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The Fire Weather information is based on
the Canadian Forest Fire Weather Index (FWI) System and 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
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This is a numerical rating of the moisture content of surface litter and other cured fine fuels. It shows the relative ease of ignition and flammability of fine fuels. The moisture content of fine fuels is very sensitive to the weather. Even a day of rain, or of fine and windy weather, will significantly affect the FFMC rating. The system uses a time lag of two-thirds of a day to accurately measure the moisture content in fine fuels. The FFMC rating is on a scale of 0 to 99. Any figure above 70 is high, and above 90 is extreme.
Duff Moisture Code
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DMC is a numerical rating of the average moisture content of loosely compacted organic layers of moderate depth. The code indicates the depth that fire will burn in moderate duff layers and medium size woody material. Duff layers take longer than surface fuels to dry out but weather conditions over the past couple of weeks will significantly affect the DMC. The system applies a time lag of 12 days to calculate the DMC. A DMC rating of more than 30 is dry, and above 40 indicates that intensive burning will occur in the duff and medium fuels. Burning off operations should not be carried out when the DMC rating is above 40.
Drought Code
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The DC is a numerical rating of the moisture content of deep, compact, organic layers. It is a useful indicator of seasonal drought and shows the likelihood of fire involving the deep duff layers and large logs. A long period of dry weather (the system uses 52 days) is needed to dry out these fuels and affect the Drought Code. A DC rating of 200 is high, and 300 or more is extreme indicating that fire will involve deep sub-surface and heavy fuels. Burning off should not be permitted when the DC rating is above 300.
Initial Spread Index
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This indicates the rate fire will spread in its early stages. It is calculated from the FFMC rating and the wind factor.
The open-ended ISI scale starts at zero and a rating of 10 indicates high rate of spread shortly after ignition. A rating of 16 or more indicates extremely rapid rate of spread.
Build Up Index
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This index shows the amount of fuel available for combustion, indicating how the fire will develop after initial spread. It is calculated from the Duff Moisture Code and the Drought Code.
The BUI scale starts at zero and is open-ended. A rating above 40 is high, above 60 is extreme.
Fire Weather Index
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Information from the ISI and BUI is combined to provide a numerical rating of fire intensity – the Fire Weather Index. The FWI indicates the likely intensity of a fire.
The FWI is divided into four fire danger classes:
Low 0 – 7, Medium 8 – 16, High l7 – 31, Extreme 32+
The system ratings for High and Extreme are:
| FFMC | DMC | DC | ISI | BUI | FWI | |
| HIGH | 70-90 | 30-40 | 200-300 | 10-16 | 40-60 | 17-31 |
| EXTREME | 91 + | 41 + | 301 + | 17 + | 61 + | 32 + |
Structure of the FWI System
The diagram below 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.
