Understanding Fire Weather

Last Review/Updated: June 3, 2005

Fire Weather Indices

Hazard Rating
 FFMC
Fine Fuel Moisture Code		 DMC
Duff Moisture Code		 DC
Drought Code		 ISI
Initial Spread Index		 BUI
Build Up Index		 FWI
Fire Weather Index		 HFI
Head Fire Intensity
Low 0-76 0-21 0-79 0-1.5 0-24 0-4.5 1-2
Moderate 77-84 22-27 80-189 2-4 25-40 4.5-10.5 3
High 85-88 28-40 190-299 5-8 41-60 10.5-18.5 4
Very High 89-91 41-60 300-424 9-15 61-89 18.5-29.5 5
Extreme 92+ 61+ 425+ 16+ 90+ 29.5+ 6


These codes and indices are indicators of the dryness of the forest fuels, and give relative measure of the burning conditions that can be expected for a "standard" fuel type. Low numbers mean wet, high numbers mean dry. The individual fields are:
  1. Fine Fuel Moisture Code (FFMC)
    Range: 0 - 101

    The dryness of the smallest forest fuels (surface litter, leaves, needles, small twigs, etc). Derived from yesterday's FFMC, and the local noon dry bulb temperature, relative humidity, wind speed, and 24 hour precipitation.

  2. Duff Moisture Code (DMC)
    Range: 0 - Unlimited

    The dryness of the medium sized surface fuels and upland duff layers (approximately 2 - 10 cm). Derived from yesterday's DMC, and the local noon dry bulb temperature, relative humidity and 24 hour precipitation.

  3. Drought Code (DC)
    Range: 0 - Unlimited

    The dryness of the largest surface fuels and deep duff layers (approximately 10+ cm). Derived from yesterday's DC, and the local noon dry bulb temperature and 24 hour precipitation.

  4. Initial Spread Index (ISI)
    Range: 0 - Unlimited

    A relative measure of how quickly a fire can be expected to spread. Derived from the FFMC and wind speed.

  5. Build Up Index (BUI)
    Range: 0 - Unlimited

    A relative measure of the amount of fuel available for combustion. Derived from the DC and DMC.

  6. Fire Weather Index (FWI)
    Range: 0 - Unlimited

    A relative measure of potential fire intensity; or energy available to be released. The FWI is a good indicator of overall fire danger. Derived from the BUI and ISI.

  7. Head Fire Intensity (HFI)
    Range: 1 - 6

    Head fire intensity is a numerical ranking of difficulty of control for specific fuel types. Flame length is a main visual manifestation.