Fire Weather Index


FWI: 22.5

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 (88)
Fire Weather Index: HIGH
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 (342)
Fire Weather Index: EXTREME
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 (1250)
Fire Weather Index: EXTREME
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 (4)
Fire Weather Index: MODERATE
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 (406)
Fire Weather Index: EXTREME
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+.