Australian Storm Spotters' Guide

5. Wall clouds, rotation and Tornadoes

As a thunderstorm becomes stronger and develops an organised inflow, its main updraught may begin to rotate slightly. This is sometimes seen as broad rotation of the cloud base beneath the main updraught or in the circular nature of the wall cloud. An example of a rotating cloud base is shown here (5.1) from near Adelaide. In most cases in the southern hemisphere, rotation will be in a clockwise direction when viewed from a distance. If however, you are looking directly upward, changing perspective will make this rotation appear anti-clockwise.

5.1 Photograph by R. Geytenbeek.

Weaker tornadoes are formed primarily by "tightening-up" of a rotating updraught. They occur as the storm intensifies to a maximum and are found right under the updraught core, sometimes without a significant wall cloud. Weaker tornadoes are most likely during mid-summer storms but may also accompany squall lines, mainly in southern parts of Australia. They are none-the-less significant events and may produce narrow strips of severe wind damage. This example (5.2) from Dimboola, Victoria occurred in December 1992.

5.2 Photograph by K. Reynolds.

Stronger tornadoes typically occur with late spring/early summer severe storms and have a more complex cause. It is speculated that at a certain stage in the storms life-cycle, a particularly intense updraught pulse partially blocks the prevailing wind aloft and deflects air down toward the surface. The downward surge interacts with the updraught to produce a tight rotational motion (in much the same way as rolling a pencil between your hands). This "spinning motion" is then tilted into an upright position and enhanced as it moves toward the ground as a tornado.

This photograph (5.3) of a strong tornado, accompanied by a well developed wall cloud, killed two people near Sandon in Victoria in 1976. The point of contact with the ground is marked by a cloud of dust and debris. The large "prong" attached to the left of the wall cloud later formed a second funnel.

5.3 Photograph by I. Kuiper.

The typical funnel shape of a tornado is formed when moist air condenses within the lower pressure of the rotating column of air. Under relatively dry conditions, a funnel cloud may not form and the presence of a tornado at the surface may only be indicated by a mass of debris. In this example (5.4) from Northam, WA, the tornado is made visible over pastoral land by a well defined dust funnel.

5.4 Photograph by J. May and C. Crane.

This interesting tornado (5.5) at Port Hedland, WA is made visible by an inner "inverted" dust funnel surrounded by an outer shell, most likely comprised of water vapour.

5.5 Photograph by P. Mudra.