Scud is a general term for low, ragged cloud fragments that form beneath or near a storm base in humid air, often drifting or rising toward the parent cloud but not organically connected to any rotating feature. In cloud classification it is called fractus, and near storms it takes the form of stratus fractus or cumulus fractus.
How it forms
Scud develops in a shallow layer of very humid, cool air near the ground. Rain evaporating out of a storm's precipitation shaft moistens and slightly cools the air beneath the base. Small parcels of that saturated air are stirred upward by turbulence or by the storm's inflow, and they condense into ragged, low-hanging fragments that sit well below the main cloud base.
In stronger inflow the fragments can be drawn upward toward the storm base and merge with it, giving a false impression of a rapidly developing lowering. In weaker conditions they simply drift, dissipate, and reform in the same general area.
How spotters interpret it
Distinguishing scud from a wall cloud is one of the most useful skills for a field spotter. A wall cloud is attached to the rain-free base under the updraft, is broadly organized, and often shows persistent rotation. Scud is smaller, ragged, patchy, and its motion is chaotic rather than organized. If you can see clear sky between the fragment and the actual cloud base, or if the fragment appears and disappears in seconds, it is almost certainly scud.
Scud that gets sucked upward into a wall cloud is not itself a tornado warning sign. What matters is whether the wall cloud above is rotating and persistent.
Where it misleads
Scud has caused countless false funnel reports because a ragged fragment viewed from the side can look like a tapered lowering. Waiting a full minute before calling anything in and looking for organized, sustained rotation prevents most of these errors.
Under a shelf cloud, scud is also common along the leading edge as warm moist air is lifted and rapidly saturates. This is a normal part of outflow structure and is not by itself a tornado indicator.