blew up, the Prescott Hotshots noticed a strange calm that often precedes a plume-dominated situation and radioed the overhead team that they were pulling out. The Perryville inmate crew and the Navajo Scouts crew were warned of the danger as well, but they were in exactly the wrong spot. The downdraft hit right in front of them and, funneled by the contours of the canyon rim, drove the fire straight toward them. Half the inmate crew escaped, as did the entire Navajo Scouts crew. The rest of the fire fighters dived into their shelters and waited for the flame front to pass over them.
From laboratory tests, researchers know that the adhesive that holds fire shelters together starts to melt at six hundred degrees Fahrenheit. That causes the fiberglass and aluminum layers to delaminate, in turn leading to rips and holes in the shelter. After the Dude fire, investigators found hard hats that had melted, leather gloves that had shrunk down to a couple of inches, and fire shelters that had begun to delaminate. Six men died, all from breathing superheated air. In all six cases they died because they had not deployed their shelters properly or because they had tried to leave them too quickly. Some of the survivors also suffered terrible burns, which suggest another possibility. The men who left the safety of their shelters may have done so because they thought they were dying inside them.
Mike Rieser and I sat on a rock outcrop eating our lunches and watching the fire. The sun was very hot and trees torched occasionally down in the valley. Every five minutes the Skycrane went by with a tremendous whump of rotors and released another two thousand gallons of Spring Creek Lake. Clouds began to move in above us, and Rieser said that they were called lenticular clouds and that the sculpted tops meant that the upper-level winds were over one hundred miles an hour. If those winds made it down to ground level, they would have a catastrophic effect on the fire. It was his job as branch director to watch out for things like lenticularis formations or castellanus or stage three cumulonimbus or anything else that might make the fire blow up. Two days earlier Rieser had pulled two entire divisions out of a canyon because he hadn’t liked the way the fire was behaving.
“We might not look like we’re doing much, but if you’re down in the valley digging, you don’t see what’s coming at you,” he said. “Air can rise off a fire, cool in the upper atmosphere, and rush back down. It superheats as it comes down and can overpower the wind field. That’s called a plume-dominated fire. It defies prediction. It’s what killed the people at the Dude fire; the ’shot crews recognized plume-dominated conditions, but the convict crew didn’t. No one could get word to them in time.”
Stage three cumulonimbus, thunderclouds, are a particular hazard. Not only do they introduce more lightning into the situation, but the air beneath their thirty-thousand-foot heads is extremely unstable. They can generate downward-moving winds of as much as one hundred miles an hour that hit the ground and spread out in a tremendous circle. The effect is to intensify the interaction among all three components of what is known as the fire triangle: fuel, oxygen, and heat. Ground winds spread dry, hot air through as yet unburned fuels, resulting in more fire and more heat, which in turn circulate more air, spreading the fire even faster. The result is a feedback loop that can be brought under control only if the relationships within the fire triangle break down—either by lowering the temperature of the fire or by depriving it of fuel. Water and retardant drops can bring the temperature of a fire down, and fire lines can deprive it of fuel. Otherwise, the fire spreads until the weather changes or there’s nothing left to burn.
In the Northern Rockies wildfire is usually started by lightning. Any lightning strike that reaches the ground can cause an explosion, but only lightning with a continuous current can start fires. In the Northern Rockies it has been estimated that one lightning stroke in twenty-five is a cloud-to-ground stroke capable of starting a fire. The inital bolt from cloud to ground moves relatively slowly, at one two-thousandths the speed of light, but it returns at one-tenth the speed of light and heats the gases inside it to fifty thousand degrees Fahrenheit. That much energy hitting a tree instantly raises it way past the ignition point, and often the tree just explodes. Flaming chunks of wood are hurled into the forest, and if the conditions are right for fire, the flames take hold.
Fire is a chemical reaction that releases energy in the form of light and heat. In the case of a wood fire, the energy was originally derived from the sun during photosynthesis and stored in the plant as cellulose and lignin. Heat—from a fire that is already burning or from a lightning strike—converts the cellulose and lignin into flammable gases, which are driven out of the wood and combined with oxygen in a process called rapid oxidation. At the base of any flame there is a clear band of superheated gases that have not yet ignited, a thin blue area of ignited gases, and a broad yellow band of incandescent carbon particles. The heat generated by this process continues to drive flammable gases out of more fuels, which burn and generate more heat. The heat also drives out moisture that might impede combustion. As long as there is sufficient air, fuel, and heat to ignite more fuel, the fire will keep advancing. As long as the fire keeps advancing, the fire triangle remains stable and continues to create the conditions necessary for fire.
