Unravelling the mystery of fire whirls, the common phenomena that are hard to spot
Ben Deacon and Kate Doyle, Wed 11 Oct 2017
Footage of spectacular fire whirls are all over the internet, and last month in Western Australia. But what exactly are they?
We asked Andrew Sullivan, a senior fire researcher from the CSIRO, to give us a bit more information about the mysterious phenomena.
Dr Sullivan was quick to down play the hype.
"Fire whirls are very common," he said.
But he did admit they were rather spectacular.
What are fire whirls?
"You know them as dust devils," Dr Sullivan said.
"You see them quite frequently. Even if you stand in a car park on a hot day you will see a dust devil go by.
"It's the same physical mechanism. It's just that in this instance it is travelling over a fire ground and picking up flame and ash and debris."
Dr Sullivan explained that as a dust devil travelled over burning ground, the flames got caught up in the fire whirl and could be carried to great heights.
"They look quite spectacular and they can be quite big," he said.
"There is footage from a fire in the US where it is in the order of 40 or 50 metres."
What causes fire whirls?
Fires can induce vortices because there is a lot of heating that goes on, but fire whirls are not solely produced by fire.
"Say you've got a hill. The sun is hitting the sunward side of the hill and heats up the hill," Dr Sullivan said.
"The side of the hill that's facing away from the sun is not so hot, so you get hot air rising on the sunward side and, because it's next to cooler air that's not moving so quick, the fluid dynamics induce a vortex."
Basically, when there are patches of air moving at different speeds, they can cause the shear and spin needed for a dust devil or fire whirl.
But hill slopes are just one example. Vortices can form anywhere there is the right shear, and they often occur on flat ground.
When do we see fire whirls?
Dr Sullivan said fire whirls were seen more commonly in grass fires because there was nothing in the way to impede the view of them.
"You need to be able to see the fire ground," he said.
"Even though they do occur in forest fires, because of the trees you don't get to see them very often."
Are they dangerous?
"There is evidence for very, very large vortices that actually do damage. The fires that came into Canberra  were associated with a very large, tornado-like structure," Dr Sullivan said.
"They can get very big and they can be very destructive."
Dr Sullivan said fire whirls could also be dangerous during prescribed burns.
"If you do get a fire whirl, what it can do is transport burning material outside the area they want to burn," he said.
"[The embers] get lifted up in the vortex and dropped somewhere else."
Dr Sullivan said sometimes the fire whirl itself could move outside the burn area, taking flames with it.
But most fire whirls were short lived.
"Most fire whirls that you see don't last very long. Five-second wonders and they are gone," Dr Sullivan said.
"Certainly one that is long enough for you to get your phone out and take a video of is probably worth getting video of."
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