A System for Fire Danger
The Canadian Forest Fire Weather Index System, developed by C.E. Van Wagner over decades of research, distills the complex relationship between weather and fire behavior into a elegant hierarchy of six components. From four simple noon weather observations — temperature, relative humidity, wind speed, and 24-hour rainfall — the system tracks fuel moisture at three timescales and predicts fire behavior through three indices, culminating in the composite FWI number.
Moisture Codes
The three moisture codes track fuels at different depths and response times. The Fine Fuel Moisture Code (FFMC) represents surface litter moisture with a timelag of about 2/3 of a day — it responds to this afternoon's weather. The Duff Moisture Code (DMC) tracks loosely compacted organic matter with a roughly two-week response. The Drought Code (DC) represents deep compacted organic layers with a 52-day timelag, capturing seasonal drought that persists through brief rain events.
Fire Behavior Indices
The Initial Spread Index (ISI) combines FFMC with wind speed to predict fire spread rate — it responds exponentially to wind when fuels are dry. The Buildup Index (BUI) combines DMC and DC to estimate total fuel available for combustion, capturing whether a fire will burn intensely into deep organic layers or remain superficial. The final FWI combines ISI and BUI into a single number proportional to fireline intensity.
Global Standard
Though developed for Canadian boreal forests, the FWI system has been adopted as the global standard by the European Forest Fire Information System (EFFIS), used across Southeast Asia for haze prediction, and applied in South America, Australia, and Africa. Its success stems from capturing universal physics — the same relationships between humidity, temperature, wind, and fuel moisture govern fire behavior in ecosystems worldwide, requiring only regional calibration of danger class thresholds.