See How the Dixie Fire Created Its Own Weather

Over its lifespan, the Dixie fire generated eight firestorms and displayed other extreme behaviors, including at least one fire whirl, a sort of mini-tornado. David Peterson, a meteorologist who tracks fire-driven thunderstorms at the U.S. Naval Research Laboratory, called Dixie “the most prolific producer of pyroCbs” in the United States this summer.

These storms create “a very dangerous situation for our firefighters on the ground,” said Edwin Zuniga, a public information officer for Cal Fire, California’s firefighting agency.

When wildfires are driven by wind, they usually move in a predictable direction. “If the wind is blowing from the southwest, we know the fire will be travelling to the northeast,” Mr. Zuniga said. But fires that are “plume-dominated” — like those creating and sustaining their own weather systems — are more volatile. They spew their own erratic winds and can throw embers long distances, causing spot fires miles away.

Faced with pyroCb clouds, firefighters are usually forced to pull back from a blaze. “It’s really nearly impossible to contain at that point,” Mr. Zuniga said.

Since it began in July, when a small cluster of flames was discovered near downed power lines, the Dixie fire has burned through nearly a million acres across the Sierra Nevada, triggering mass evacuations and destroying thousands of homes, businesses and other structures, including much of the town of Greenville.

Today, the fire is almost fully contained, though pockets still smolder.

Competing forces helped Dixie grow into a monster, said Ryan Bauer, a fire manager at Plumas National Forest. Strong winds often pushed flames across the rugged terrain of the Sierra. But even when the winds were calm, the fire created its own momentum, kicking up powerful plumes and pyro-clouds.

Recent extreme heat and drought had turned California’s overgrown forests into potent fuel for either scenario. “The stage was set for fire to spread no matter what the conditions,” Mr. Bauer said. “It was just a matter of whether it was going to be wind-driven that day or plume-dominated.”

Wildfires on the West Coast have grown larger and more intense in recent years, especially in California. Experts attribute the trend to a combination of factors, including decades of aggressive fire suppression that left forests dangerously overgrown, and climate change, which has parched the landscape, priming it to burn at record scale.

Fire clouds have also become increasingly common on the landscape. In California, where powerful infernos have burned close to population centers, snapshots of the mushroom-like clouds have spread widely on social media.

Dr. Peterson’s team recorded 33 pyroCb clouds in the American West this year, tying a record set in 2013.

It’s not yet clear whether there is a sustained long-term trend toward more fire-fueled storms, in part because the record of these events is still relatively short: An effort to consistently track pyroCb formation across the globe, based on satellite observations, dates back about a decade.

What is clear is that the ingredients necessary for firestorm activity — drier landscapes that support larger, more intense fires; more atmospheric instability, which aids the development of thunderstorms; or both — are becoming more common in many parts of the world as human-caused climate change pushes temperatures higher.

Following a historic heat wave early this summer, Western Canada experienced a particularly explosive run of pyroCb activity. On two occasions, the powerful storm clouds grew so tall — the largest reached an altitude of 57,000 feet — that they broke through the lower atmosphere and injected smoke into the stratosphere. The smoke particles can linger in this layer for months and make their way around the globe, which can even lead to some temporary regional cooling.

Firestorm clouds have regularly been documented in Western North America, Southern Australia and across much of Siberia, where wildfires are seasonal events. But extreme fire weather is now occurring in less expected places, too: A pyroCb cloud formed north of Athens this year, part of a disastrous outbreak of summer wildfires.

“We’re creating an environment that favors these positive feedbacks, where the fire makes itself worse,” said Dr. Lareau, an assistant professor at the University of Nevada, Reno. “It tips the balance between what may have been an ordinary fire in decades past and a fire that can grow into a megafire.”