Fire Weather index - FWI
Information from the ISI (Initial Spread Index) and BUI (Build Up Index) is combined to provide a numerical rating of fire
intensity called 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 17-31 Extreme 32+.
Build -Up Index -- BUI
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.
Initial Spread Index -- ISI
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.
Duff Moisture Code -- DMC
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 -- DC
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.