Forest fire weather index
The Forest fire weather index is an estimation of the risk of wildfire developed by the Canadian Forest Service in the late 1960s. It is in use in Canada, France, Australia, and several other countries. It uses a variety of fuel moisture measurements and weather indices to calculate potential fire behaviour in wildland environments.
History and development
Efforts to understand the risk and potential severity of forest fires began in Canada in 1925 under J.W. Wright. Four different systems were developed over the next decades. By the late 1960's, researchers with the Canadian Forest Service began a project to harmonize and simplify the different systems. The current system was released in 1970, and underwent only small changes in calculation and a switch to metric measurements over the next decades. in 2025, the FWI is being updated to reflect non-forest fire environments, such as grassland and peat. The new updates will allow more fine-grained data, such as hourly weather readings, to be integrated into the system.The FWI is part of a larger set of measurements - the Canadian Forest Fire Danger Rating System.
Description
The index is a whole number that ranges between 0 and 20 in France and up to above 30 in Canada. It is computed from five components. The first three components are numeric ratings of the moisture content of litter and other fine fuels, the average moisture content of loosely compacted organic layers of moderate depth, and the average moisture content of deep, compact organic layers. The last two components are the rate of fire spread if fuel is available for combustion, and the frontal fire intensity.Individual components
Fine Fuel Moisture Code : This measures the moisture level in the smaller, surface materials that are often the first fuels to burn in a fire. Depth is 0-2 cm. This can include needles, dry grasses, small twigs and branches, dead leaves, and other small pieces of organic matter on the forest floor. The FFMC is a good indicator of how easily a new fire could start from an ignition source such as lightning.Duff Moisture Code : The DMC measures moisture levels in the organic soil just below the surface, as well as moisture in medium-sized woody material, such as small logs. Depth is 2-7 cm. The DMC gives an indication of how deeply a surface fire will penetrate.
Drought Code : Long-term moisture deficits in deep soil layers are indicated by the Drought Code. It is measured from 8-18 cm in depth below ground level. It also indicates the flammability of large logs and trees. The DC is most effected by extended temperature trends and long-term rainfall levels.
Build-Up Index : This number represents a combination of the DMC and the DC, and indicates how much fuel is available for combustion.
Initial Spread Index : The ISI factors in wind speed and the FFMC numbers to give an indication of how fast a fire might spread once ignited.
The essential information needed to calculate this index is:
- the humidity of the air at the beginning of the afternoon ;
- the temperature in the middle of the afternoon ;
- the 24-hour total precipitation ;
- the maximum speed of the average wind.