A fire danger rating system measures the variable elements which cause day to day changes in fire risk, and interprets the information gained. To interpret the system, the three fuel moisture codes and the three behavior indices need to be understood. Each code and index is a numerical rating related to likely fire behavior. 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 behavior using past and present weather effects on ground level fuels. The moisture codes reflect the net 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. The moisture content of fine fuels is very sensitive to the weather.