Once and for all — trees do not explode

I asked a fire scientist

Elkhorn Fire
Elkhorn Fire. Shasta-Trinity National Forest in northern California. September 3, 2020. Photo by Mike McMillan.

September 15, 2020  |  7:45 a.m. PDT

Yesterday President Trump flew in to Sacramento McClellan Airport to receive a briefing on the wildfires ravaging the state. Before he met with the Governor and fire officials he stepped before microphones and provided his opinion about what led to the numerous fires in Oregon and California.

“There has to be good, strong, forest management,” he said, “which I’ve been talking about for three years with the state so hopefully they’ll start doing that.”

(The federal government owns nearly 58 percent of California’s 33 million acres of forestlands, while the state owns 3 percent.)

Then the President talked about exploding trees:

“But with regard to the forest, when trees fall down after a short period of time, about 18 months, they become very dry, they become really like a match stick and they get up you know there’s no more water pouring through and they become very, very they just explode. They can explode.”

The myth of exploding trees may have originated with a classic film about wildfires, “Red Skies of Montana” which showed firefighters being harassed by exploding trees, thanks to movie magic. Then the book “Young Men and Fire” mentioned “the occasional explosion of a dead tree”.

In my 33 years of fighting wildland fires I never saw or heard a tree explode, and I don’t know a reputable firefighter that has.

In 2016 after the late Senator John McCain talked about the Chedeski and Wallow fires in Arizona and “trees literally exploding as the fuels that have accumulated around the bases of the trees burns up,” I reached out to the firefighter community asking if anyone had ever seen a tree explode. No one said they had.

When lightning strikes a tree it can explode when the moisture inside is converted to steam in a millisecond. And maple trees can explode in below freezing temperatures when the sap freezes. There are unconfirmed reports that eucalyptus trees in Australia can explode in a fire but I’m not convinced this is true. I understand that heated gasses or sap can shoot out of a crack in a eucalyptus tree and can be ignited during a fire.

Frank Carroll, in a comment on yesterday’s article about the President’s remarks, said he possessed a video shot with a wildlife camera of a tree exploding in a fire. He also said, “Rothermel theorized that the moisture in the tree is superheated and caused the rapid expansion of gasses that go boom.”

(UPDATE: Mr. Carroll sent me the video, with permission to post it on YouTube. You can judge for yourself if it shows an explosion.)

Dick Rothermel fire research Ember Award
Dick Rothermel was given the Ember Award for his wildland fire research, at a conference in Missoula in 2014. Photo by Bill Gabbert.

Since his name had been invoked, I reached out to Richard Rothermel, who is a retired fire scientist. In 1972 he and others developed the Forest Service’s first quantitative, systematic tool for predicting the spread and intensity of forest fires, which introduced a new era in fire management. And surprisingly, it is still the main tool being used today. Many researchers have produced alternative models, but none have made it into the hands of firefighters on a widespread basis.

When Mr. Rothermel began researching the behavior of wildland fires, he had just been downsized from a shuttered Department of Defense program that had been working to develop a nuclear-powered airplane.

I told Mr. Rothermel what was said about him, and asked for his response. He replied within a few hours:

“I have been out of the fire business for a long time and I don’t recall discussing exploding trees. Thats not to say I didn’t, but theorizing the problem now, here is what I think. There may be different concepts of what it means for a tree to explode. One could be that the foliage suddenly bursts into flames due to a massive amount of heat engulfing the tree. That I believe could happen.

“The other which your question prompts me to believe is what is meant by an exploding tree is for the trunk to become super heated sufficiently to cause the moisture in the tree to suddenly become steam with resulting expansion which would shatter the tree. In my years at the fire laboratory I never heard anyone report seeing this or finding evidence of it.

“The problem is the timing, a tree at the fire front could be engulfed in both convective and radiant heat which would transfer heat to the tree’s surface very fast, but the heat would then have to be transported by conduction to the moisture in the cambium layer. Conduction is a very slow method of heat transfer in woody material. In the situation under discussion the fire would be spreading extremely fast and the fire front would have moved on before the heat could have time to boil the water in the cambium layer and cause a steam explosion at the fire front. What could happen after the front has passed and the fire continues to burn if fuels are available is anybody guess.”

