They start from the smoky tendrils of wildfires trailing into the atmosphere. Forming first as small, white clumps of clouds, in as little as 30 minutes they can become imposing thunderstorms.
“It’s hard to overstate how dark they become,” says Nicholas McCarthy, a wildfire scientist at the University of Queensland in Australia, of the pyrocumulonimbus clouds that grow from intense wildfires.
Called pyroCbs for short, or often just fire storms, the dangerous atmospheric phenomena can worsen fires by generating intense winds, transporting embers, and striking lightning in yet- untouched regions.
During California’s infamous Carr fire in 2018, pyroCbs grew from three to seven miles across in just 15 minutes and spawned a fiery tornado. These fire storms have been seen in places plagued by deadly wildfires like Portugal, Texas, and Arizona.
As the planet warms, large-scale fires are becoming more frequent, and wildfire seasons are getting longer. Australia, which this year experienced its driest spring and hottest year, has experienced increasingly dangerous wildfires. Scientists say the fire storms there may increase as well, creating a dangerous positive feedback loop that scorches already-dry land.
How these storms form
Exactly when and how fire storms spring to life can be difficult to predict, says Mike Flannigan, a professor of wildland fire at the University of Alberta.
“They are incredibly intense and erratic,” he notes.
“Everything that goes into these phenomena are at their worst when these fires are occurring,” says Mike Fromm, an expert on pyroCbs at the U.S. Naval Research Lab.
The same conditions that can lead to devastating wildfires—hot, dry air and intense wind—are also the most likely to produce fire-borne storms.
As the air over a fire becomes intensely hot, it creates an upward-moving rush of wind called an updraft that funnels smoke into the atmosphere like a smokestack. As the air rises, it cools and condenses, forming clouds. The higher it lifts, the more likely it is to form a thunderstorm, says Flannigan.
“These storms create their own wind field because they have such a violent updraft. It’s a very turbulent environment,” he says.
Once formed, pyroCbs can look similar to intense thunderstorms, but they have key differences. They tend to produce lightning with positive, rather than negative charges, which lasts longer and allows the strikes of lightning more time to set the ground aflame. Fire storms also tend to stagnate, staying parked over the fires that create them. And perhaps most notably, fire storms rarely produce the precipitation desperately needed to squelch massive fires.
“They produce almost no precipitation whatsoever,” says Fromm. “One of the ironies of the pyroCb is that because it’s generated by fire, the smoke seeding changes the microphysics to the point where precipitation doesn’t form.”
The climate question
Australia is particularly at risk of feeling the direct impacts of climate change. Since 2005, the country has experienced its 10 hottest years on record.
Whether it be a single hurricane, flood, or fire, scientists can’t attribute any one single weather event to climate change, and instead look for trends showing how weather patterns have changed over time. Fromm says his research hasn’t definitely shown such a trend yet, but their research remains ongoing.
Over the past year, Australia has had more fire-generated storms than it has in the past 20 years. As hot, dry conditions persist in the country, scientists expect pyroCbs to continue forming.
“With climate change, we should see higher-intensity fires, and with higher-intensity fires, one would expect to see more of these storms,” says Flannigan. “I expect more in the future.”
In a paper publish in July of last year, McCarthy and his study co-authors found that Australia’s changing climate could put more people and habitat at risk of seeing fire storms.
What’s still unclear is the long-term effects of the storms. Scientists know they’re capable of spreading fires and worsening wildfire conditions, but Fromm says the pyroCbs may block out the sun in localized areas, creating a cooling effect.
Australia is still in the beginning of its wildfire season, and meteorologists say the fires could continue for months.