Forest fire weather index

The Forest fire weather index (FWI) (French: indice forêt météo, IFM) 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.

Daily Fire Weather Index forecast

History and development

Efforts to understand the risk and potential severity of forest fires began in Canada in 1925 under J.W. Wright.^[1] 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.^[1] 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.^[2]

The FWI is part of a larger set of measurements - the Canadian Forest Fire Danger Rating System (CFFDRS).

Description

How the elements of the FWI work together

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.^[3]

Individual components

Fine Fuel Moisture Code (FFMC): This measures the moisture level in the smaller, surface materials that are often the first fuels to burn in a fire.^[4] Depth is 0-2 cm.^[5] This can include needles, dry grasses, small twigs and branches, dead leaves, and other small pieces of organic matter on the forest floor.^[6] The FFMC is a good indicator of how easily a new fire could start from an ignition source such as lightning.

Duff Moisture Code (DMC): 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.^[5] The DMC gives an indication of how deeply a surface fire will penetrate.^[4]

Drought Code (DC): Long-term moisture deficits in deep soil layers is indicated by the Drought Code. It is measured from 8-18 cm in depth below ground level.^[5] It also indicates the flammability of large logs and trees.^[4] The DC is most effected by extended temperature trends and long-term rainfall levels.

Build-Up Index (BUI): This number represents a combination of the DMC and the DC, and indicates how much fuel is available for combustion.^[4]

Initial Spread Index (ISI): The ISI factors in wind speed and the FFMC numbers to give an indication of how fast a fire might spread once ignited.^[4]

The essential information needed to calculate this index is:

• the humidity of the air at the beginning of the afternoon (when it has its lowest value);
• the temperature in the middle of the afternoon (when it has its highest value);
• the 24-hour total precipitation (from noon to noon);
• the maximum speed of the average wind.

Usages

The FWI is one of the most used fire indexes worldwide. It is used in Europe, North and South America, and in parts of Asia.^[7]

Australia

The FWI was adapted for use in Australia, with large FWI values most commonly associated with high wind speeds, followed secondly by low relative humidities and then thirdly by high temperatures.^[8]

France

This index is computed for 40 zones of France, three are in the Landes (département with the biggest forest of France). Météo France sends the IFM to civil defense authorities, which allows the prepositioning of men and vehicles, especially air units. The computed index is revised every year to take new data into account.^[9]

Canada

Each provincial forest fire prevention authority is calculating the FWI for the different regions under their jurisdiction. The Daily Severity Rating (DSR) is a numeric rating of the difficulty of controlling fires, based on the Fire Weather Index, that is calculated afterward. It reflects more accurately the expected efforts required for fire suppression.^{[citation needed]}

New Caledonia

A daily fire weather outlook is issued twice a day to alert the fire authority and help local firefighters to get organized. Provided information is based on FWI values computed at 20 weather stations representative of the climatic diversity of the country.^[10]

Croatia

FWI has been adopted in 1980s at Meteorological and Hydrological Service of Croatia where it is computed daily and used as a predictor in forest fire risk assessment. Final product is calibrated and ranked in 5 risk classes, from very low to very high. Product is daily forwarded to civil defense authorities. ^[11]

See also

• National Fire Danger Rating System
• Haines Index