Fire Danger Maps
A Fire Danger Rating determination is the cumulative effort of the NFDRS, taking into account current and antecedent weather, fuel types, and the state of both live and dead fuel moisture.
Local managers have much flexibility in local application of NFDRS. They may select up to four fuel models from 20 (broadly covering grass, timber, brush, and slash). Staffing levels may be based on one of several NFDRS indexes, though about 90% use the Burning Index (BI) which is related to potential flame lengths for the selected fuel type. Staffing class breakpoints are set by local managers from historical fire weather climatology.
The adjective class rating is NFDRS's method of normalizing rating classes across different fuel models and station locations. It is based on the primary fuel model cataloged for the station, the fire danger index selected to reflect staffing levels, and climatological class breakpoints. All this information is provided by local station managers.
Dead fuel moisture responds solely to ambient environmental conditions and is critical in determining fire potential. Dead fuel moistures are classed by timelag. A fuel's timelag is proportional to its diameter and is loosely defined as the time it takes a fuel particle to reach 2/3's of its way to equilibrium with its local environment. Dead fuels in NFDRS have four timelag classes:
Four vegetation greenness maps are derived weekly from Normalized Difference Vegetation Index (NDVI) data observed by satellites and provided by the EROS Data Center, U.S. Geological Survey. These maps have a 1 kilometer (.6 mile) spatial resolution. Maps with historical references (RG and DA) are based on the years 1989 through 1995. The derived maps are:
Keetch-Byram Drought Index. A soil/duff drought index that ranges from 0 (no drought) to 800 (extreme drought) and is based on a soil capacity of 8 inches of water. Factors in the index are maximum daily temperature, daily precipitation, antecedent precipitation, and annual precipitation.
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Lower Atmosphere Stability Index Maps
The Lower Atmosphere Stability Index, or Haines index, is computed from the morning (12Z) soundings from RAOB stations across North America. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels. The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior.
Haines Indices range from 2 to 6 for indicating potential for large fire growth:
Lightning Ignition Efficiency
Ignition efficiency is calculated by algorithm, using the NFDRS 100 hour fuel moisture and specialized fuel type maps for the western United States to indicate probability of ignition, given occurrence of lightning.
Bradshaw, Larry S.; Deeming, John E.; Burgan, Robert E.; Cohen, Jack D., compilers. 1984. The 1978 National Fire-Danger Rating System: technical documentation. Gen. Tech. Rep. INT-169. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 44 pp.
Burgan, Robert E. 1988. 1988 revisions to the 1978 National Fire-Danger Rating System. Res. Pap. SE-273. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station. 39 pp.
Deeming, John E.; Burgan, Robert E.; Cohen, Jack D. 1977. The National Fire-Danger Rating System - 1978. Gen. Tech. Rep. INT-39. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 63 pp.
Haines, D.A. 1988. A lower atmospheric severity index for wildland fire. National Weather Digest. Vol 13. No. 2:23-27.
Keetch, John J; Byram, George. 1968. A drought index for forest fire control. Res. Paper SE-38. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station. 32 pp. (Revised 1988).
USDA Forest Service. 1995. Weather Information Management System User's Guide. Washington, DC: U.S. Department of Agriculture, Forest Service, Fire and Aviation Management.