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The Park Fire spews so much smoke into the atmosphere that it creates fire tornadoes and another strange natural phenomenon: the pyrocumulonimbus cloud, or PyroCb.
Why it is important: Storm clouds of smoke make dangerous wildfires like the Park Fire in the state’s north even more unpredictable. PyroCbs can produce lightning that ignites more fires around the fire that caused the clouds. And as the planet warms, they seem to be occurring more frequently.
The science behind it: In a fire like the Park Fire, the heat from the flames rises and hurls smoke particles tens of thousands of meters into the atmosphere. As they rise, the air cools and expands. Water then condenses on the smoke particles and a cloud forms.
All over the world: PryoCb plumes are appearing all over the world, from Australia to Siberia, as fuels become drier and temperatures warm, and were also spawned by the monster fires in Canada this summer.
With over 600 square miles burned so far, the Park Fire is already one of the largest wildfires in California – and it’s far from under control. Driven by strong winds, the fire has eaten through dead vegetation and spewed smoke high into the atmosphere. In fact, there’s so much smoke and rising hot air that it’s creating fire tornadoes and one of the strangest natural phenomena on Earth: the pyrocumulonimbus cloud, or PyroCb.
It’s a thundercloud of smoke that makes dangerous wildfires like the Park Fire in the state’s north even more unpredictable. PyroCbs can create lightning that ignites more fires around the fire that started the clouds. And as the planet warms, PyroCbs seem to be becoming more common, as they are sparked by the largest and most violent wildfires, which themselves are getting worse. “PyroCbs are such massive, almost volcano-like eruptions,” said Rajan Chakrabarty, an aerosol researcher who studies the clouds at Washington University in St. Louis. “These PyroCbs create their own fire weather.”
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The Park Fire was made massive by the extremely dry fuel. This part of California had not seen fires for decades, so a lot of plant life had accumulated and dried out under the summer sun. The very low humidity helped to strip the vegetation of what little moisture remained, turning the landscape into a tinder pile.
A fire this large and intense is a breeding ground for pyrolysis forces – miracles of fire physics. As a fire like the Park Fire burns – and even more fiercely as climate change brings higher temperatures and drier fuels – the heat from the flames rises, hurling smoke particles tens of thousands of feet into the atmosphere. As the air rises, it cools and expands. Water then condenses on the smoke particles and the cloud forms.
The rising air masses in a PyroCb form a kind of cavity near the ground that sucks in more air and creates winds that help the flames spread. The most intense wildfires use so much oxygen that they can somewhat smother themselves, but PyroCb winds blow more gas into the firestorm. It’s “a self-perpetuating process,” said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research. “Because the more intense it is, the more oxygen rushes in, which means the more intense it is, the more oxygen rushes in. So you can imagine how that works.”
At the same time, a towering PyroCb can create a downdraft that makes winds at the surface even more unpredictable. “The convection causes a lot of chaos, so it becomes very difficult to predict where the air is coming from and where it’s going,” said Payton Beeler, an atmospheric scientist who studies PyroCbs at the Pacific Northwest National Laboratory in Richland, Washington. That, in turn, leads to chaotic fire behavior as these winds drive flames across the landscape at different speeds and in different directions.
The smoke from a PyroCb extends far beyond the fire that created it. “Some of the aerosols that get into the upper troposphere or lower stratosphere stay there for up to six to eight months,” Beeler said. “And they can basically be transported across the hemispheres.”
Soot from a pyroCb cloud isn’t exactly behaving up there either. In a paper published last week in the journal Nature Communications, Beeler found that soot from a pyroCb cloud absorbs up to twice as much visible sunlight as soot from smaller fires or from urban sources like coal burning. “Particles in the pyroCb cloud tend to have really, really thick layers of organic matter,” Beeler said, “and that’s different from soot from other sources.”
This causes light to be better absorbed and temperatures in the atmosphere to rise. “It’s like a black sweater – it absorbs all the sun and heats up the surrounding area,” said Chakrabarty, who co-authored the study with Beeler.
Why this happens in PyroCb clouds is not yet clear to scientists. It could be because there is something special about the way a PyroCb-generated wildfire burns, or because there is a secondary process going on within the cloud that coats the particles with more organic matter. (Organic matter in this case comes from the burning of vegetation by the fire.)
Another unanswered question is whether pyroCb clouds are already becoming more common because climate change is making wildfires more intense, or whether scientists are getting better at spotting them using satellites, or whether both are at play. PyroCb clouds are appearing all over the world, from Australia to Siberia, as fuels get drier and temperatures get warmer. The monster fires in Canada this summer also spawned them. “They seem to be occurring more frequently,” Beeler said. “Whether that’s a result of the warming climate or better identification, I think it’s probably both. But the effects seem to be very long-lasting and far-reaching.”
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