NFDRS Indices Interpretation
The following is a brief explanation of the National Fire Danger Rating (NFDRS) indices as used by Georgia Forestry Commission (Table 1). For more details, please refer to the materials in the references.
1: NFDRS for
RH(%): Relative Humidity in percent
IC: The Ignition Component (IC) is an index within 1988 National Fire Danger Rating System. IC relates the probability that a fire that requires suppression action will result if a firebrand is introduced into a fine fuel complex. Theoretically, on a day when the ignition component registers a 60, approximately 60% of all fire brands that come into contact with wildland fuels will require suppression action.
BI: Dividing Burning Index (BI) by 10 gives a reasonable estimate of flame length in feet at the head of a fire. Burning Index is fuel model dependent. In Georgia Forestry Commission, this is the basis for Class Day. Byram (1959) developed an equation to relate flame length and fireline intensity, FL=0.45(I)0.46 where FL is flame length in feet and I is fireline intensity in BTU per second per feet. Table 2 (Deeming, 1977) can be used as a guide to relate Burning Index and fire behavior
Table 2: Fire behavior, Controllability and Fireline Intensity Table
Class Day: Class day describes the potential for wildland fires. Georgia Forestry Commission uses Table 3 as the determination percentile for all the class day. Since BI is fuel model dependent, different BI determination points are arrived.
Table 3: Class Day determination points
KBDI: Keetch-Byram Drough Index (KBDI) measures moisture in deep duffs or upper soil layers. The relative dryness of soil is important in fire suppression. KBDI varies from 0 (Wet) to 800 (Dry).
Wind (mph): 10-minute averaged wind speed in mile per hour measured at 20 feet and wind direction in 8-point compass, i.e. N, NE, E, SE, S, SW, W, NW and calm.
Mx_Wind(mph): Wind gust in mile per hour measured at 20 feet and wind direction in 8-point compass, i.e. N, NE, E, SE, S, SW, W, NW and calm. Mx_Wind has to be at least as strong as Wind and they should be in the same direction.
Rn24(inch): Rainfall amount in the last 24 hours in inches.
Dur(Hr): Duration of rainfall in hours if there is any.
Sow: State of weather
Table 4: Code for State of weather
Temp(°F): Temperature in Fahrenheit
Td(°F): Dew point temperature in Fahrenheit
Tmax(°F): Maximum temperature in the last 24 hours in Fahrenheit
Tmin(°F): Minimum temperature in the last 24 hours in Fahrenheit
RHMax(%): Maximum relative humidity in the last 24 hours in percent
RHMin(%): Minimum relative humidity in the last 24 hours in percent
HrbGF: Herbaceous greenup factor (HrbGF) expresses the actual greening and curing of live herbaceous vegetation. HrbGF varies from 0 (completely cured) to 20 (completely green).
In Georgia Forestry Commission, we changed greenup factors automatically by the time of the year and KBDI. In winter, greenup factor is set to 0. In summer, greenup factor is set to 20. In between, greenup factor is moved up or down 3-4 points every 30 days or so. In order to account for dry periods, drought adjusted greenup factor based on KBDI is adopted from Burgan (1988) (Table 5). If greenup factor at the time of the year exceeds the drought adjusted greenup factor, greenup factor will be reduced to the drought adjusted greenup factor. If not, green up factors are used as is. For example, if KBDI is 395 in summer when greenup factor should be 20, the greenup factor will adjusted to 10 due to drought. If KBDI is 395 in winter when green up factor is 0, the greenup factor will still stay at 0.
Table 5. Drought Adjusted greenup factor based on KBDI
WdyGF: Woody greenup factor (WdyGF) expresses the actual greening and curing of live woody vegetation. WdyGF varies from 0 (completely cured) to 20 (completely green). Please refer to HrbGF for details.
1-Hour/10-Hour/100-Hour/1000-Hour: Dead fuel moisture content in fuel that takes 1 hour/10 hours/100 hour/1000 hour to lose or gain 63% of the difference between the dead fuel itself and the surrounding atmosphere. Fuel moisture percent is computed by dividing the weight of "water" in the fuel by the oven-dried weight of the fuel and then, multiplying by 100 to get to percent. Gerogia Forestry Commission sets 1-hour to be the same as 10-hour. 10-hour dead fuel moisture is measured using an electronic fuel stick manufactured by Forestry Technology System. 100-hour and 1000-hour dead fuel moisture are calculated
X1000: Predictor value for 1000-Hour
Herbaceous: Fuel moisture in live herbaceous vegetation. Please refer to "1-Hour" on fuel moisture computation. This value is calculated in our system.
Woody: Fuel moisture in live woody vegetation. Please refer to "1-Hour" on fuel moisture computation. This value is calculated in our system.
SC: Spread Component (SC) is the forward rate of spread at the head of the fire in feet per minute. Since 1 chain=66 feet and there are 60 minutes in an hour, so 1 feet per minute is 0.9 chain per hour. Thus, multiply the value of SC by 0.9 gives the spread rate in chain per hour.
EC: Energy Release Component (EC) is the potential available energy per square foot at the head of the fire in BTUs per square foot.
Introduction to Nation
Fire Danger Rating System from
Weather Information Management System (WIMS) user's manual
Remsoft FireWeatherPro user's manual
Bradshaw, Larry S.;
Deeming, John E.;Burgan, Robert E.;Cohen,
Jack D., compilers. The 1978 National
Fire-Danger Rating System;technical
documentation. General Technical
Burgan, Robert E.; 1988
Revisions to the 1978 National Fire-Danger Rating System. Res. Pap. SE-273.
Byram, George M. Combustion of
forest fuels. In: Forest fire control
and use. 2nd ed.
Deeming, John E.; Burgan,
Robert E.; Cohen, Jack D. The National Fire-Danger Rating System – 1978. 1988
Revisions to the 1978 National Fire-Danger Rating System. General Technical Report INT-39.