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Wildfires are forces of nature, devastating thousands of acres of land, causing evacuations and destroying any building in their path.
They also produce their own weather, making them unpredictable and dangerous for those firefighters working to contain and douse the flames.
A wildfire generates its own wind as it grows in size and burns more biomass (plants and trees). Huge quantities of air are needed to support the chemical reaction, according to Bret Butler, research mechanical engineer at the U.S. Forest Service’s Rocky Mountain Research Station in Missoula, Montana. This creates high winds, or localized in-drafts, which pull air into large fires.
“We have measured in-drafts at the base of flames greater than 60 mph,” Butler said. “Such strong winds can overpower the synoptic winds [large-scale winds] associated with the fire and cause the fire to move in unexpected directions with unexpected intensity.”
The impacts of the in-drafted air can be exacerbated in mountainous terrain, Butler said.
“As fires burn on slopes, the in-draft can be impeded by the slope resulting in a low pressure region on the upward slope side of the flame front,” he said. “As slopes exceed 30 to 35 percent, this low pressure region can result in flames attaching to the slopes leading to significant increases in fire spread rates and associated fire intensity.”
The changes are difficult to predict and when they occur, they can happen so rapidly with potentially dangerous implications to public and firefighter safety.
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“This is very similar to a growing thunderstorm, and when coupled with atmospheric instability, thunderstorm clouds can actually form over a particularly large and potent fire, similar to a volcano, and are called ‘pyrocumulus,’” said AccuWeather Meteorologist Evan Duffey, who is also a Pennsylvania volunteer firefighter.
Fires can also create their own vortexes, also known as firewhirls and firenadoes, Duffey said.
“Firewhirls are created by turbulent air being picked up by the hot rising air of a fire,” Duffey said. “It's a complicated physical process, but the turning air rising over the fire becomes a self-sustaining process.”
It usually takes a substantial fire to produce firewhirls, because of the vertical velocities needed to create that self-sustaining process, Duffey said.
“They are a particular problem for firefighters because they generally lead to extreme spotting, by transporting burning material well away from the fire's flank, starting new secondary fires,” Duffey said.
There are many documented cases of firewhirls on the scale of hundreds of feet in diameter, Butler said.
The concern with these phenomena is that once they form, they can travel some distance from where they formed.
“There are documented cases of large trees being uprooted, damage to buildings and vehicles by these [tornadoes],” Butler said.
Weather conditions, such as heat, dryness and high winds, can not only set the stage for high fire danger, but they can also dictate how fires evolve, Duffey said.
“A fire season setup can be determined by weather features such as drought, early frosts and freezes and periods of excessive heat,” Duffey said. “Once wildfires are underway, the weather, in combination with the terrain profile and the fuel conditions, impacts the intensity and spread of a wildfire.”
When it comes to fighting a wildfire, the wind direction is probably the most critical factor affecting strategy, Duffey explained.
“A sudden shift in wind direction will move the head, or worst part of the fire, to a new area,” Duffey said. “If firefighters are on the left flank of a fire and it shifts 90 degrees, changing the flank to the head of the fire, then those firefighters are in immediate danger.”
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