But Latham and other researchers have for years mapped, modeled, and studied blazes in an effort to learn what makes them rage. Fires come in three strengths, each phase an escalation of the last. Ground fires creep through the organic debris and decomposing litter in the soil, usually causing the least overall damage, while surface fires clear loose shrubbery and detritus from the forest floor. The dead stuff goes first, and then when all the moisture is cooked out of the green wood, it goes too. Treetops can explode as the sudden intense heat ignites the gases inside them.
Winds can carry embers miles ahead of the advancing flames.
Crown fires typically travel at one to five miles per hour, but a steep hillside and a strong wind can turn one into a 15-mph blaze, often belching out a downdraft that sounds like a freight train. Some U.S. forests have adapted to devastating crown fires that sweep through once every 100 years or so. Forest Service scientists have in recent decades developed computer programs that take into account such factors as terrain, wind, fuel, and elevation to plot a blaze's expected course. But out on the front lines, a raging crown fire is still difficult to predict. Although it may take centuries for the land to return completely to its pre-fire state, parts of the forest start springing back to life right away.