Scud clouds, a type of fractus cloud, are low, detached, irregular clouds found beneath cumulonimbus clouds. These clouds are often ragged or wispy in appearance. 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.
Scud clouds are formed as the cooler (and often more moist) downdraft of a thunderstorm lifts the relatively warm air near the surface. These clouds condense as the warm, moist air saturates through ascent and is pushed outward from the storm. Scud clouds are very commonly found on the leading edge of a storm front. In this area of a storm, scud are commonly associated with shelf clouds.
Scud clouds may also form when an updraft ingests precipitation-cooled air from the updraft. Scud forming in this region of the storm, if moving laterally, will tend to move inward towards the dominant updraft. Rising scud my condense and organize into a wall cloud.
