Weather
Parameters
Wind calm mean speed
8
Average daily wind on a normal map (classic and large). A light breeze that stays below WIND_FIRE_CALM_SPEED, so it never affects fire — wind is present everywhere, but only Velaria’s is strong enough to bite.
Source: NOAA NCEI — Comparative Climatic Data, Average Wind Speed at US cities
Wind calm speed sd
4
Day-to-day spread of the daily mean wind on a normal map. With the cap at WIND_FIRE_CALM_SPEED, calm maps drift between still air and a gentle breeze and never cross the line where wind drives fire.
⚠️ Source pending
Wind windy mean speed
24
Average daily wind speed on the windy river map (Velaria). Set just above the long-run means of the windiest major US cities (Dodge City ~13.9 mph ≈ 22 km/h, Amarillo ~13.5 mph) so the map reads as genuinely, dangerously breezy rather than merely above-average. It is ~3x the calm-map mean and, unlike it, sits above the fire threshold.
Source: NOAA NCEI — Comparative Climatic Data, Average Wind Speed at US cities
Wind windy speed sd
9
Day-to-day spread (standard deviation) of the daily mean wind around WIND_WINDY_MEAN_SPEED. At ~9 km/h the map sees calm-ish days back down near a light breeze (~12 km/h) and gusty, gale-leaning days past 40 km/h — the wide day-to-day swing real daily wind records show. This is the long-run spread; wind.ts shrinks the per-day random kick to account for the persistence below so the realized spread still lands here.
Source: NOAA NCEI — Comparative Climatic Data, Average Wind Speed at US cities
Wind windy max speed
60
Upper clamp on the windy map’s daily wind so the random walk stays realistic (a strong gale, not a hurricane). The fire multiplier is separately capped by WIND_FIRE_MAX_MULTIPLIER, which binds first — speeds past ~48 km/h already sit at the ceiling — so this mostly just keeps a displayed wind speed sane.
⚠️ Source pending
Wind daily persistence
0.7
How strongly one day’s wind carries into the next — the lag-1 autocorrelation of daily mean wind speed. Real daily wind series are strongly autocorrelated (windy days cluster), so the model is an AR(1) process: each day reverts toward the regime mean but keeps ~70% of yesterday’s departure from it. This is what makes the wind evolve from the previous value instead of jumping around as white noise.
Wind fire calm speed
12
Wind at or below which there is no effect on fire (the multiplier is 1), and the speed a normal map’s wind is capped at. ~12 km/h is the top of a Beaufort ‘light breeze’; below a gentle breeze the wind doesn’t measurably push a fire front, and the empirical ‘forward rate of spread ≈ 10% of wind speed’ rule only takes over once wind is actually driving the fire.
Wind fire spread doubling speed
18
Extra wind (above WIND_FIRE_CALM_SPEED) that DOUBLES a fire’s chance to start and to spread. McArthur’s forest fire-danger meter roughly doubles fire danger / rate of spread for every ~18-20 km/h of added wind, so the multiplier is 2^((wind - WIND_FIRE_CALM_SPEED) / 18): ~1.6x at the windy map’s 24 km/h average, ~2.9x on a 40 km/h gusty day.
Source: Noble, Bary & Gill (1980) — McArthur’s fire-danger meters expressed as equations
Wind fire max multiplier
4
Hard ceiling on the wind fire multiplier. Past roughly gale force the real rate of spread stops rising and can even fall — the flame is bent flat and the front disrupted (McArthur’s data rolls over near 11 m/s ≈ 40 km/h), and very strong winds also suppress new ignitions. Capping at 4x keeps a windy day dangerous without spawning a runaway firestorm when a rare gust stacks on top of a heatwave.
Source: USDA Forest Service — Exploring fire response to high wind speeds: rate of spread and ignition