Fire Danger Rating System

A fire danger rating system converts daily weather, fuel, and topography information into operational indices that summarize how easily wildfires will ignite, spread, and burn intensely. Two systems dominate worldwide practice: the U.S. National Fire-Danger Rating System (NFDRS), documented by Deeming, Burgan, and Cohen, and the Canadian Forest Fire Weather Index (FWI) System, whose structure Van Wagner formalized in 1987. Both begin by tracking the moisture content of fuels of different sizes — fine fuels respond to weather within hours, while heavy logs and deep duff respond over weeks — and then feed these moisture estimates, together with wind and fuel characteristics, through a chain of subindices that estimate rate of spread, fuel consumption, and fire intensity. The end product is a small set of numbers and danger classes (from Low to Extreme) that agencies use to set preparedness levels, issue public warnings, position resources, and impose restrictions.