Nasa calls them the “fire-breathing dragon of clouds”.
Aerial images of the McKinney fire taken this week captured an increasingly common phenomenon: a nearly 50,000ft plume known as a pyrocumulonimbus.
The fire, which is raging nearly uncontained in California’s Klamath national forest, is just the latest this year to produce the clouds. They are akin to fire-triggered thunderstorms, explained Derek Mallia, a researcher at the University of Utah who recently co-authored a paper showing how smoke plumes are getting taller, and they are of deep concern to firefighters.
Typically, thunderstorms are initiated by large-scale storm systems like a low pressure zone or a cold front. But if a fire is large enough and there is enough moisture in the atmosphere, it can create its own thunderstorm where smoke mixes with the resulting thunderstorm cloud. “Essentially, the fire is creating its own weather,” Mallia said.
Pyrocumulonimbus clouds are feared for several reasons. When they form above fires, the clouds can make the blaze below spread even faster. Sometimes they can also create their own lightning, which can spark more fires. Fire-triggered clouds tend to be drier and produce less rain, Mallia explained. “This can be problematic as pyrocumulonimbus clouds can produce a lot of wind, which can result in unpredictable fire behavior underneath the cloud and even cause new fires from lightning strikes.” That’s what happened in British Columbia during the summer of 2021, when one fire generated 700,000 flashes of lightning, as many as the province typically gets in one year.
They also can cause fires to move in erratic ways, because they create a vacuum between the cloud and the ground. The hotter a wildfire burns, the more rising air it produces, said David Peterson, a meteorologist at the US Naval Research Laboratory, during a virtual press conference in 2021. “These are pushing smoke upward at extreme velocities, such that they’re injecting smoke at altitudes above the cruising altitude of jet aircraft,” said Peterson. “So we’re talking 50, 60,000 feet, potentially.”
Pyrocumulonimbus clouds are becoming more common and are throwing more pollutants into the upper atmosphere. As fires become larger, with more dry fuels, they produce more heat and smoke, Mallia said.
In some cases, the tall plumes and fire convection can boost aerosols into the stratosphere, like a moderate volcanic eruption – pushing particles into the jet stream that can affect air quality at continental scales. That’s what happened when smoke from fires on the west coast was pushed all the way to New York City. Sometimes the clouds can be so thick that they blot out the sun.
In 2019, Nasa researchers flew a plane through a fire cloud to understand the chemistry of smoke to better understand its impact on air quality and climate.
Scientists are still learning about the fire-triggered storms. Mallia and his colleagues hope to develop ways to predict when pyrocumulonimbus clouds will appear, to help fire managers and firefighters.
And they want to know how the clouds could cause more warming in the atmosphere. Smoke lofted into the lower stratosphere could affect the climate, Mallia said. Smoke particles contain a lot of black carbon, which can result in warming when exposed to sunlight at high altitudes.
“It’s possible that if we see more smoke being injected into the stratosphere, this could result in further warming of our atmosphere,” he said. “If we see this more frequently, this could result in longer-term warming effects.”