UPDATE, September 18, 2020  |  1:32 p.m. PDT:

I first wrote about the myth of trees exploding in 2016 when a senator talked about “trees literally exploding”. I noted then that at least one book and a movie also propagated the myth, and I wanted to dispel it.

When Mr. Trump said trees can explode, I decided to write a followup to the 2016 article, hoping to clear up any confusion, since trees do not explode.

Some of the readers of Wildfire Today were offended by what they saw as criticism, and bent over backwards to make what he said seem reasonable. A person might wonder if they would have put up as strong a defense if the same words had been spoken by a different President.

Bill Gabbert
Wildfire Today
Wildfire News & Opinion, since 2008

Engineer who worked on plans for nuclear-powered airplane later developed the fire spread model

Throwback Thursday: the origin of the model for predicting the spread of wildland fires

Today we may take it for granted that tools are available that can estimate how a fire, unplanned or prescribed, will spread across a landscape. It is not an exact science because there are far too many variables than can realistically be accounted for, at least with the technology available today. But in 1972 when Dick Rothermel and others developed the Forest Service’s first quantitative, systematic tool for predicting the spread and intensity of forest fires, it introduced a new era in fire management. And surprisingly, it is still the main tool being used today. Many researchers have produced alternative models, but none have made it into the hands of firefighters on a widespread basis.

Dick Rothermel fire research Ember Award
For his research and contributions to understanding how fires spread, Dick Rothermel was given the Ember Award at caonference in Missoula in 2014. Photo by Bill Gabbert.

After Mr. Rothermel developed the mathematical model, others used the information to make the concept more user-friendly and to analyze complex scenarios. Behave, software burned onto a custom made chip in a hand-held Texas Instruments 51 calculator, and later BehavePlus for personal computers, became must-have tools for fire behavior analysts. FARSITE added the ability to predict spread across variable terrain, vegetation, and weather. Rare Event Risk Assessment Process (RERAP) estimates the risk that a fire will reach a particular place before it dies. FireStem estimates tree mortality based on fire behavior and intensity. And there are many others.

When Mr. Rothermel began researching the behavior of wildland fires, he had just been downsized from a shuttered Department of Defense program that had been attempting to develop a nuclear-powered airplane.

Below is an excerpt from an excellent article by Gail Wells for the March, 2008 edition of Fire Science Digest, a publication of the Joint Fire Science Program.

[Jack] Barrows, [the first director of the fire laboratory in Missoula when it opened in 1960], went looking for researchers. He learned that General Electric was closing a laboratory in Idaho Falls where engineers had been working on a defense project to develop a nuclear-powered airplane. The government scrapped the program in 1961, and a handful of highly trained engineers and scientists were suddenly up for grabs.

“GE wanted to see that we got as good a placement as we could,” Rothermel recalls. “So we all wrote resumes, and Jack got hold of these, and he said it was like a Sears and Roebuck catalog of people.” Barrows hired four of the GE scientists: Hal Anderson, a physicist; Stan Hirsh, an electrical engineer; Eric Breuer, a technician; and Dick Rothermel.

Their hiring represented a departure from Forest Service custom. Up until that time, fire research had been pretty much the domain of foresters, who are used to looking at their work through the lenses of biology and silviculture. Gisborne was a forester; Barrows was a forester. But Barrows recognized that fire is a physical process, and that physical scientists and engineers could contribute much to the emerging science of fire behavior.

Rothermel, then barely into his 30s, was glad to join Barrows’s staff. He had a bachelor’s degree in aeronautical engineering from the University of Washington. During the 8 years since he’d graduated, he had worked in the engineering of nuclear systems in Albuquerque and then in Idaho. (Rothermel later went on for a master’s degree in mechanical engineering from Colorado State University.)

