Fire Weather Map Keys



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.

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Dead Fuel Moisture


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:

  • 1-hr: Fine flashy fuels, less than 1/4" diameter. Responds quickly to weather changes. Computed from observation time temperature, humidity and cloudiness. 
  • 10-hr: 1/4 to 1" diameters. Computed from observation time temperature, humidty, and cloudiness, or may be a standard set of "10-Hr Fuel Sticks" that are weighed as part of the fire weather observation.
  • 100-hr: 1 to 3" diameter. Computed from 24 hour average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges. 
  • 1000-hr: 3 to 6 " diameter. Computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.

 

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Live Fuel Moisture -- Greenness Maps

 

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:

  • Visual Greenness Maps - portray vegetation greenness compared to a very green reference such as an alfalfa field or a golf course. The resulting image is similar to what you would expect to see from the air. Normally dry areas will never show as green as normally wetter areas.
  • Relative Greenness Maps - portray how green the vegetation is compared to how green it has been historically (since 1989). Because each pixel is normalized to its own historical range, all areas (dry to wet) can appear fully green at some time during the growing season.
  • Departure from Average Greenness Maps - portray how green each pixel is compared to its average greenness for the current week of the year. 
  • Live Moisture Maps - portray experimental live vegetation moisture with values ranging from 50 to 250 percent of dry weight.
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Drought Maps


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.

 

  • KBDI = 0 - 200: Soil moisture and large class fuel moistures are high and do not contribute much to fire intensity. Typical of spring dormant season following winter precipitation. 
  • KBDI = 200 - 400: Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity.
  • KBDI = 400 - 600: Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively.
  • KBDI = 600 - 800: Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.

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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:

  • 2 : Very Low Potential -- (Moist Stable Lower Atmosphere)
  • 3 : Very Low Potential
  • 4 : Low Potential
  • 5 : Moderate Potential
  • 6 : High Potential ------ (Dry Unstable Lower Atmosphere)
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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.

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References


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.

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