clouds. These clouds are often ragged or wispy in appearance.
When occurring under nimbostratus, pannus clouds are typically associated with precipitation, appear darker than the cloud layer above them, and often move quickly.
For cumulonimbus, when caught in the outflow (downdraft) beneath a thunderstorm, scud clouds will often move faster than the storm clouds themselves. When in an inflow (updraft) area, scud clouds tend to rise and may exhibit lateral movement ranging from very little to substantial.
In convective situations, the updraft of a thunderstorm pulls in warm, moist air from near the surface. As the air rises and cools, the moisture in the air condenses into precipitation which falls towards the front of the storm with the forward flank downdraft. Pannus or scud clouds are very commonly found on the leading edge of the forward flank gust front. In this area of a storm, scud are commonly associated with shelf clouds.
Pannus clouds may also form when an updraft ingests precipitation-cooled air from the rear flank downdraft. Scud forming in this region of the storm, if moving laterally, will tend to move inward towards the dominant updraft. Rising scud may condense and organize into a wall cloud.
Pannus clouds can often be mistaken for a developing tornado, landspout, or waterspout. The difference is determinable by observing the presence or absence of rotation (not just movement) of the scud clouds. If rotation is present, then a tornado, landspout, or waterspout is possible, and the more intense the rotation, the more likely.