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Pannus, or scud clouds, is a type of fractus cloud at low height above ground, detached, and of irregular form, found beneath nimbostratus, cumulonimbus, altostratus and cumulus clouds. These clouds are often ragged or wispy in appearance. When caught in the outflow (Vertical draft) beneath a thunderstorm, scud clouds will often move faster than the storm clouds themselves. If the parent cloud that scud clouds pair with were to suddenly dissipate, the pannus cloud accessory would not be able to be told apart from a fractus cloud formation.
When in an inflow (Vertical draft) area, scud clouds tend to rise and may exhibit lateral movement ranging from very little to substantial.
Pannus clouds may also form when an updraft ingests precipitation-cooled air from the 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. The key to differentiating between a scud cloud, wall cloud, or funnel cloud is to find signs of rotation or to determine if they are attached to the base of the thunderstorm.
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.
Newnham, E. V. (1929). METEOROLOGY. Science Progress in the Twentieth Century (1919-1933), 23(92), 608–613.
Timothy. (n.d.). Scud Clouds - Weather - Creepy Clouds That Form Under Thunderstorms • Tim’s Weird & Wonderful World. Retrieved November 8, 2021, from https://timswww.com.au/scud-clouds-weather-creepy-clouds-that-form-under-thunderstorms/
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