Canadian Fire Weather Index (FWI) System
The standardized empirical model used across Canada and internationally to rate fire danger based on noon meteorological observations.
How It's Calculated — The FWI Chain
Four weather inputs feed into moisture codes, which combine into fire behaviour indices, producing the final FWI danger rating.
WEATHER INPUTS (measured at noon)
MOISTURE CODES
FFMC
Fine Fuel Moisture Code
Moisture in litter & fine fuels — predicts ease of ignition
0 – 101
DMC
Duff Moisture Code
Moisture in loosely compacted decomposing matter — medium-depth duff
0 – 300+
DC
Drought Code
Deep organic layer moisture — indicates long-term drought & smouldering potential
0 – 800+
FIRE BEHAVIOUR INDICES
ISI
Initial Spread Index
FFMC + Wind → expected rate of fire spread
0 – 50+
BUI
Buildup Index
DMC + DC → total fuel available for combustion
0 – 200+
FINAL OUTPUT
FWI
Fire Weather Index
ISI + BUI → overall fire intensity rating. The single number decision-makers rely on to judge current fire danger.
0 – 100+
Danger Rating Scale
0 – 4
🟢 Low
Fires unlikely to start or spread. Most ignitions self-extinguish.
5 – 11
🔵 Moderate
Fires start easily from accidental causes. Creeping or gentle surface fires.
12 – 21
🟡 High
Forest fuels ignite easily. Moderate-intensity fires with intermittent crown involvement.
22 – 37
🟠 Very High
High-intensity fires. Rapid spread, crowning, and spotting expected.
38+
🔴 Extreme
Explosive fire growth. Extreme behaviour including fire whirls and long-range spotting.
Data Source & Literature
FWI data on the Detection Map is sourced from the BC Wildfire Service SFMS (Spatial Fire Management System) API, which provides daily FWI grid values across British Columbia derived from Environment and Climate Change Canada weather observations.
The FWI System was developed by the Canadian Forest Service and is described in: Van Wagner, C.E. (1987). Development and Structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35. Canadian Forestry Service, Ottawa.
