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69 W/NE, 70 W/SE In 69, the back
end of an LP-type storm is a single cylindrical updraft column. In a way, the
whole base has become a lowering (note tail cloud lower right), but there are
signs that a smaller focus within it (seen as a marginally lower step below
the base) is the real lowering. Storms where the whole updraft base is lowered
are usually seen in drier regions. The opposite case (70) is a large, low, damp-looking
front-side lowering. It was briefly rotating, but with undercutting outflow
from the left (hence the angle) it soon lifted.
The wall cloud and rotation
Ordinary updrafts contain no rotation.As they become stronger and have an organized
inflow leading up to them, rotation may exist and this is evident in the circular
structure of a wall cloud. The term wall cloud has loosely become associated
with any lowering under a severe storm. The following features, when individually
present, suggest a true wall cloud that you should watch closely and report:
- a circular shape, indicating rotation
- a "tail" (a tail cloud)- a cloud extension or tube
pointing toward the rain curtain, indicating a strong drawing-in of the moist
outflow air
- laminar bands above, where it joins the main cloud
base, indicating the presence of a rotating updraft (mesocyclone)
- a tiered structure, stepping down in stages below
the main base also indicating rotation
- prongs, or lower portions on the left and right edges,
the result of strong rotation (the bathtub drain effect)
- a shadowy curl below it (a rain foot) from the edge
of the rain curtain curving up into it, indicating a controlled inflow despite
very strong outflow there (the sharp outflow curves bruptly upward into the
updraft).
A wall cloud can form from nothing in less than ten minutes! Initially, the
rain-free base has nothing under it. Then, you will suddenly see hunks of scud
forming in mid-air under the base, or see tufts of lower cloud extending from
the base. More scud forms, rises, then joins the base to form a lowering. The
lowering takes a more solid shape, rounds out, and begins to rotate as a wall
cloud.
The circular structure implies rotation, but this is not caused by a tornado.
The circular motion is part of the spiral updraft column, the mesocyclone, within
the storm. Its rate of rotation (as seen by you) will depend on how large and
low the wall cloud is, but averages about one full revolution every minute or
two. That may appear slow to you, but up there the cloud is actually
moving around at 100-150km/hr! Many severe storms exhibit this motion for short
periods or even an extended phase without forming funnels or tornadoes. However,
if this motion increases in speed or seems to have a single, rotating
hot-spot within the wall cloud, a funnel may soon form and a tornado can follow.
(The possibilities are discussed in the section on tornadoes, page 33 .)
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Created :
2002-08-26
Modified :
2002-12-19
Reviewed :
2002-12-19
Url of this page : http://www.msc.ec.gc.ca /education/severe_weather/page28_e.cfm
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