Fire tornadoes: a rare weather phenomenon
'Firenadoes' are rare weather events that occur when a unique set of conditions come together.
LIKE PHENOMENA OUT OF a fantasy film, fire whirls are mysterious and highly dangerous tornadoes.
Known as fire devils, fire tornadoes and even 'firenados', they can come in different sizes and intensities, and are formed in different ways depending on environmental conditions.
Most commonly, fire whirls occur when hot, strong winds, often whipping through trees, come into contact with already raging bushfires. Updraughts of hot air catch the fire and surrounding winds send it whirling into the air, sucking up debris and flammable gases.
CSIRO fire researcher Dr Andrew Sullivan says that in order for a vortex to form, a 'shear' must be present, which is a flow (for example a prevailing fire or wind) coming perpendicular to another.
"If there is already a shear present, whirls will form regardless of the size of the fire. If there isn't a shear present, then larger fires will tend to induce them," Andrew says. "Depending on the intensity of the fire, you can get updrafts. This forms a shear layer moving in two directions that induces a vortex."
A fire tornado in Alice Springs in September 2012. (Credit: Chris Tangey)
Fire tornadoes: a rare but spectacular phenomenon
These fire vortices can be seen at fronts of bushfires, often forming around hills, where the lee air is relatively still, and shear occurs. Although not uncommon, whirls rarely last more than a few minutes, and are usually between 2m and 10m tall.
Chris Tangey, manager at Alice Springs Film and TV, was location scouting on 11 September 2012 when he was lucky enough to witness a desert bushfire give rise to towering fire whirls.
"There were big red pillars and big black pillars," he says. "They'd pick up the fire and become a tower of fire."
Whirls of the magnitude Chris witnessed are indeed rarely seen, indicating the intense heat that must have been generated by the fire, which had been burning for about a week before he filmed the whirls. He said there were multiple large whirls, lasting for over 40 minutes.
"They pull up whatever's around, so sometimes they'd be fire, ash, smoke or sand. They were like shape shifters," Chris says. "One would fade out and another would start."
Fire tornadoes and open terrain
Chris's observations mirror those of Californian fire fighter Royal Burnett's 2008 report on fire whirls.
"Fire whirls and dust devils seem to form more often at low wind speeds in open terrain," wrote Royal. "It's common to see dust devils and small fire whirls form on recently burned ground. The heat from the fresh burn plus the added solar heating makes for low-level instability and any errant gust of wind can produce a whirl."
Although Chris maintains there was no wind present, fire researcher Andrew suggests that, "in this case, the shear could have formed between the updraught above the fire and the relatively still air around it".
He adds that whirls "don't need a lot of wind, but there is a horizontal vortex also present, and it often can't be seen".
The general consensus is that drier fuel leads to more rapid combustion and energy release. This explains why, as Royal Burnett also observed, whirls are more common in the desert or with drought conditions.
"An unusual event needs an unusual reason," he says. "The key is the intensity of the heat. They weren't whirly-whirlys that got caught up in the fire; they were created by the fire."
Fire tornadoes form when one flow is perpendicular to another and creates a vortex of heat.
(Credit: Chris Tangey)
Fire tornadoes still not understood
During his research, Andrew has witnessed many fire whirls, but has found difficulty in figuring out the cause of each event, not to mention how the fire whirls behave once formed.
"The question is always 'Did the fire cause or augment the vortex?'" he says.
Whirls commonly arise during flash burns (a sudden, intense fire), and they are quite powerful says Andrew. He has seen dry logs and debris being thrown substantial distances by fire whirls.
"The key aim for flash burn operators is to not allow enough heat to be generated for the fire whirls to form," he says.
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