Weather and wildfires share a close relationship. Certain weather conditions are known to ignite wildfires: High temperatures and low humidity dry out the landscape, lightning strikes can spark a flame, and fast-moving winds spread flames across nearby desiccated land.
But wildfires also spawn their own weather systems, including pyrocumulonimbus clouds—which NASA has called the “fire-breathing dragon of clouds” for the thunderbolts they hurl at Earth, fueling further blazes and sometimes even fire tornadoes.
Fire weather has contributed to the scale of several historic conflagrations, including the 2009 Black Saturday bushfires that burned more than a million acres across Australia, and the wildfires across the West Coast of the United States in 2020. Here’s what causes firestorms—and why they’re becoming more common in a warming world.
How firestorms get started
Firestorms form through a convective process, in which heat rises through the air. When a column of moist air over a fire becomes intensely hot, it rushes up into the atmosphere where it cools and condenses into droplets, creating fire clouds called pyrocumulus clouds. Unlike the fluffy white cumulus clouds that also are created by convection, fire clouds are grayish or brown because of the ash, smoke, and particulate matter that get swept into the updraft. The tops of these clouds can reach nearly six miles high.
As a fire grows, the updraft funnels smoke and particulates ever higher into the lower stratosphere, forming even bigger pyrocumulonimbus clouds. They look similar to regular thunderclouds, but these clouds—also called pyroCbs—are far more devastating. They remain tethered to the fire that spawned them, spewing embers and lightning that continue to fuel the fire. These clouds also tend to produce lightning with a positive rather than negative charge, which makes the storm last longer, and they rarely form rain to help extinguish a wildfire.
Pyrocumulonimbus clouds also occasionally trigger fire tornadoes, which form when the updraft twists and stretches the air that is being sucked into the sky at high speeds. These fiery whirls tend to last only a few minutes and reach no more than 150 feet tall. But with wind speeds of up to 140 miles an hour, they can do serious damage to anything in their path.
Fire weather helped fuel some of the devastating wildfires in recent history. In 2009, the Black Saturday bushfires in the Australian state of Victoria generated clusters of pyroCb clouds that stretched more than nine miles high and ignited new fires that contributed to the fire’s spread across more than a million acres of land. The Black Saturday fires killed 173 people, the greatest loss of life from fire since Australia’s colonization in 1788.
In 2017, an even bigger wildfire in the forests of British Columbia produced five fire thunderstorms nearly simultaneously. They blew smoke up to 14 miles into the stratosphere as black carbon in the smoke absorbed the sun’s energy and heated the plume, making it rise faster and farther. Multiple studies have shown that the plumes were comparable to those of a moderate volcanic eruption and remained in the atmosphere for almost nine months.
In the United States, California has also seen several intense pyrocumulonimbus events. During the Carr fire near Redding in July 2018, a fire tornado spinning at speeds of up to 143 miles an hour was responsible for four of the eight deaths associated with the fire. In August 2020, several possible fire tornadoes were reported in Northern California during a record-breaking fire season rife with firestorms.
Firestorms also contribute to the damaging effects that wildfires have on human health. The gases and particles that make up wildfire smoke have been linked to respiratory and cardiovascular illnesses, and firestorms can exacerbate the blazes and increase levels of smoke.
Fire weather and climate change
As climate change stokes bigger and more intense wildfires, scientists believe the planet will experience a rise in firestorms. In 2019, Australia saw as many fire-generated storms as it had seen in the 20 preceding years. On September 7, 2020, smoke from a pyrocumulous cloud near Fresno, California, shot 10 miles into the stratosphere, a record for a fire in North America that likely released significant carbon emissions.
In fact, scientists believe these firestorms are responsible for “a huge volume” of the pollutants in the upper atmosphere. But there’s still much that’s unknown about how firestorms might contribute to climate change, including whether their plumes damage the ozone layer that protects Earth from ultraviolet radiation and whether they might actually have a temporary cooling effect on the planet by blocking sunlight—a phenomenon seen in volcanic eruptions. Answering these questions will be key to understanding the true consequences of firestorms in a warming world.