- Written by Nadia O’Carroll
The basic structure of convective vortices, such as waterspouts and tornadoes, is created by an intense low pressure, which causes an increase in wind speed, the winds then rotate into a vortex flow. Large-scale tornadoes are influenced by the Coriolis effect and rotate cyclonically ie in the Northern Hemisphere rotate counterclockwise and in the Southern Hemisphere rotate clockwise. However the rotational direction of smaller formations such as waterspouts are primarily effected by the ambient conditions.
Tornadoes are not common in Australia but they do occur, in 1971 three people were killed by a tornado at Kin Kin near Gympie and in 1992 the most intense tornado recorded in Australia - with a Fujita rating of F4 - occurred at Bucca, west of Bundaberg. Waterspouts are more common and are seen in tropical and subtropical oceans around Australia and were described by Sir Joseph Banks during Captain Cook’s 1770 journey of discovery on the Endeavour.
Waterspouts can occur over oceans, lakes and rivers, they appear as a funnel-shaped whirling column of air and water vapour extending between the water surface and cloudbase. The water vapour that forms this funnel cloud is a result of condensation, not the upward suction of water from the water surface.
Waterspouts develop in five discreet overlapping stages. Firstly there is a raised dark spot on the water surface which indicates contact of a complete funnel of air between the cloud system and the water's surface. Then a spiral pattern of light and dark bands of water rotate around the dark spot. When wind speed reaches around 65 kms/hour a spray vortex, or cascade, is formed, often rising several hundred metres into the air. Next, the fully visible waterspout reaches from the water surface to the cloud, at this point the waterspout reaches maturity and maximum intensity. The final stage is decay as rain begins to fall from the parent cloud and cools the supply of warm air feeding the waterspout
There are two types of waterspouts
The most common are fair weather (non tornadic) waterspouts - formations that are caused by the convection action of towering cumulus clouds that draw humid air upwards in a spiralling vortex. They are static or slow moving, dissipate quickly and usually exhibit wind speeds less than 70 kms/hour.
Less common but larger, more long lasting, severe and therefore more dangerous are tornadic waterspouts which are basically tornadoes over water. These form when the downdraft from a supercell (thunderstorm) drags a condensation funnel down from the base of the storm to the water surface.
Waterspouts are spectacular but best seen at a distance and should definitely be avoided at sea.