Understanding Fire Weather
These are important terms used when referring to fire weather.
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:
- 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.
- Duff Moisture Code (DMC)
Range: 0 - Unlimited
The dryness of the medium-sized surface fuels and upland duff layers (approximately 2 to 10 cm). Derived from yesterday's
DMC and the local noon dry bulb temperature, relative humidity and 24-hour precipitation.
- 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.
- 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.
- Build Up Index (BUI)
Range: 0 - Unlimited
A relative measure of the amount of fuel available for combustion. Derived from the DC and DMC.
- 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.
- 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.
- Wildfire Ignition Probability Prediction (WIPP)
Range: 0% - 100%
The Wildfire Ignition Probability Prediction (WIPP) model calculates the probability that a firebrand or spark falling on the forest floor will “persist” and sustain itself long enough to become established as a fire.
Updated: May 12, 2010