Why Fires Move Faster Up a Hill than Down
Paradise, California's Camp Fire accelerated as it moved uphill. U.S. Department of Agriculture scientists conduct an experiment to discover why.
Unlike the humans that fight them, wildfires move much faster uphill. On a slope, air rushes in more easily from the downhill than the uphill side. If the slope is sufficiently steep, all of the air may come in from the downhill side, pushing the flames into the slope and putting them in contact with more fuel.
Why Fires Move Faster Up a Hill than Down
Published: May 6, 2019
Mark Finney: This entire building is devoted to fire. It’s actually a pretty complex phenomenon. It’s not something that you can just observe it and figure out how it works.
Narrator: Forest Service scientist Mark Finney showed us how they unlock the mysteries of fire.
Finney: So, this is the burn chamber right here. Everything we do in here is designed to look at how fires behave.
Narrator: On this day, he and a dozen of his colleagues are preparing for a test on a large tilting burn bed of precisely cut cardboard.
Finney: What we can do is put these on the burn bed in any kind of density that we’d like, to engineer whatever kind of fire we’d like: how long we want it to burn, how long the flames are, how fast it spreads.
Narrator: The burn table brims with instruments that measure pressure and temperature, taking samples 500 times a second. And there are cameras everywhere.
Today, they are trying to understand more about how a fire spreads uphill, as it did near Paradise.
Finney: Okay, go ahead. Line of fire guys, good line. Okay.
Narrator: The flames are the visible sign of rapid oxidation, or fire. Fire requires dry flammable material, or fuel, oxygen and a heat source to create this chemical chain reaction. Wildfires add two additional elements, weather and topography, that determine how the flames will grow and spread.
Finney: The reason we’re measuring this is that you can actually get fires to accelerate extremely quickly going uphill.
Narrator: Unlike the humans that fight them, wildfires move much faster uphill.
Finney: When you have a slope, you can’t get air in from the uphill side as easily as you can get in from the downhill side. If the slope is sufficiently steep, all of the air is coming in from the downhill side.
Narrator: With air fanning the fire exclusively on the downhill side, the flames get pushed into the slope, putting them in contact with more fuel. The tilted, climbing fire transfers a lot of heat to the trees and brush ahead of the flames, preheating and drying them, making them more combustible.
Finney: You get very, very effective heat transfer and a very much faster spreading fire.
Narrator: This is what happened when the Camp fire started. The wind funneled the flames into a steep canyon, and they rapidly accelerated uphill, right toward Paradise.
PRODUCTION CREDITS
Inside the Megafire
Produced by: Will Toubman
Produced and Edited by: Brian Truglio
Produced and Directed by: Miles O'Brien
Digital Producer: Ari Daniel
© WGBH Educational Foundation 2019
