The forest floor is a grid of fuel cells. Each step, a burning cell has a chance to ignite each unburned neighbour; that chance scales with the neighbour's own fuel load, drops sharply with fuel moisture (wet fuel absorbs heat instead of igniting), and is boosted downwind — real ground fires race with the wind and creep against it. A cell with a heavy fuel load burns hotter and longer; if the local fireline intensity crosses a threshold it "torches" into the canopy (shown in red) instead of staying a low, easily-managed ground fire.
P(ignite) = k · fuel_neighbour · wind_factor(θ) · (1 − moisture/40)
duration ∝ fuel_neighbour (heavier fuel = longer, hotter burn)
- Control line — clears a strip of cells down to bare mineral soil (no fuel), the standard tool for stopping a spreading fire or bounding a burn unit.
- Prescribed burn — deliberately ignites a strip under mild, chosen conditions (usually higher moisture, lower wind) so it burns off accumulated fuel in a controlled low-intensity pass, reducing the fuel available to a future wildfire.
- Fuel moisture — the seasonal/weather variable land managers watch most: prescribed burns are timed for moisture high enough to keep intensity low and containment easy, but low enough that the fuel still carries a flame.
- Wind — elongates the fire into a narrow, fast-moving head fire downwind, with slower flanks and a nearly stationary back fire upwind — try turning the direction slider and watch the burned-area shape stretch.
Real-world relevance: this is the core logic behind fire-danger rating systems and prescribed-burn planning — the same fuel bed can be a manageable, ecologically beneficial burn on a moist, calm day, or an uncontainable wildfire on a dry, windy one.