Fire Weather Index (FWI) - Anglais

What is FWI/IFM?

The Fire Weather Index (FWI) is a meteorological index developed in Canada to assess fire danger based on atmospheric conditions. It is based on physical principles and is designed to provide a consistent assessment of fire risk across the country. The FWI is widely used internationally for fire danger forecasting.

What parameters does it take into account?

The FWI is based on 4 main components:
  • ISI (Initial Spread Index) : assesses the initial rate of fire spread, dependent on wind speed and humidity of the FFMC.
  • FFMC (Fine Fuel Moisture Code): assesses the moisture content of fine surface fuels (grasses, dry needles), influencing ignition and initial fire spread.
  • Wind
  • BUI (Build-Up Index) : assesses the fuel loading potential, combining DMC/IH (Humus Moisture Index) and DC/IS (Dryness Index).
  • DMC (Duff Moisture Code) : evaluates the moisture content of moderately deep organic layers, indicating the ability of a fire to smolder below the surface.
  • DC (Drought Code) : assesses the moisture content of deep layers of organic soils, reflecting prolonged droughts.
These indices are calculated from daily meteorological data, including temperature, relative humidity, wind speed and 24-hour precipitation.

Index scale:

The FWI is a unitless numerical index, typically ranging from 0 to 180, although there is no fixed upper limit. Specific thresholds may be defined based on region and season to indicate hazard levels.
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  • It is based on simple, observable and predictable meteorological variables.
  • Allows for a uniform assessment of fire danger across different regions.
  • Does not take into account topography (slope, exposure).
  • Does not take into account the type of local vegetation, except by calibration.
  • May overestimate the risk immediately after rain if the wind remains strong.
  • It takes into account many factors, and therefore it is difficult to understand the final result.
  • The index becomes saturated when there are extreme values.

Please note that the displayed FWI is calculated from the meteorological parameters selected by the user (temperature, humidity, wind, precipitation), including those provided by Météo-France.
However, we do not directly use the FWI values ​​calculated by national authorities. Therefore, there may be slight differences—generally less than 15%—between the visible FWI components and those displayed on the government portal.


Scientific references:

  • Van Wagner, C. E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Canadian Forestry Service, Technical Report 33.
  • Natural Resources Canada. (n.d.). Canadian Forest Fire Weather Index (FWI) System. Retrieved from  Canadian Wildland Fire Information System | Canadian Forest Fire Weather Index (FWI) System 
  • Climate Data Guide. (n.d.). Canadian Forest Fire Weather Index (FWI). Retrieved from  Canadian Forest Fire Weather Index (FWI) | Climate Data Guide