Late last week the Carr Fire, the largest of 17 major wildfires currently burning across California, jumped the Sacramento River and began burning in the city of Redding. As residents evacuated, a scene out of a disaster movie unfolded: a massive tornado of smoke and fire expanded in size, reaching up to 18,000 feet high as it moved through parts of Redding for almost an hour, blowing at nearly tornado-strength speeds, reports Allie Weill at KQED. Images of the so-called firenado have haunted social media ever since.
So what, exactly, is going on in the apocalyptic scene, and what is a “firenado?” There are many names for the whirling masses of ash, dust and live flame. In general, they are known as fire whirls but are also called fire devils, fire twisters or fire tornadoes, though experts discourage that name since they are not formed in the same way as tornadoes. Tornadoes pop into existence when conditions are just right as warm moist air near the surface of the Earth rises into the cooler air above. If that updraft is set spinning by strong winds and is then tilted vertically by a thunderstorm, it can spawn a tornado.
But fire whirls are created by the same process that creates dust devils, the much smaller columns of rotating air that appear in hot weather. Marc Lallanilla at LiveScience explains fire whirls form when high ambient temperatures and hot air from a fire combine, creating a column of superheated air. That air begins to rotate and as is rises, though researchers aren’t sure exactly what sets the air spinning. The rotating column flips into the vertical position, with the force of the rising air pulling ash, embers and flames up into the sky with it. Angular momentum, the same process that causes a figure skater to spin faster and faster as they pull their arms in, causes the chimney to concentrate and spin faster, creating the apocalyptic spectacle. Josie Rhodes Cook at Inverse reports that the spinning air also feeds fresh oxygen to the burning core sometimes found at the center of the whirl.
In general, fire whirls only extend a few hundred feet into the sky and last only a few minutes. They are not uncommon during wildfires, but since they flit into and out of existence quickly, it’s often difficult to capture images of them. Which makes the Redding fire whirl a true and terrifying oddball. The intensity of the Carr Fire and the high ambient air temperatures, which have been near triple digits, likely added extra energy to the whirl. “It’s very rare as well to have these really persistent long-lived events like that,” Neil Lareau, atmospheric scientist at the University of Nevada, Reno, tells Weill. “To get a big one like this is really scary.”
The whirl is so big that some meteorologists are dropping their objection to the term “fire tornado” to describe the event. “I’m not particularly fond of the term,” Brenda Belongie, lead meteorologist of the U.S. Forest Service's Predictive Services in Northern California, tells Mark Kaufman at Mashable. “But it works because of the strength of the fire whirl, the size — and the destructiveness is not unlike the power of a tornado.”
Other large fire whirls were recorded last year as well and may have helped spread the Thomas Fire and North Bay Fires, among the most destructive in state history. The Redding fire whirl was larger than those and was particularly scary because it appeared near a highly populated area.
So far, the Carr fire has destroyed 1,000 homes in Shasta County, including neighborhoods in Redding, has scorched, 113,000 acres, and killed six people, making it the seventh most destructive fire in California history. Luckily, reports NPR’s Scott Neuman, the strong winds that made the fire difficult to control have died down and firefighters currently have the blaze 30 percent contained.