Severe Weather Watcher Handbook

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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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