Severe Weather Watcher Handbook

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96-97  SW/E  A wide view of this LP supercell shows the entire storm - from the high-based compact core to the flaring anvil above. Rain and hail are falling overhead and to the right, but are not heavy enough to block the light. There were rumbles of thunder but not a single bolt! A slender F0-F1 tornado (see closeup in 97) lifted and reformed repeatedly for 20 minutes but never had any signs of a wall cloud above it. Although the anvil was streaming by above, the main updraft remained almost in the same location for an hour due to extreme regeneration at the back of the storm.

A significant distinction must be made between a tornado as a visible funnel extending to the ground and the associated violently rotating column of air. When we see a tornado we are usually looking at its condensation funnel, a cloud condensed in the very low pressure within the vortex. However, many tornadoes have short or incomplete condensation funnels and some none at all. This can fool us into thinking the tornado hasn't reached the surface. A tornado can be doing major damage (often made visible by the debris cloud at the ground) with few visible signs of its presence in the sky above.

Stages in tornado development

Most tornadoes and funnel clouds will be preceded by a rotating wall cloud. You can see the rotation by watching the bits of cloud along the edge of the lowering moving slowly around it. The motion is usually clockwise when looking up.

If the entire wall cloud rotates faster and lowers further,a large,wide tornado can form without a typical funnel. However, most cases will have a funnel cloud form under the wall cloud, usually between the centre and south edge of it. The funnel will be smooth-edged and tapered to a point. Some larger funnel clouds may appear as a rounded bulb or diffuse V-shaped cone.

If the funnel is wide or close to the ground it is likely already a tornado and you should check carefully for debris. Thinner funnel clouds may hang in the air for several minutes, then disappear again. If they lengthen downward it will only be a minute or two before ground contact is made. But it must be stressed that a tornado can exist without a long funnel or any signs of rotation in the air. Indeed, often the first visible sign of a tornado touchdown is a cloud of debris at the ground.

98-99  NW/E  The SW back end of this classic supercell has a small, circular wall cloud and F2 tornado. The funnel is a mere rounded bulbous extension, but the vortex is obvious from the large debris cloud being stirred up. After 5 minutes (99), the tornado has lifted (no more debris visible) even though the condensation funnel is now much longer.

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