Fire Hazard Index

Note: This is a quasi-subjective index based on weather thresholds that the local fire centers consider to represent high-danger fire conditions. These thresholds are temperatures above 90°F, relative humidities less than 20%, and wind speeds greater than 15 mph. Based on these thresholds, the index is defined as

where T is temperature (°F), rh is relative humidity (%), and spd is wind speed (mph). Note that FHI = 1 when the temperature, humidity, and speed equal the thresholds. Any negative values are set to zero, so the above formula ensures that FHI = 0 when temperatures are below 60°F and humidities are above 40%.

The FHI is subjective to the extent that it is not based on quantitative fire behavior theory. Moreover, it has not undergone any rigorous testing to determine whether it actually correlates well with the true fire danger. Its only intention is to highlight periods when the WRF model is forecasting high temperatures and wind speeds in combination with low humidities.