October 22, 2013
A bizarre weather phenomenon known as a fire cloud is likely to form over the fire raging between Lithgow and Bilpin if weather conditions deteriorate from Tuesday, as predicted, say fire analysts.
The unique weather phenomenon – also called a pyrocumulus – occurs when a large fire is coupled with an unstable atmosphere.
Rural Fire Service spokeswoman Natalie Sanders said the State Mine fire, which stretches across more than 42,000 hectares, had generated a pyrocumulus last Thursday and similar, unstable conditions were forecast for Wednesday.
”If these fires are still going strong, there’s a potential for that to happen again,” she said.
Andrew Sullivan, a leading bushfire behaviour researcher with the CSIRO, said the phenomenon required a fire generating a significant amount of heat.
As it burned, the hot air it released would rise as a column into the atmosphere. As air moved upwards, it was quickly replaced by cooler air, a process that produced a convection column.
In very large bushfires, these hot-air columns could be ”enormous” and rise high into the atmosphere carrying a large amount of water vapour – one of the main combustion products of fire.
An unstable atmosphere meant that the column could rise higher and higher into the atmosphere, where the temperature was cool enough for the water vapour to condense into a pyrocumulus cloud, said Dr Sullivan, a senior research scientist.
”It’s a cloud formed by the presence of the fire,” he said.
While the clouds typically did not contain enough water to rain and extinguish the fire, under certain conditions they could generate lightning.
”In some cases, the lightning has hit the ground downwind of the fire and started new fire,” he said.
For a large convection column to form, a significant amount of heat needed to be released from the fire, a process that was related to the amount and rate the fire burned through fuel.
While extensive fires with large convection columns could generate strong winds, they did not change the behaviour or direction of the fire, Dr Sullivan said.
”There is this confusion [where] people think a fire has gotten so big that it’s created its own weather and it can do whatever it wants,” he said. ”Well, it doesn’t.
”A fire will still follow the laws of physics and move in the direction the prevailing wind is driving it.”
But, under certain circumstances, fires could generate vortices, or fire tornadoes, with very high wind speeds that could cause significant damage.
The Sydney Morning Herald