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A towering cloud of hot air, smoke and humidity that reached the heights of an airliner and created lightning. Flame fronts pushed by the wind which swept over the landscape, often exceeding the firebreaks. Even, perhaps, a rare tornado of fire.
The Bootleg fire in southern Oregon, spurred by months of drought and last month’s scorching heat wave, is the largest wildfire so far this year in the United States, having already burned down over 340,000 acres, or 530 square miles, of forest and grassland.
And at a time when climate change is making forest fires bigger and more intense, it’s also one of the most extreme, so big and hot that it affects the winds and disrupts the atmosphere.
“The fire is so big and generates so much energy and extreme heat that it changes the weather,” said Marcus Kauffman, spokesperson for the state’s forestry department. “Normally the weather predicts what the fire will do. In this case, the fire predicts what time will do. “
The Bootleg Fire has been burning for two weeks, and for most of that time it exhibited one or more forms of extreme fire behavior, causing rapid changes in winds and other conditions that caused the flames to spread rapidly. in the forest canopy, ablaze entire stands of trees at once and embers blown long distances, quickly setting off point fires elsewhere.
“It’s kind of an extreme and dangerous situation,” said Chuck Redman, a forecaster with the National Weather Service who has been to the fire command headquarters to provide the forecast.
Fires so extreme that they generate their own weather patterns disrupt firefighting efforts. The intensity and extreme heat can force the wind to bypass them, creating clouds and sometimes even generating fiery tornadoes – eddies of heat, smoke and strong wind.
The catastrophic Carr fire near Redding, Calif., In July 2018 was one of those fires, devastating 130,000 acres, destroying more than 1,600 structures and killing at least eight people, some of whom were killed. attributed to a fire tornado with winds reaching 140 miles per hour which were captured on video.
Many forest fires get bigger quickly and the Bootleg Fire is no exception. In the early days it grew by a few square miles or less, but in recent days it increased by 80 square miles or more. And almost every day, erratic conditions forced some of the estimated 2,200 firefighters to retreat to safer locations, further hampering efforts to bring it under control. More than 75 houses and other structures burned down.
On Thursday evening, along its northern edge, the fire jumped over a line that had been treated with a chemical retarder, forcing firefighters to back down. This was just the latest example of a fire going beyond a firebreak.
“This fire is a real challenge, and we envision a sustained battle for the foreseeable future,” said Joe Hessel, incident commander for the forestry department.
And it is likely that it continues to be unpredictable.
“The behavior of fire is a function of fuels, topography and weather conditions,” said Craig B. Clements, director of the Wildfire Interdisciplinary Research Center at San Jose State University. “It usually changes from day to day. Sometimes minute by minute.
Mr Redman said almost every day the blaze created large updrafts of hot air, smoke and humidity called pyrocumulus clouds, some of them reaching as high as 30,000 feet. One day, he says, they saw one of these clouds collapse, which can happen in the early evening when the updraft stops.
“All that mass has to come down,” he said, forcing the air on the surface outward, creating strong, gusting winds in all directions that can spread a fire. “It is not a good thing.”
Last Wednesday, however, conditions led to the creation of a larger and taller cloud called pyrocumulonimbus, which is similar to a thunderstorm. It probably reached an altitude of about 45,000 feet, said Neil Lareau, who studies wildfire behavior at the University of Nevada, Reno.
Like a thunderstorm, the huge cloud created lightning, worrying firefighters because of their potential to start new fires. It may also have brought precipitation.
“Some of these events are raining down on themselves,” said John Bailey, professor of forestry at Oregon State University.
Rain can be a good thing, by dampening some of the fuels and helping to slow the fire. But by cooling the air closer to the surface, rain can also create dangerous downdrafts, Dr Lareau said.
There have also been reports of fire whirlpools, small eddies of air and flames that are common to many forest fires and are often mistakenly described as fire tornadoes. The vortices of fire are small, perhaps a few tens of feet in diameter at most, and last from seconds to minutes.
But Dr Lareau said there were indications that the Bootleg Fire could have created a true tornado of fire, which can reach several thousand feet in diameter, having wind speeds in excess of 65 miles per hour, stretch thousands of feet in the air and last much longer. “Looks like he’s produced a pretty big rotation,” he said.
Fire tornadoes occur when a plume of warm air rises inside a fire, which draws more air from the outside to replace it. Local topography and differences in wind direction, often caused by the fire itself, can cause this incoming air to spin, and stretching the air column can cause it to spin faster, such as a figure skater pulling her arms to increase it. turn.
Mr Redman said incident command had not received any reports of a tornado of fire. “But it is entirely possible” for a fire to occur in such a large and intense blaze, he said. “When we get these extreme events, these are things we have to watch out for.”
Other types of extreme fire behavior are more common. But the duration of the Bootleg Fire’s extreme behavior has stunned some of those who fight it.
“It’s day in and day out of this extreme behavior and explosive growth,” Mr. Kauffman said. “And you can’t really fight fire under these conditions. It’s too dangerous.”
The root cause of most extreme behavior is the enormous amount of heat that the fire gives off.
The amount of heat is related to the dryness of the fuel – trees and other plants, dead and alive. And fuels in southern Oregon, as well as much of the west, are extremely dry, a result of the severe drought that is affecting most of the region.
Dr. Clements compared it to a campfire. “You want the tinder and the driest logs to light this fire,” he said. “Same thing in a forest fire. That is why we are watching the drought.
If the vegetation is wet, part of the energy of the combustion is used to evaporate its moisture. If there is no moisture to evaporate, the fire burns hotter. “More heat is released,” he said. “The flames are bigger. “
Oregon was also hit in late June by an extreme heat wave, when record high temperatures in some places were beaten to 9 degrees Fahrenheit. This dried out the vegetation even more. In southern Oregon, fuels were as dry as they would be in late summer in a more normal year.
“We had a lot of fuel that was ready to burn,” Dr. Bailey said.
What would help end the extreme behavior, and possibly the fire itself, is widespread good rain. But that doesn’t seem to be in sight.
“We don’t see any significant relief at least next week,” Mr. Redman said. “But I don’t think we can do any worse.”
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