Fire Weather Index: Information from the Initial Spread Index
and the Build Up Index is combined to provide a numerical rating of fire intensity. The FWI indicates the fire risk for the hottest part of the day (afternoon).
The FWI
uses a warning system which is divided into five fire danger classes. Key components used in the calculations are temperature, humidity, wind speed and recent history of rainfall. Thresholds are:
Low < 14, Moderate < 35, High < 64,
Very High < 122, and Extreme > 122.
Interpretation of the other five (FWI noted above) FWI Codes and Indices
To interpret the system
the three fuel moisture codes and the three behaviour indices need to be
understood. Each code and index is a numerical rating related to likely fire
behaviour. The scales start at zero, and except for the Fine Fuel Moisture Code
which has a maximum of 99, all are open-ended. Low ratings indicate high
moisture content, and ratings rise as moisture content decreases. Ratings rise
as fire weather becomes more severe.
Fuel Moisture
Codes
The FWI System evaluates fuel moisture content and relative
fire behaviour using past and present weather effects on ground level fuels. The
moisture codes reflect the net effects of daily moisture gains and
losses.
Fine Fuel Moisture Code (87)
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. Low (0.0 - 80.9),
Moderate (81.0 - 87.9), High (88.0 - 90.4), Very High (90.5 - 92.4), Extreme 92.5+
Duff Moisture Code (38)
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. Low (0.0 - 12.9), Moderate (13.0 - 27.9 ), High, (28.0 - 41.9), Very High (42.0 - 62.9 ), Extreme 63.0+.
Drought Code (182)
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. Low (0.0 - 79.9), Moderate (80.0 - 209.9),
High (210.0 - 273.9), Very High (274.0 - 359.9), Extreme 360.0+.
Fire Behaviour Indices
The three
behaviour indices (fire weather index noted above) are relative to the fuel moisture content. They indicate what
a fire is likely to do. The lower the moisture content, the higher the Fuel
Moisture Codes, and the higher the Fire Behaviour Indices ? and the more active
the fire will be.
The following fire behavior indices are used to calculate the Fire Weather Index.
Initial Spread Index (3)
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.Low (0.0 - 3.9), Moderate (4.0 - 7.9),
High (8.0 - 10.9), Very High (11.0 - 18.9), Extreme 19.0+.
Build Up Index (50)
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. Low (0.0 - 18.9), Moderate (19.0 - 33.9),
High (34.0 - 53.9), Very High (54.0 - 76.9), Extreme 77.0+.
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