The Fire Weather Index or FWI combines measurements of wind, temperature and humidity to calculate a number of indicators which are then combined to give an overall score (FWI).
Fine Fuel Moisture Code:
The Fine Fuel Moisture Code (FFMC) is a numeric rating between 0 and 101 of the moisture content of litter and other cured fine fuels. This code is an indicator of the relative ease of ignition and the flammability of fine fuel. It is calculated using wind, temperature, relative humidity and rain.
A rating of:
• under 58 suggests low probability of fire ignitions
• 58 - 74 suggests 50% of fires will ignite
• 74 to 86 suggests all fires will rapidly ignite
• over 86 suggests extreme fire behavior
Duff Moisture Code:
The Duff Moisture Code (DMC) is a numeric rating between 0 and 150 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. It uses temperature, relative humidity and rain in it's calculation.
A rating of
• greater than 30 is high fire risk
• greater than 40 is extreme fire risk
Drought Code:
The Drought Code (DC) is a numeric rating between 0 and 800 of the average moisture content of deep, compact organic layers. This code is a useful indicator of seasonal drought effects on forest fuels and the amount of smoldering in deep duff layers and large logs.
A drought code of:
• 200 is a high risk of fire
• 300 is a very high risk of fire
• 350 is an extreme risk of fire
Initial Spread Index:
The Initial Spread Index (ISI) is a numeric rating between 0 and 100 of the expected rate of fire spread. It combines the effects of wind and the FFMC on rate of spread without the influence of variable quantities of fuel. It uses the FFMC rating and Wind in it's calculation.
An ISI of greater than 10 will see rapid spread of fires.
Buildup Index:
The Buildup Index (BUI) is a numeric rating between 0 and 200 of the total amount of fuel available for combustion. To calculate it combines the DMC and the DC.
A BUI of greater than 40 is a high fire risk with a BUI of greater than 60 is an extreme fire risk.
Fire Weather Index:
The Fire Weather Index (FWI) is a numeric rating between 0 and 150 of fire intensity. It combines the Initial Spread Index and the Buildup Index. It is suitable as a general index of fire danger.
The ratings are based on the following FWI scores:
• FWI score of between 0 and 11.9 is a Low to Moderate of fire
• FWI score of between 12.0 and 24.9 is a High risk of fire
• FWI score of between 25 and 49.9 is a Very High risk of fire
• FWI score of between 50 and 74.9 is a Severe risk of fire
• FWI score of between 75 and 99.9 is a Extreme risk of fire
• FWI score of more than 100 is a Catastrophic risk of fire