To properly identify tougher cases, watch for a minute or so to look for rotation, both in the cloud and in the debris near the ground. Talk with your colleague. Scud clouds are small, detached, wind-torn clouds that often form near thunderstorms. Scud clouds can change shape rapidly and sometimes take on the appearance of a wall cloud or funnel cloud. Remember, though, that wall clouds are attached to the cloud base and funnel clouds always rotate.
Roll clouds forma long the gust front as the cool outflow lifts warm, moist air.
They take the shape of horizontal tubes, and in some cases, you can actually see a horizontal rolling motion. You probably will see considerable motion in the shelf cloud's base. The motion will be turbulent in nature, though, without the persistent, organized rotation seen in true wall clouds. Shelf clouds are more common than roll clouds and can define areas of strong downburst winds but usually are not favorable locations for tornado development. Us Copyright 1998 Topeka Capital-Journal.