A plume-dominated fire, more commonly known as a fire storm, is this same cycle writ large. In this case the three legs of the fire triangle not only provide the conditions for fire but amplify one another in an apocalyptic feedback loop—a “synergistic phenomenon of extreme burning characteristics,” as it is known. As in all fires, heat generates wind, which makes the fuels burn hotter, generating more wind. If a high fuel load is introduced into this loop, a convection cell of smoke and gases can be set in motion that overrides the local wind patterns. During World War II Allied bombers intentionally started fire storms in the German cities of Hamburg and Dresden; in that case the high fuel load was densely packed houses ignited by thousands of tons of ordinance. Once the convection engine has started, it is nearly impossible to stop. Entire stands of trees torch as one. Tornadoes twist through the interior of the storm. Superheated fuels appear to combust spontaneously in a phenomenon called “area ignition.” Such a fire can rip through well over one hundred thousand acres of timber in one day.
Another sort of apocalypse, equally destructive, is the running crown fire. Crown fires occur when flames climb so-called ladder fuels into the treetops and are swept along by high winds. In 1967 a running crown fire crossed the Idaho panhandle on a four-mile front that incinerated sixteen miles of timber in nine hours. The Sundance fire, as it was called, was calculated to have burned at rates of up to 22,500 British thermal units per square foot per second. By comparison, 500 Btu is the outer limit of what humans can control; 1,000 Btu describes potential fire storm conditions. The Sundance fire was estimated to release the energy equivalent of a twenty- kiloton Hiroshima type of bomb exploding every ten minutes.
Not all big fires are fire storms, of course, and not all fire storms are big. The Steep Creek blowup on the Lowman fire had the physical characteristics of a fire storm but was limited in area; the Foothills fire developed several convective columns over heavy timber but went on to become a wind-driven fire that ripped through two hundred thousand acres in two days, making it one of the biggest fire runs ever. In the Northern Rockies there are a host of winds that push fire: jet stream winds that drop down over mountainous areas; chinook winds that plunge downslope because of an air pressure differential; cold fronts that move in for twenty-four hours at a time; and, of course, unstable air associated with the fire itself. Unstable air rises and falls with the atmospheric conditions, whistling up canyons, sheering over ridges, bending around solitary trees or boulders to start whirls up to four thousand feet high. Any of those winds, in the wrong situation, could cause a blowup and kill people. That was why Mike Rieser was on a ridgetop watching the clouds rather than down in some canyon fighting the fire.
Hotshots have been known to complain that overhead—the men and women who risk other people’s lives—do not do enough. Not only that, but hotshots believe that many overhead have never really fought fire and therefore can’t be trusted to make life-and-death decisions. Sometimes that is true; there are the inevitable instances of ’shot crews simply saying, “No, we won’t go in that canyon,” or, “No, we won’t try to hold this ridge.” More often, however, the members of a command team, like Rieser, have worked their way up from grunt-hood to positions of authority over the course of years, if not decades. Rieser has fought fire for nearly twenty years and had two extremely close calls (that he told me about). Once he and his crew fell asleep after cutting line all night and were almost burned over; another time he was caught in a chaparral fire outside Los Angeles. Chaparral fires are extremely volatile because, invariably, the fuels are bone dry, the terrain is steep, and the winds are terrible: Santa Anas that hit seventy miles an hour for days on end. It was in 1979, and Rieser was on a type two crew that, he says, had violated just about every watch out rule in the book.
“We were backfiring off a road two-thirds the way up a ridge,” he said. “We couldn’t get a good burn because of a marine air intrusion. We double-shifted into the next day and got a dominating Santa Ana wind, and the fire just blew up. We were right in a canyon, it was acting like a natural chimney, and the flame front was on us in about ten minutes.”
In those ten minutes the crew managed to jump onto a tanker truck and make it to a marginally safe area at the intersection of two dirt roads, not big enough to qualify as a legitimate safe area but better than nothing. They parked the truck and crouched down between it and a road cut. The crew were so rattled that they were reading
