3 key factors that cause 'coldies', or winter tornadoes
WHEN the 2010 tornado hit Lennox Head, it was June, and perhaps not the usual time of year to expect such a weather event.
But according to the Bureau of Meteorology, some parts of Australia -- usually southern areas -- experience tornadoes all year round.
"In the cooler months, the weather in southern Australia is characterised by bursts of wind and rain from cold fronts sweeping up from the Southern Ocean, followed by periods of relatively calm weather under the influence of high-pressure systems," BoM explains on its website.
"Occasionally, these cold fronts will bring short bursts of intense and violent weather in the form of tornadoes.
"These tornadoes generally occur between the months of May and September, and affect many parts of southern Australia.
"Known by meteorologists as 'coldies' these tornadoes live by the creed 'live fast and die young'.
"Unlike the tornadoes of the United States which can exist for hours, travel hundreds of kilometres and occur mostly during the warmer months, coldies form and collapse in the space of about 10-30 minutes.
"They consist of rotating columns of air that move across the ground at 50-80 km/h, with typical damage paths being anything from less than a kilometre to several kilometres long and very narrow.
"Often they are only 20-50 m wide, but sometimes can be 100-150m wide."
BoM says the localised and short-lived nature of the systems makes them difficult to predict.
They can even be difficult to identify on radar, due to their small size.
"The ingredients for a cool season tornado to form can be found within the broad air movements associated with a cold front, but things need to be just right for one to form," BoM explains.
"The three key factors are:
"Low-level wind shear (the change of wind speed and/or wind direction between the ground and around 1 km above the earth's surface);
"Capacity of the atmosphere to support deep vertical development of cloud; and
"Alifting mechanism, usually a cold front, to initiate cloud development."
If all of these conditions exist at the same time in the same place, then there is a chance that a cool-season tornado will form.