Fosberg Index - The FWI (Fire Weather Index) is defined by a quantitative model that provides a nonlinear filter of meteorological data which results in a linear relationship between the combined meteorological variables of relative humidity and wind speed, and the behavior of wildfires. Thus the index deals with only the weather conditions, not the fuels. Several sets of conditions have been defined by Fosberg (Fosberg, 1978) to apply this to fire weather management. The upper limits have been set to give an index value of 100 if the moisture content is zero and the wind is 30 mph. Thus, the numbers range from 0 to 100 and if any number is larger than 100, it is set back to 100. The index can be used to measure changes in fire weather conditions. Over several years of use, Fosberg index values of 50 or greater generally appear significant on a national scale. The SPC fire weather verification scheme uses the Fosberg Index, but with a check for both temperature (60F) and adjective fire danger rating (3-High, 4-Very High, 5-Extreme). Fosberg index values are displayed in increments of 10 starting at 50 through 100 with the color pink indicating values of 50 or 60, dark orange indicating values of 70 or 80, while values of 90 and 100 are shown in bright orange.

The Fosberg Index, orginially called the Fire Weather Index (Fosberg, 1978), was created to meet management needs for timeliness of weather information and for a meaningful interpretation of the short time and close space weather impacts on fire management. It is a non-linear filter of meteorological data developed by first transforming temperature and relative humidity to equilibrium moisture content, then transforming the equilibrium moisture content to combustion efficiency. The index is approximated by F = D((Rate of Spread) (Energy Release)) ^0.46