How Stuff Burns
It's 90 degrees in the shade; at 20 percent relative humidity, throats instantly feel dry.
A test burn in the Fire Lab's wind tunnel.

A tiny spark flares in the deep pine needles. Fanned by an unceasing 10-mph wind, the spark becomes a frisky curtain of flame smoking along at more than 2 feet a minute. A few moments later a sooty steel door opens and Greg Lovellette pokes his head through. "You can come out now," he says. I climb out of the world's only forest fire research wind tunnel smelling like a weenie roast and glad to get some fresh air.

Greg set this fire in a carefully-measured batch of pine needles to test new fire retardants here at the Rocky Mountain Research Station's Fire Science Lab. From the borax first tried in the 1950s to the advanced gels and foams Greg is testing, a fire retardant really can do just one of two things: cool the fuel or deprive the flames of oxygen.

This is the the "fire triangle," the central equation of fire fighting, at work in the real world:

Remove or weaken any one of these elements and the fire collapses like a two-legged stool. Each part affects the other -- very dry grass doesn't take much heat to ignite, while wetter grass or brush will only burn after they get hot enough to dry out.

Running on Fumes

We normally think of the fuel as something solid, like a tree trunk, that catches on fire when it gets hot enough. But that's not quite true -- and what it means to "catch on fire" is complicated.

As the fuel heats up, it gives off flammable gases that react with the oxygen in the air to form carbon dioxide and water. A candle flame is a burning bubble of vaporized wax, and a tree trunk is basically a really big candle. So it's not the solid part of the fuel that burns at all.

Since burning is about moving gases, fire behavior on a large scale is one of the messiest physics problems around.

An active crown fire by night.

Bret Butler knows this all too well. He started out studying coal burning in power plants and is now revising the fire lab's fire-spread model to account for the basic physics of burning -- such as how the heat produced by wildfires affects their behavior.
Bret says all flames in a wildfire are about the same temperature -- around 1,800 degrees Fahrenheit. Big fires feel hotter because more flames create more total heat. One of the things he wants to know is how much heat is needed for a fire to change from a relatively manageable ground fire to a much more intense crown fire.

Baked Canada

Bret got his closest look yet at a crown fire during a first-time-ever experiment in Canada's Northwest Territories in 1997. In three separate 5-acre stands of pine and spruce, researchers set up heat sensors, high-speed movie cameras, video cameras, infrared imagers, smoke sampling devices and even some small structures. They then torched the trees with a flame-thrower.

Even the world's leading fire experts were surprised by how hot it got. Bret's instruments recorded over 100 times the heat you'd feel on a sunny tropical beach. One entire stand burned in a matter of minutes.

Bret, meteorologist Larry Bradshaw, and others are using the results to build computer simulations to aid firefighters in predicting how their blaze will act. Further experiments will help them match their computer-generated results with real-world measurements. "People make decisions based on these things and their lives depend on it," Larry says. "You can't just throw in a weather model because it makes nice pictures."