Home Ignition Zone — Wind-Driven Spread & Firebrand Ballistics (2D)

Approaching Wildfire

Wind speed 8 m/s

Fire line intensity 1200 kW/m

Defensible Space Zones

Zone 1 clearance (1.5–9 m) 40%

Zone 2 clearance (9–30 m) 20%

Live Readouts

Nearest front distance

45.0 m

Embers landed / spot fires

0 / 0

Structure ignition probability

0.0%

Head-fire vs backing-fire front

45.0 m / 45.0 m

How this 2D model differs

The 3D scene collapses the whole wildfire perimeter to one shrinking radius. This top-down 2D model instead runs a genuine spatial perimeter split into 72 independent angular sectors, so the front's shape emerges from wind and fuel rather than being drawn as a uniform ring — and every firebrand is a separately integrated projectile, not a canned travel formula.

Per-sector spread rate (anisotropic, wind-driven):
  alignment = −cos(φ)                (φ = sector's compass angle)
  windFactor = max(0.05, 1 + (wind/10)·alignment)
  R(φ) −= R0·windFactor·(1−clearance)·(I/1200)·dt

Firebrand vertical fall (quadratic drag → terminal velocity vT):
  dw/dt = g − (g/vT²)·w²            (w = downward speed)
Firebrand horizontal drift (relaxation toward wind speed):
  dvh/dt = (wind − vh)/τ

Flame length (Byram, 1959):  Lf = 0.0775·I^0.46
Loft height:                 H  = 3·Lf
Launch speed:                 vz0 = √(2g·H)

Head / flank / backing fire — the sector directly upwind of the house (alignment ≈ +1) is a head fire pushed by the wind and closes fastest; the downwind sector (alignment ≈ −1) is a backing fire crawling against the wind, exactly as real fire behavior guides describe.
Firebrands — launched from the upwind arc of the burning perimeter with a plume speed set by Byram's flame-length formula, then flown frame-by-frame under real gravity, quadratic air drag (settling to a terminal fall speed) and wind-relaxed horizontal drift — so spotting distance genuinely grows with both wind speed and fire intensity instead of being read off a lookup formula.
Zone 1 / Zone 2 clearance and Zone 0 toggle work exactly as in the 3D model: they set local fuel factor (1 − clearance) at whatever point the front or a landed ember currently occupies.
Ignition probability still follows the same exponential survival model, fed by radiant flux from the closest point of the (now irregular) front and by embers landing inside each zone.

wind →

Active fire front Cleared (treated) zone Fuel-heavy (untreated) zone Firebrand in flight