The FWI was evaluated for several seasons before it was introduced for the 1980-81 fire season. The fire weather index calculation structure. Interpretation of 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. 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 nett effects of daily moisture gains and losses. 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.