“I had the option of staying on [at GE] and working on a lot of programs, but with the cancellation of the atomic-powered airplane, nothing sounded that appealing,” he says. “And then I heard about this laboratory, and they said they had two wind tunnels and a combustion lab where you could control the atmosphere, temperature, and humidity. I thought, “Wow, that’s an opportunity!” Rothermel worked with Hal Anderson to get the new lab’s equipment calibrated and running smoothly. Then they began a set of experiments in the wind tunnel and combustion chamber, testing the effects of wind and moisture on various fuels and determining how fast a fire would spread under different conditions.


Given their training, it made sense to Rothermel and Anderson to approach the task as an engineering problem. Says Rothermel: “The idea was, if we could develop a way of describing the fuels, the weather, the topography, and something about the fire, and be able to put that into what we call a mathematical model, and if we described all these things properly, the model would integrate it and produce answers. It would tell you the resulting fire intensity, rate of spread, flame length, these sorts of things.”

Rothermel, Anderson, and Bill Frandsen, another physicist on the project, adapted an approach developed by an early Forest Service fire researcher, Wally Fons, which turned on the concept of conservation of energy. A fire spreads by igniting a series of little fires in the fuel ahead of it. The ignitions are driven by convection, radiation, and conduction. Even if it’s unknown which mode is operating in a given instance, the rate of heat transfer can be measured. The researchers reasoned that if they knew how much fuel was ahead of a fire, how big and how densely packed the fuel particles were, and how much moisture the fuel contained, then they could figure out how much energy would be needed to transfer enough heat to bring the fuel up to the ignition point. They could then calculate the rate of ignition that would carry the fire as it spread. The model would also have to account for the critical variables of wind speed and slope of the ground.

Because of the limitations of wind tunnels and combustion chambers, the model is forced to make certain assumptions that don’t hold in real life. For example, it assumes that the fuel is continuous and evenly distributed and burns uniformly. It further assumes that the fire is carried primarily by dead plant material and that only moisture will stop it.

The Rothermel model “describes very well a fire burning in a field of wheat,” says Bret Butler, a mechanical engineer at the Fire Sciences Lab whom Rothermel hired in 1992. “As you get further away from that uniformity, the less accurate it becomes.”

More significantly, the researchers had no basis for modeling the endless spatial variability that actually exists in a forest. So there was no way to simulate a fire’s movement through clumpy, discontinuous trees and shrubs. There was also no way to model a crown fire, one that leaves the surface and moves up into the crowns of trees. These were significant and universally acknowledged shortcomings.

Fire research scientists throughout the world are working on developing more accurate surface-fire spread models, but at this point all of them are too complicated to be used in an operational system. The beauty of Rothermel’s model, says Butler, “is that it’s simple—it can be run quickly with a low-capability computer.”

(end of excerpt)

What made me think of Mr. Rothermel was a graphic distributed on Twitter today by the National Weather Service. It is a fancy, colorized version of the figure in his 1972 paper that depicts how heat is transferred in a fire.

wildfire research dick rothermel
Graphic distributed by @NWS that is based on Dick Rothermel’s 1972 paper.

But of course Mr. Rothermel’s contributions are far more complex than this graphic.

Below is a screenshot from his paper where he describes Propagating Flux, just one of many elements of his mathematical fire spread model.

rothermel propagating flux

And here is his summary of equations for the model:

Summary equations Rothermel's 1972 paper fire model
Summary of equations from Rothermel’s 1972 paper.

Epilogue 1: The current administration has expressed a desire to zero-out the budget for the Joint Fire Science Program, the organization that published the 2008 article. 

Epilogue 2: Mr. Rothermel was one of the 655 attendees at the Fire Continuum Conference in Missoula last month. 

Wildfire briefing, June 4, 2014

Up to 20 large air tankers this year

There could be as many as 20 large air tankers on exclusive use contracts this year, the largest number since 2009. This increase follows the downward spiral in the number since there were 44 in 2002. Fire Aviation has the details, but below is a list.

