Australian Storm Spotters' Guide
6. Other rotating structures and features
A waterspout looks like a slender tornado, but occurs over water. They are occasionally seen near the coast in the late summer and autumn when cool, unstable air passes over warmer waters. Nearby topography and other effects, allows local concentration (or convergence) of the airflow, which results in vigorous updraughts "tightening up" into spinning columns. This example (6.1) shows twin waterspouts over the warm waters of the Gulf of Carpentaria.
A similar phenomenon over land is the landspout, seen here (6.2) below a large cumulus cloud near Cleve, South Australia. Note the lack of an anvil on the top of the cloud and the absence of any wall cloud. The mechanism which forms a landspout is similar to the waterspout except in this case, relatively cool air passes over hot ground.
Whenever air rises rapidly at the edge of cool moist outflow, lower cloud fragments, known as scud, will form. Many low, dark patches of scud can be seen in the middle of this photograph (6.3) below a ragged shelf cloud. The rain curtain behind makes the scene appear even more ominous. Scud can best be distinguished from funnel clouds or tornadoes because it is less smooth-edged, more transient and rising (or moving in a straight line) rather than rotating.
This "tornado-like" structure (6.4) was seen on the leading edge of a wind squall, located just ahead of a ragged shelf cloud. The feature lasted for several minutes but was not rotating and was most likely just an unusual feature of the shelf cloud itself.
6.4 Photograph by A.Treloar.
Another common deception is provided by distant shafts of falling precipitation, especially when silhouetted as here (6.5). Virga, or rain which evaporates before reaching the ground, often looks like dark, tapered extensions below a cloud base and thus, like a tornado. A closer inspection, however, shows they are diffuse and soft-edged.
Another ominous-looking cloud feature, is mamma, or mammatus clouds. These rounded pouches or bulges protruding from an anvil base are shown here (6.6) in a good example from Brisbane. Mammatus have long been associated with severe weather, but their presence merely indicates descending pockets of small droplets or ice crystals from an anvil surface. In fact sometimes they can be observed below middle level cloud sheets.