The royal commission that is investigating the Black Saturday fires of February 7 in Australia was told by wildfire behavior expert Dr. Kevin Tolhurst that spot fires occurred a record 35 kilometers (21 miles) ahead of the main fire. Dr. Tolhurst also had some other interesting observations about fire behavior during the fires. Two examples are spot fires occurring off the flanks of a fire, and researchers studying fire behavior of small fires and extrapolating that to assume fire behavior on large fires would be similar. The latter was an issue during the 1988 Yellowstone fires when crown fire behavior did not match the existing prediction models.
Here is an excerpt from The Australian:
Dr Tolhurst said researchers had developed “warped ideas” about bushfire behaviour because they had studied comparatively small fires. This had flowed through to training of fire fighters and advice given to the public.
Dr Tolhurst, who has studied extended video of the February 7 fires, said most people expected a fire front that was like a “wave of fire”, which would pass in a matter of minutes. But during the Black Saturday disaster fire activity had lasted for hours in some cases, with up to an hour when radiated heat remained a danger.
“There’s no one front of fire,” Dr Tolhurst said. “It’s not a continuous wave of fire going through.”
The strong wind change caused by the arrival of a cold front late in the afternoon of Black Saturday created a “horror situation”.
“That’s the worst situation you can have,” Dr Tolhurst said.
Although cold fronts can bring cooler conditions, bushfires burn just as intensely for a number of hours after their arrival. The change in wind direction to a southerly or south-westerly will “blow out” the eastern flank of any fire burning at the time, pushing it in a new direction on a much wider front.
The size of the new fire front and the speed it travels will “catch people out”, Dr Tolhurst said.
He said 80 per cent of the damaged caused by bushfires in Victoria occurred after the arrival of a cold front and change in wind direction. Dr Tolhurst said the Black Saturday fires produced huge convection columns and pillars of smoke that made them burn more intensely.
The convection columns caused air to be sucked into the fires at ground level, creating localised cyclonic winds of up to 120kph and snapping trees off three or four metres above the ground. Higher winds created by fires could carry smoke and embers in a direction different from the prevailing wind that was driving the main fire front. This meant spot fires could occur not just ahead of the main front, but off the fire’s flank. On Black Saturday spot fires occurred up to a record 35 kilometres (21 miles) ahead of the main fire, Dr Tolhurst said.
It also made it difficult attempt to judge whether a fire was coming towards you by looking at the direction the smoke was blowing. Someone in the path of the fire could be “in the clear” in terms of seeing smoke.