DEFINITION:

The Fosberg Fire Weather Index (FFWI), also known as the Fire Weather Index (FWI), is a fire weather index created to measure the potential influence of weather on a wildfire based on model output of temperature, wind and relative humidity.

The index represents expected flame length and fuel drying based on model output fields of temperature, wind and humidity. Large values of the FWI imply high flame lengths and rapid drying. The values were designed so that a index rating of 100 is equal to a moisture content of 0 and a wind speed of 30 mph. Larger combinations of these values still result in an index of 100.

FFWI = n*[(1+U^2)^.5]/0.3002

where U=wind speed in mph and n=moisture damping coefficient.

n=1-2(m/30)+1.5(m/30)^2-0.5(m/30)^3

where m=equilibrium mositure content.

for h < 10 %

m=0.03229 + 0.281073h - 0.000578hT

for 10% < h <= 50%

m=2.22749 + 0.160107h - 0.01478T

for h > 50%

m=21.0606 + 0.005565H^2 - 0.00035hT - 0.483199h

where T=temperature in F and h=relative humidity in percent.