Wildland Firefighting (3rd Edition)
Chapter 1 - Wildland Fire Behavior: Fuel, Weather, & Topography
"By the Numbers"

  • The National Fire Danger Rating system (NFDRS) lists 20 standard fuel models.
  • The NFDRS models are aligned with 13 fire-behavior models to predict large area daily fire danger ratings.
  • NFPA 1574, Aids to Determining Fuel Models for Fire Behavior, provides a complete explanation of fire behavior models.
  • Fuels exposed to heat from the sun can reach 150oF.
  • Most wildland fuels must be heated between 400oF to 700oF to ignite.
  • Live fuels usually have a moisture content from 35% to 250% of their dehydrated rate.

Live Fuel Moisture Content
Moisture Content / Stage of Development
300% / Fresh foliage, annuals
200% / Maturing foliage (still developing)
100% / Mature foliage & older perinnials
50% / Entering dormancy, coloration starting, some leaves have dropped
< 30% / Completely cured (dead fuels)
  • Dead plants have a moisture content between 1.5% to 30%.
  • One fire behavior prediction system uses the following to determine the time it takes for moisture content to equalize with that of the surrounding air.

Classification / Diameter of Fuel
1 hour / <1/4"
10 hour / 1/4" to 1"
100 hour / 1" to 3"
1,000 hour / 3" to 8"
  • Fuels in full sun can contain up to 8% less moisture than those in the shade.
  • Surface fuels include brush up to 6 feet in height.
  • Aerial fuels include brush over 6 feet in height.
  • Fuel loads can range from 1 ton to more than 500 tons per acre (Grass: range from 1/4 ton to 1 ton per acre, Timber: can be up to 600 tons per acre).
  • 1,000 hour fuels, roughly 42 days, is of little value for fire behavior forecasts done in 12 hour increments.
  • Light fuels are short grass and light brush up to 2 feet in height.
  • Medium Fuels include brush up to 6 feet in height.
  • Heavy fuels include heavy continuous brush more than 6 feet in height.
  • 14 firefighters were killed in the 1994 SouthCanyon fire near Glenwood Springs, Colorado.

Atmospheric Layers
Layer / Distance above ground level
Troposphere / 0-10 miles
Stratosphere / 10-20 miles
Mesosphere / 20-50 miles
Thermosphere / 50-700 miles
  • An average lapse rate is 3.5oF per 1,000 feet of elevation.
  • Stable atmosphere lapse rates are 5.5oF or less per 1,000 feet of elevation. Above 5.5oF is unstable.
  • Within an inversion layer, temperature may increase as much as 25oF in 250 vertical feet.
  • Daytime sea breezes usually extend 5 to 6 miles inland but may extend up to 25 miles from the coast.
  • For each pound of water condensed from vapor, more than 1,000 Btu of heat energy is released.
  • A thunderstorm cell can be up to 10 miles in diameter. Multiple cells may converge to form a mass that extends more than 50 miles.
  • Downdraft winds produced by thunderstorms range from 25-35 mph (most common) up to 60 mph.
  • Winds may shift as much as 180 degrees between the time of a storm's approach and its departure.
  • Fuels in sun may be more than 50oF than those in shade.
  • Temperature change in dry air is approximately 5.5oF per 1,000 feet in altitude.
  • When relative humidity is low (< 30%), moisture is drawn out of fuels, dehydrates them, and makes them susceptible to ignition.
  • Fire with humidity of 30% or less burn freely. Less than 10% burns vigorously.
  • Gradient winds occur at approximately 1,500 feet above the earth's surface.
  • Upslope winds early in the day average 3 to 5 mph.
  • Up-canyon or valley winds (developed upslope winds) average 7 to 10 mph.
  • Downslope winds average 5 to 7 mph and are present after sunset.
  • Along the PacificCoast, sea breezes may reach speeds of 10 to 30 mph.
  • Firewhirls range in size from 1 to 2 foot up to small tornado in size and intensity.
  • Fire behavior timeline:

Time / Fire Behavior
10 am to 6 pm / Fire Behavior most erratic
6 pm to 4 am / Wind usually moderates, air cools, relative humidity rises, and fuels begin to absorb moisture
4 am to 6 am / Fire activity at its lowest
6 am to 10 am / Wind increases, temperature rises, controlling fire is more difficult
  • Elevation divided by Distance times 100 = % of slope.
  • Latitude is measured in degrees from 0 through 90, North and South of the equator.
  • Longitude is measured in degrees from 0 through 180, East and West of the Prime Meridan.
  • General survey map breakdown:

Ranges / North/South rows of Townships
Townships / 6 miles square
Section (36 per township) / 640 acres
Quarter-Section / 160 acres
One acre = 209 square feet or about 70 x 70 yards
  • Each side of a square acre is about 2/3 the length of a football field.
  • The agonic line is 0 degrees declination.
  • Spot fires from sparks and burning embers can occur more than 1 mile away.
  • Forward rate of spread doubles twice when moving from timber to brush and twice again when moving to from brush to grass. Spread slows conversely when moving from lighter to heavier fuels.
  • Rate of spread doubles for every 20% increase in slope.
  • Rate of spread doubles for every 10 mph of wind above 10 mph.
  • The burning index (BI) rates the expected fireline intensity from 0 to 110.

Suppression Limitations
Flame Length / Suppression
< 4 feet / Attacked at head or flanks
4 to 8 feet / Heavy equipment and/or aircraft needed
8 to11 feet / Serious control problem, attack on head may be ineffective
> 11 feet / Indirect attack, major fire runs probable