The Canadian Forest Fire Weather Index (FWI) System
The Canadian Forest Fire Weather Index (FWI) System consists of six components that account for the effects of fuel moisture and wind on fire behavior.
The first three components are fuel moisture codes and are numerical ratings of the moisture content of litter and other fine fuels, the average moisture content of loosely compacted organic layers of moderate depth, and the average moisture content of deep, compact organic layers.
The remaining three components are fire behavior indexes which represent the rate of fire spread, the fuel available for combustion, and the frontal fire intensity; their values rise as the fire danger increases.
Fine Fuel Moisture Code
The Fine Fuel Moisture Code ( FFMC )is a numerical rating of the moisture content of litter and other cured fine fuels. This code is an indicator of the relative ease of ignition and flammability of fine fuel.
Duff Moisture Code
The Duff Moisture Code ( DMC ) is a numerical rating of the average moisture content of loosely compacted organic layers of moderate depth. This code gives an indication of fuel consumption in moderate duff layers and medium-size woody material.
The Drought Code ( DC ) is a numerical rating of the average moisture content of deep, compact, organic layers. This code is a useful indicator of seasonal drought effects on forest fuels, and amount of smouldering in deep duff layers and large logs.
Initial Spread Index
The Initial Spread Index ( ISI ) is a numerical rating of the expected rate of fire spread. It combines the effects of wind and the Fine Fuel Moisture Code on rate of spread without the influence of variable quantities of fuel.
The Buildup Index ( BUI ) is a numerical rating of the total amount of fuel available for combustion that combines the Duff Moisture Code and the Drought Code.
Fire Weather Index
The Fire Weather Index ( FWI ) is a numerical rating of fire intensity that combines the Initial Spread Index and the Buildup Index. It is suitable as a general index of fire danger throughout the forested areas of Canada and Eurasia.
Structure of the FWI System
The diagram below (Fig.1) illustrates the components of the Canadian Forest Fire Weather Index 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 numerical ratings of relative wildland fire potential.
Figure 1. Structure of the FWI System.
Source: Natural Resources Canada, Canadian Forest Service
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