The Rothermel Equation
Richard Rothermel's 1972 model remains the foundation of wildfire behavior prediction in the United States and beyond. The model decomposes fire spread into reaction intensity (heat released per unit area of fuel bed), propagating flux (fraction of heat directed ahead), and heat sink (energy needed to bring adjacent fuels to ignition temperature). Wind and slope enter as multiplicative factors that tilt the propagating flux forward.
Wind and Slope Coupling
Wind is the dominant driver of surface fire spread. The model's wind factor increases nonlinearly with midflame wind speed, reflecting how turbulent convection efficiently preheats fuels downwind. Slope acts similarly — uphill spread accelerates because flames lean into unburned fuel above, while downhill spread is retarded. On steep terrain with aligned wind, the combined effect can produce spread rates an order of magnitude faster than calm, flat conditions.
Fuel Moisture and Ignition
Fuel moisture content is the primary inhibitor of fire spread. The Rothermel model computes a moisture damping coefficient that reduces reaction intensity as moisture increases, reaching zero spread at the moisture of extinction (typically 12-40% depending on fuel type). Live fuel moisture adds additional complexity, as living vegetation can either resist or promote fire depending on seasonal drying cycles.
From Spread Rate to Fire Behavior
Spread rate alone doesn't capture fire danger. Byram's fireline intensity combines spread rate with fuel consumption to yield heat release per unit fire front length — the key metric for suppression difficulty. Flame length, derived from intensity via Byram's equation, determines whether hand crews, engines, or only aerial resources can engage the fire. This simulation links all three quantities so you can explore the full chain from fuel conditions to operational fire behavior.