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Canadian Forest Fire Danger Rating System

Canadian Forest Fire Weather Index (FWI) System

The Canadian Forest Fire Weather Index (FWI) System in its current form has been in use across Canada for the past 30 years. The system uses daily weather observations to estimate the moisture content of three different fuel classes and uses these to generate a set of relative indicators of potential rate of fire spread, fire intensity, and fuel consumption.

The calculated components of the FWI System are based on daily measurements of dry-bulb temperature, relative humidity, 10-meter open wind speed, and 24-hour accumulated precipitation, all recorded at noon local standard time. Although based on noon measurements, the fuel moisture codes and related fire behavior indices of the FWI System are considered representative of the daily peak in fire danger, generally considered to be around 1600.

The three fuel moisture models within the FWI System are in fact dynamic bookkeeping systems that keep track of the drying and wetting of three distinct fuel layers in the forest floor. Although the system models actual moisture in these fuels, the codes output by the system have been developed such that rising code values indicate decreasing levels of actual fuel moisture.

The FWI System was formally laid out in its current form by Van Wagner in the 1987 Canadian Forest Service (CFS) publication FTR-35, The Development and Structure of the Canadian Forest Fire Weather Index System.

Fine Fuel Moisture Code

The Fine Fuel Moisture Content FFMC) represents the moisture of litter and other cured fine fuels in a closed forest stand. The nominal depth of this fuel layer is 1.2 cm, and the nominal fuel load is 0.25 kg/m².

Duff Moisture Code

The Duff Moisture Code (DMC) represents moisture in the layer of loosely compacted decomposing matter on the forest floor. The nominal depth of this layer is approximately 7 cm with a dry fuel load of 5 kg/m².

Drought Code

The Drought Code (DC) represents the moisture in deep, compact organic matter with a nominal depth of about 18 cm and a dry fuel load of 25 kg/m².

Initial Spread Index

The Initial Spread Index (ISI) is a combination of wind speed with the moisture content of the fine fuel layer and is a general indicator of rate of fire spread.

Build-Up Index

The Buildup Index (BUI) is a combination of the fuel moisture components (DMC and DC) that have a longer response time and is an indicator of the total fuel available for combustion.

Fire Weather Index

The Fire Weather Index (FWI) is a combination of generalized spread (ISI) and consumption (BUI) indicators and, as such, is a relative estimate of the potential intensity of a fire.

The FWI system was formally laid out in its entirety by Van Wagner in the 1987 CFS publication FTR-35, The Development and Structure of the Canadian Forest Fire Weather Index System.

Structure of the FWI System

The diagram below illustrates the components of the FWI System. Calculation of the components is based on consecutive daily observations of temperature, relative humidity, wind speed, and 24-hour rainfall. The six standard components provide numeric ratings of relative wildland fire potential.