The Physics of Wildfire Spread

The fire triangle and how wood actually burns

Fire needs three things at once — fuel, oxygen and heat — and removing any one puts it out, which is exactly how firefighting tactics work: water removes heat and moisture, firebreaks remove fuel, retardants deny oxygen contact. Wood itself does not burn directly; it first undergoes pyrolysis, thermal decomposition at 200–300°C that releases flammable gases (CO, hydrocarbons, H₂) and leaves charcoal behind — it's the gases that burn. Moisture content is the single biggest predictor of fire behaviour: fuel at 30% moisture barely ignites, fuel at 4% moisture ignites explosively, because every percent of water must be evaporated before combustion can start.

How heat reaches the next tree

Fire spreads because heat reaches unburned fuel ahead of the flame front through two dominant mechanisms. Radiation — infrared from the flame heating and drying nearby fuel — scales with temperature to the fourth power, so even a small increase in flame temperature dramatically raises radiant flux; it dominates in dense, calm-wind forests. Convection — the rising hot-gas column bending forward in wind to pre-heat fuel ahead — typically accounts for 50–70% of spread rate in most fires. Conduction, by contrast, is slow and largely irrelevant to wildfire behaviour.

Rothermel rate of spread (1972):
R = I_R · ξ · (1 + φ_W + φ_S) / (ρ_b · ε · Q_ig)
  I_R  = reaction intensity     φ_W, φ_S = wind & slope multipliers
  ξ    = propagating flux ratio  ρ_b·ε·Q_ig = heat of ignition per volume

Slope, wind, crown fires and spotting

Wind bends flames toward unburned fuel, replenishes oxygen at the flame base, dries fuel on the windward side, and carries burning embers kilometres ahead — spotting. Slope creates the same geometry without any wind at all: fire running uphill has its flame naturally inclined over the fuel above, so a 20° slope roughly doubles spread rate and 45° slopes can multiply it tenfold; slope aligned with wind is the most dangerous combination. Surface fires that reach a "ladder" of young trees or dense shrubs can climb into the canopy as a crown fire, spreading 2–10× faster than a surface fire and becoming nearly impossible to suppress directly. During Australia's 2009 Black Saturday fires, embers were spotted up to 35 km ahead of the main front, which itself moved at up to 25 km/h.

Fire weather and a warming climate

Fire-danger indices combine temperature, humidity, wind and drought; the most dangerous conditions worldwide share a signature — hot (>35°C), dry (<20% relative humidity) and windy (>40 km/h), whether it's California's Diablo/Santa Ana winds or the hot northerly ahead of a cold front in south-eastern Australia. Climate change stretches these conditions in three ways: higher temperatures raise the vapour-pressure deficit that dries vegetation (each 1°C of warming roughly doubles burned area in fire-prone regions), earlier snowmelt and later autumn rain extend the fire season by weeks per decade, and accumulating multi-year droughts dry out deep fuels that normally resist ignition. Australia's 2019–2020 "Black Summer" burned 24 million hectares — an area comparable to the entire United Kingdom — and attribution studies found it 30% more likely because of anthropogenic warming.

Frequently asked questions

Try it live

Everything above runs in your browser — open Forest Fire Spread, ignite a cell, and watch a cellular-automaton wildfire cross a forest as you tune density, ignition probability and regrowth. Nothing is installed, nothing is uploaded.