Large air tankers-2014


Galahad fire on the north rim grows to 2,702 acres

The National Park Service is managing, rather than completely suppressing, the Galahad Fire in Grand Canyon National Park. But one thing in their favor is a mile deep gorge, the Canyon itself, that provides a pretty good barrier on the south side. Below is the most current 3-D map — more details are in our main article about the fire.

map of the Galahad Fire
3-D map of the Galahad Fire, looking south at 8:20 p.m. June 4, 2014. Incident Management Team, and Google Earth.

Richard Rothermel receives award

Dick Rothermel receives Ember Award
Dick Rothermel receives Ember Award May 21, 2014 at the Large Fire Conference in Missoula, MT. Photo by Bill Gabbert.

At the Large Fire Conference in Missoula Dick Rothermel received the International Association of Wildland Fire’s Ember Award for Excellence in Wildland Fire Science. It was presented by Dr. Domingos Viegas, who said in part:

…Dick Rothermel started his activity in the field of forest fires in the sixties and was a leading member of a research team working on fire retardants and fire behavior research at the World famous Missoula Fire Research Laboratory for almost thirty years. Dick brought his engineering approach to develop a very innovative and comprehensive research program on fire spread. Based on the past work from his team and from others that preceded him, Dick used his skills and intuition to identify the most relevant factors of fire spread, to formulate quantitative relationships among them in a quite general form and develop the Mathematical Model on Fire Behavior Prediction. The famous report presenting the Rothermel’s Model as it is universally known was published in 1972 as we all know, is possibly one of the most cited documents in forest fire literature. The Rothermel model was incorporated in a consistent system designated Behave that was released in another historical report in 1983.

IAWF scholarships

The International Association of Wildland fire recently announced the award of two scholarships to students studying wildland fire. The recipients were Miltiadis Athanasiou, a PhD student at the National and Kapodistrian University of Athens, Greece, and Trisha Gabbert, a MSc student at the South Dakota School of Mines and Technology, USA. (As far as I know, the author of this artcle is not a relative of Ms. Gabbert.) More information.

More Atlantic articles about wildland fire

On May 28 we pointed our readers to two excellent articles at The Atlantic about wildland fire. One was a very well done video about fire research activities at the Missoula Fire Sciences Laboratory. The other was an article that summarized many of the story arcs about the Yarnell Hill Fire, and was written by Brian Mockenhaupt. 

I met Brian last year a couple of days before the memorial service in Prescott, Arizona for the 19 firefighters who died in the Yarnell Hill Fire. He was beginning to research wildland fire and the issues surrounding the Yarnell Hill fatalities. A former Army infantryman, he had spent  a lot of time in recent years writing about the military and the wars in Iraq and Afghanistan. “Wildfires”, he said, “are a new area for me, and yet I’ve already seen some striking parallels in command and control, and in the after-action review process in looking at what went wrong, whether in battle or fire.”

Having read Wildfire Today, Brian asked if he could interview me. It turned out that we were both going to be at the memorial service, so when it was over, and after I interviewed Tom Harbour, the Director for Fire and Aviation for the U.S. Forest Service, I met with Brian for about half an hour as thousands of mourners filed out of the Tim’s Toyota Center arena.

He said recently that one of the things he remembered about our conversation on that memorable, sad day, was that I told him about firefighters’ “slide files”. Our young firefighters today may not even know what a slide is. Before digital photography there were two frequently used ways to take pictures and and process film. The most common was to create a negative from the film and then make a paper print. The other was to use slide film, which was processed into a positive right on the film, and placed in a cardboard holder. The sides, as they were called, could be put in a slide carousel, a circular tray that could hold up to 140 slides. It could then be projected onto a large screen for an audience. When the El Cariso Hot Shots developed the first comprehensive 32-hour training program for rookie wildland firefighters in 1972, we produced a slide program that was synced to recorded audio on a Wollensak cassette recorder. Later it was converted to VHS video tape, thankfully — because setting up and using the slide projector and the tape recorder was a pain in the ass. Sometimes human error would cause the slides to get out of sync with the audio if they were not started correctly.

