CB cell (fig. 1-15). It may extend along the entire outer edge of the base of the CB cell, or may angle out slightly ahead of the CB base with the top of the outflow boundary. Although with weaker gust fronts, the roll cloud may appear rough, ragged, or bumpy, under certain conditions the roll cloud may appear very smooth.   Roll   clouds   indicate   that   thunderstorm downrush has occurred and that LLWS may be present. The turbulent action along the cold air outflow boundary may produce small-scale vortices on the ragged base of the roll cloud.  These  vortices  sometimes take on the appearance of small, ragged funnel clouds. The public commonly mistakes these vortices for funnel clouds and occasionally reports them as funnel clouds. These vortices are known as cold-air funnels. Rarely, a cold-air funnel will develop sufficiently to reach the surface.  It does not have the strength of a true funnel cloud or tornado, and is about as powerful as a strong dust devil. By itself, it may be able to pick up objects, such as trash cans or to shake a small camp trailer.  Usually,  the  damage  associated  with  sightings of cold-air funnels is caused by the much more powerful straight-line winds in and behind the outflow boundary (gust front) that produced the cold-air funnel. Another phenomenon associated with the outflow boundary is a dust cloud on or near the earth’s surface. This phenomenon is frequent in desert areas and is fairly common in other areas after a dry spell. The dust often appears to be rolling outward and upward from the ground as it moves over an observer. It is associated with the first gust of a thunderstorm’s gust front. A  wall  cloud,  a  sometimes  hollow,  generally circular patch of cloud with a ragged bottom edge, may lower from the base of a CB cell. A wall cloud is usually much smaller than the base of the CB cell, and will usually form in the right rear quadrant of the CB cell with respect to the CB's movement. When viewed from the side of the CB, the wall cloud usually is under the rear portion of the cell, where the billowy cloud tops appear to be tapering from the anvil top downward toward the rear portion of the cell. A slowly rotating or spinning wall cloud is an indication of a very strong thunderstorm   and   impending   funnel-cloud development. Funnel clouds (tuba) usually form on the edge of the wall cloud or near the wall cloud at the rear of the storm. In the early stages of development, the funnel cloud may be only a rotating rounded bulge extending Figure 1-15.—Roll cloud formation on cumulonimbus. 1-15


 


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