How do wildfires spread?
A dropped cigarette on a dry hillside can become a fire front that outruns you uphill. Pull a forest apart into its fuel layers, cook a twig until it catches, light a hillside and steer the wind and slope, then try to stop the fire before it reaches the village.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A forest ready to burn. To a fire, a forest is fuel in layers: duff and roots, leaf litter and dry grass, shrubs that act as ladders, and the canopy. A fire needs heat, fuel and oxygen together (the fire triangle). How it behaves depends on fuel, weather and terrain. Dead leaves and needles follow the air: on a hot, dry afternoon they fall to a few per cent moisture within an hour.
- How wood catches. Wood does not burn directly. Heat first boils off its water, and the temperature stalls at 100 °C until the water is gone. Then the dry wood breaks down into flammable gases (pyrolysis), and at roughly 320 °C those gases catch. Dry fuel needs about 700 kJ per kilogram to light and living green fuel several times more. Thin fuel heats in seconds, so fires start in grass and needles.
- How fire travels. A fire front moves by heating the fuel ahead of it: by radiation, by hot gas that wind bends forward, and by embers that start spot fires. Wind can make a fire ten or more times faster and stretches it into an ellipse with a fast head and a creeping back. Fire runs uphill because flames lean into the slope and preheat it: roughly double the speed for every 10° by a common rule of thumb.
- Ground, surface and crown fires. Surface fires burn litter and grass, and most forest fires in India are of this kind. Ground fires smoulder in duff and peat at a few centimetres an hour, for weeks. Crown fires burn the canopy and cannot be fought head-on. A surface fire climbs into the crowns when it is intense enough to reach the lowest branches, which ladder fuels make easy. The biggest fires spin up fire whirls and build their own thunderclouds.
- Fighting fire, and using it. Firefighters mostly take away fuel. A fire line of bare soil stops a gentle fire, a backburn widens it, and helicopter water drops buy time. Once flames are longer than about 2.4 m the head cannot be held, so crews work the flanks and wait for the weather. Controlled burns in cool weather remove litter before summer, and some ecosystems need fire to renew.
- India's fires and staying safe. The Forest Survey of India says over 36 % of India's forest cover is prone to frequent fires and that almost all are started by people. It has sent satellite fire alerts since 2004. Never burn waste or drop a cigarette near a forest in the dry season, and report smoke to the forest department, 101 or 112. If a fire is below you, do not flee uphill: move across the slope or downhill, out of its path.
| Term | Meaning |
|---|---|
| Fire triangle | Heat, fuel and oxygen: a fire needs all three, and stops when one is taken away. |
| Fire-behaviour triangle | Fuel, weather and terrain: the three things that decide how fast and how fiercely a wildfire burns. |
| Fuel moisture | The water in a fuel as a percentage of its dry weight. Dead litter can fall below 5 %; this model's litter will not carry fire above about 25 %. |
| Pyrolysis | Heat breaking wood into flammable gases and char. The flame is those gases burning. |
| Rate of spread | How fast a fire front moves over the ground, in metres a minute. Wind and upward slope multiply it. |
| Spotting | Embers carried ahead of a fire by its plume and the wind, starting new fires where they land. |
| Surface, ground and crown fire | Fire in the litter and grass, fire smouldering below the surface, and fire in the treetops. |
| Ladder fuel | Shrubs, climbers and young trees that carry flames from the ground up into the canopy. |
| Fire line and backburn | A strip cleared to bare soil to starve a fire, and a fire lit on purpose from that line to widen it. |
From people who burned the land with care, to satellites that spot a forest fire and text the nearest guard.
- 1871 · The Peshtigo fire (People of Peshtigo and nearby towns, Wisconsin, USA)
- 1910 · The Big Burn (US Forest Service crews, including ranger Ed Pulaski, Idaho and Montana, USA)
- 1972 · Rothermel's fire-spread model (Richard C. Rothermel, Missoula, Montana, USA)
- 1982 · The Bambi Bucket (Don Arney, SEI Industries, Delta, British Columbia, Canada)
- 1988 · Yellowstone burns, and recovers (US National Park Service, Yellowstone National Park, USA)
- 1999 · MODIS starts spotting fires from orbit (NASA, Vandenberg, California, USA)
- 2004 · India's forest fire alerts begin (Forest Survey of India, Dehradun, India)
- 2009 · Black Saturday (People of Victoria, Victoria, Australia)
The full story, with 30 moments, charts, people and 47 sources: glassbox.how/e/wildfireclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
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|---|---|---|
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glassbox/video.mp4 |
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Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
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js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
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js/firephys.js |
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js/fire.js |
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