Storm Spotters' Handbook

Don't Be Deceived

False Funnels

There are several tornado/funnel look-alikes and other ominous thunderstorm features which can easily deceive us at first glance. Disqualifying such 'false funnels' is very important for minimising erroneous reports.

Scud and Inflow Tails

Whenever air rises rapidly at the edge of cool moist outflow, lower cloud fragments, known as scud, will form in mid-air or under the base in the shape of small points or lumps and protruding downward. These change shape constantly and do occasionally take the form of a tapered cloud that looks like a funnel. They are most common along the gust front, near ragged lowerings, or along any cool-warm boundary. They are best differentiated from true funnels by location, since a true wall cloud is absent or found elsewhere in the sky. However, they are also less smooth-edged, more transient, more ragged or fragmented, and rising rather than rotating.

The edges of wall clouds or lowerings also assume pointed shapes briefly but without classic funnel features. When such a lowered extension forms on the rainy side of the wall cloud, it may indicate overall intensification and should be watched closely.

Virga and Rain Shafts

Another common deception is provided by distant shafts of falling precipitation, especially when silhouetted. Virga, or rain which evaporates before reaching the ground (Photo 15), often looks like dark, tapered extensions below a cloud base and thus, like a funnel cloud. A quick look around the sky will show the absence of an organised thunderstorm or lowering and closer inspection of these tufts will show they are diffuse and soft-edged.

Virga in offshore storm

Photo 15. Virga in offshore storm to the right of the image. Seen from
Cottlesloe Beach, Western Australia. Photograph courtesy of John Terni.


When Looks are Deceiving

False Rotation

A turbulent, stormy sky is full of different air motions and some of them will swirl in a circular pattern without being true, tornadic rotation. Sometimes the swirl is apparent from bits of scud moving around; at other times, the cloud base will have a circular spot or hole present. These are quite common under and behind the shelf cloud or gust front when descending air currents rotate slightly. They can also be seen near or under the updraught region but will be isolated and not accompanied by any lowering, funnel, or other expected signs.

Mammatus Cloud

Another ominous-looking cloud feature is mammatus (Photo 16). These are rounded pouches or bulges protruding from an anvil base. They are dramatic, beautiful adornments, especially when side-lit. They have long been associated with severe weather but their presence merely indicates descending pockets of small droplets or ice crystals from an anvil surface. Even if the thunderstorm is severe the mammatus pose no threat at all. In fact sometimes they can be observed below middle-level cloud sheets.

Mammatus cloud

Photo 16. Mammatus cloud over Melbourne Docklands, Victoria.
Photograph courtesy of Mike Rosel.


Cloud Colourings

Thunderstorm cloud can sometimes display a greenish colour near the rain core but this is not related to tornadoes, although some observers have related it to hail. It is thought that the colour may occur due to the absorption of sunlight as it passes through a dense cloud containing very large amounts of suspended water which may, or may not, contain hail. The sun being low in the sky is thought to assist in generating the colour.

Rising Dust and Dust Devils

Any strong wind gust will lift a cloud of dirt into the air and turbulent motions will mix it upward. If the gust is a microburst or violent wind squall, the dirt will be concentrated and/or have a sharply defined forward edge. A tornado debris cloud will move more slowly, remain compact and symmetrical, and soon take the shape of a column. Without this discrete structure under the appropriate part of the thunderstorm, the rising dust is more likely due to strong straight-line winds. The one exception is a dust devil, a column of whirling dust that looks like a small tornado but occurs exclusively on fine, dry days and is usually harmless, but can sometimes cause minor structural damage to property.

Dust devil

Photo 23. Dust devil, Wirrabilla Farms, Birchip, Victoria, 12 March 2006.
Photograph courtesy of John Ferrier.


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