What I told Brian about slide files was that firefighters who learn, record mental images or slides of fire situations especially if they were unusual or surprising. They might remember the fuels, weather, and topography, and the fire behavior that was the result. The could record what tactics worked, or didn’t work, or how long it takes to hike up a 100 percent slope when it is 4 p.m. on a hot day and you’re carrying 30 pounds of gear after working hard for 10 hours. Or what the spotting distance would be, or the flame lengths when the fire hits a hand line on a ridge.

Then…. after building up a slide file from thousands of fire events, when a crucial situation is encountered the firefighter can quickly search that data base and hopefully find conditions that match what they have in front of them. Drawing on what happened before, maybe years or decades before, they can make a better prediction about what will happen next, than another firefighter could that had not encountered that “slide”. This, obviously, can reduce the chance of something unpleasant happening to the firefighter and anyone they are supervising. It can also make them more efficient, effective, and productive in their job.

When a firefighter is experiencing a given set of conditions, and especially if it is a tense, crucial situation, they should take a few seconds. Look up, down, and around, and take a slide, then carefully file it away in your carousel. You never know when you might need to replay it.

The reason Brian mentioned to me that I told him about slide files that day in Prescott, was that in researching his latest article about wildland fire, two experienced firefighters at the National Interagency Fire Center in Boise brought up the concept. He wrote about it in How to Read the Mind of a Wildfire, published today in The Atlantic.

It is a very interesting article, and ranges from tree ring research to Harry Gisborne to Richard Rothermel to reductionist fire spread models to physics-based models that require a super computer. Below is an excerpt from the piece:

…“One of the big things about fire behavior is looking at the exceptional events, the 1 in 100,” says Richard Bahr, the head fire-behavior analyst at the National Interagency Fire Center, who was trained by Rothermel in the 1980s. Those outliers have always existed, but are exacerbated today by climate change, bug infestations, invasive grasses, and fires so intense that they create their own weather. “We are living in a time that is unprecedented, with the extremes we’re seeing in temperatures, precipitation, and winds, and with that, the effects are unpredictable,” Bahr says. “If you don’t have that in your slide tray, you aren’t going to believe it.”


In Los Alamos, New Mexico, Rodman Linn showed me the next generation of fire models, which can map out fire not just in the middle of the bell curve, but out on the tails as well, where firefighters more often meet the unexpected. Linn is a researcher at Los Alamos National Laboratory, which primarily develops and studies weapons and their effects for the military. Models of weapon effects and the spread of fire share the same scientific underpinnings. For his doctorate dissertation in theoretical fluid dynamics, Linn built FIRETEC, which uses physics-based models for combustion, heat transfer, and turbulence to simulate the spread of wildfire. Unlike FARSITE, which can run on laptops, FIRETEC requires the supercomputers at Los Alamos National Laboratory.

FIRETEC doesn’t treat fire as a wall moving through a uniform fuel bed, but divides a landscape into cells as small as a cubic meter and approximates the type and condition of fuel, the terrain, and the weather within each small space. One cell might have a tree, while the surrounding cells are grass, or the wind might be blowing south in a cell, but west in an adjacent space and north in another.”

Brian also wrote an article, published May 23, about the 10 members of a fire use module that were overrun by the Little Venus Fire in 2006 and had to deploy their fire shelters. He interviewed some of the firefighters that experienced the event, for the article titled, What It Feels Like to Lie Face Down and Let a Wildfire Burn Over You.

An excerpt:

…[Lathan Johnson, the fire use module leader] did a quick headcount and came up one short. He didn’t know that one of his firefighters had panicked and split off from the group a few minutes earlier. They didn’t have time to look for her: they couldn’t outrun the fire, and if they waited any longer to deploy, they might not have enough time to get under their shelters before the wall of flame washed over them. Between them and the fire was a 30-foot rock face, which would give some shelter from the heat and flame.

