The (forest) fire weather index, in short FWI, is a number calculated to estimate the risk of wildfire in forests. The calculations are based on a Canadian empirical model that was developed in 1976.
The meteorological parameters (for our region observations taken at 12:00 UTC) necessary as input for the calculations are:
- precipitation during the past 24 hours
- temperature of the air
- relative humidity of the air
- average windspeed
Also the values of the previous day(s) are taken into account.
The organic soil layer - important for the spread of fire - is broken down as follows:
- a loose top layer, consisting of litter / fine fuels, on average 7-8 mm thickness.
- loosely compacted middle layer (decaying organic materials), thickness few centimeters.
- the deeper, more compacted organic layer, on average ~10 centimeter thickness.
From the meteorological parameters, different numbers are calculated representing the moisture content of these 3 layers:
- the FFMC (= Fine Fuel Moisture Code) is the moisture content of litter; as this layer is on top, the parameters temperature, humidity, precipitation and wind all have an impact as is the calculated value from the day before.
- 70 or more is high
- 90 or more is extremely high
- the DMC (= Duff Moisture Code) is the moisture content of the middle layer. This layer is not in direct contact with the air, hence wind does not have to be considered. Because the top layer is only few millimeters thick, other parameters such as temperature, humidity, precipitation are taken into account, as well as the value of the day before.
Compared to the FFMC, the value of the previous day is more important and has a bigger share in the final result (time constant is few days).
- 30 or more means dry
- 40 or more is considered very dry
- the DC (= Drought Code) is a number indicating the moisture content of the bottom organic layer. This layer is even more shielded from the air. Only the parameters precipitation and temperature have a proven effect on this layer.
The value of the day before is even more important and it changes (very) slowly when the weather is getting dry for some time (time constant is more than 50 days).
- a number of 200 is high
- a number of 300 is very dry and means that this organic material is available for combustion in case of wildfire.
Starting from these 3 calculated codes (FFMC, DMC, DC), other numbers are derived indicating how the fire will spread:
- from the FFMC and windspeed, the number ISI (= Initial Spread Index) is calculated. This number is an indication how fast fire can spread:
- lowest value is 0
- 10 or more shows high spread after ignition
- 16 or more is very high spread once fire started
- from the numbers DMC and DC, the number BUI (= Buildup Index) is calculated. This number is an indication how much fuel is available for combustion.
- 40 or more is high
- 60 or more is extremely high
Both ISI and BUI are combined to get the final Fire Weather Index FWI.
An adjusted FWI is also used. The adjustment is based on time of the year. For example during the month July, much “green” material is available (leaves, fresh branches, … materials with higher water content), hence the adjustment factor is 1; but during early spring, under the same dry circumstances, the “green” material is not yet available. The adjustment factor can be as high as 3, slowly decreasing when growing season starts and ramping up again during autumn.