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