“We’re going to deploy here,” Johnson told the crew. For a moment, he saw their fear and disbelief. But then they set to work, reverting to procedures that they knew well from annual training exercises.

Learning fire behavior from the experts

The Wildland Fire Lessons Learned Center is doing great work. I recently ran across a treasure trove of videos that they produced and uploaded to YouTube. Previously the LLC hosted the videos on their own site, but they have revisited that decision and are moving them to YouTube, where they are much more accessible.

The five videos about fire behavior that I embedded here have been on YouTube for one to two months but have only been viewed between 2 and 18 times as of February 3, 2012. These interviews with recognized experts in fire behavior are an attempt to capture and preserve knowledge that may help those coming up through the ranks. Maybe a viewer will learn something that might have taken them years otherwise, or it may prevent a firefighter from making a costly mistake.

Of the 12 videos in this “Learning from the Experts: Fire Behavior” series, I selected these 5 simply because I know or have been taught by these individuals, but check out the other 7 videos also.

In the video below: Richard Rothermel began research and writing on a fire spread model that is still taught in fire management classes today. This model of Rothermel’s is the basis for today’s S-490 class.

In the video below, Rob Seli addresses how to ask the right questions about the modeling tool.

In the video below, Patti Koppenol describes her process of assessing risk.
Continue reading “Learning fire behavior from the experts”

Dick Rothermel’s collaboration with Norman Maclean

Dick Rothermel
Dick Rothermel

The Missoulian has an article about how Norman Maclean, in researching the fire behavior on the Mann Gulch fire in 1979, sought out Dick Rothermel of the Fire Sciences Laboratory in Missoula when Maclean was working on his “Young Men and Fire” book. Here is an excerpt from the article:

Rothermel, an aeronautical engineer, had developed a fire-spread model at the lab in 1972. It’s a model that, while technologically enhanced over the years, remains the engine of tools used to predict fire behavior today.

At first his involvement with Maclean’s book was “something I really didn’t want to do,” Rothermel told a packed room Thursday at the fire sciences lab’s weekly seminar series.

Controversy still swirled around the Mann Gulch fire, in which 13 firefighters died, and he had no desire to reopen emotional wounds. But the tragedy was what spawned establishment of the research lab itself, which recruited Rothermel shortly after it opened in 1960.

“We set up a communication that went on for several years while (Maclean) was back at the University of Chicago, and when he’d come to Seeley Lake and hang out, he’d come and see us,” Rothermel said.

They went to work on Mann Gulch questions that Maclean felt remained unanswered. How did the fire near the Missouri River north of Helena get from a ridge above the firefighters to the mouth of the gulch below? Where did the men go and why couldn’t they escape? Did the escape fire that saved foreman Wag Dodge’s life overtake his own crew?

“We never could get it straight, in (Maclean’s) mind anyway, as to just what happened until finally I worked out a diagram,” said Rothermel.

The graph shows the rate of spread of the fire and the rate of travel of the men, and how “they finally meet in a race that couldn’t be won,” he said. Maclean used it in “Young Men and Fire.” In 1993, the Forest Service published the chart along with Rothermel’s own assessment of the day in a 10-page pamphlet called “Mann Gulch Fire: A Race That Couldn’t Be Won.” Rothermel has high praise for Maclean’s work, calling it “an almost poetic rendition of what happened that day.”

“Norman was kind of a feisty little guy, and he was an English professor,” Rothermel said, recalling the days of scientific discussion with Maclean and fellow fire scientist Frank Albini.

“Norman would look at us and we’d get into ‘rate of spread’ and ‘flame lengths’ and ‘heat content,’ and pretty soon his eyes would glaze over. He’d start saying how strong these young men were. His main thought in this book was the young men themselves, and the tragedy that occurred.”

Rothermel retired from the fire lab 15 years ago. When Norman Maclean died in 1990 the book that he had worked on for 14 years was still not finished, but his son, John N. Maclean completed some editing on the book and it was published in 1992.