Req 5 — Fire Behavior

Fire behavior describes how a wildfire reacts to environmental influences—how fast it burns, how high the flames reach, and how rapidly it spreads across a landscape.

Requirement 5a: The Fire Environment Triangle (Fuel, Weather, Topography)

The Fire Environment Triangle consists of three environmental factors that control fire behavior:

                  [ Weather ]
                 /           \
                /             \
      [ Topography ] -------- [ Fuel ]

1. Weather:
   • Wind: The most variable and dangerous factor. Wind supplies fresh oxygen, pushes flames toward unburned fuel, and carries spot embers far ahead of the main fire front.
   • Relative Humidity & Temperature: As temperature rises, relative humidity drops. Dry air absorbs moisture from dead forest fuels, making them far more flammable.
2. Topography:
   • Slope: Fire travels much faster uphill because rising heat preheats the vegetation above the fire front.
   • Aspect: South- and west-facing slopes receive more direct sunlight, making their fuels drier and warmer than north-facing slopes.
   • Chutes & Canyons: Narrow canyons create a chimney effect, funneling wind and rapidly accelerating fire spread.
3. Fuel:
   • Fuel Moisture: Dry fuels burn rapidly and intensely; moist fuels require more heat to ignite.
   • Fuel Size: Fine fuels (grass, leaves, pine needles) ignite instantly, while heavy fuels (logs, stumps) burn longer at higher temperatures.
   • Fuel Continuity: Continuous fuel beds allow fires to spread unchecked; breaks (rivers, roads, rock outcrops) disrupt fire spread.

Requirement 5a Resources

Requirement 5b: The Fire Triangle & Suppression Principles

All fires require three elements to exist, known as the Fire Triangle: Heat, Fuel, and Oxygen. Removing any single leg of the triangle immediately extinguishes the fire.

       [ HEAT ]  +  [ FUEL ]  +  [ OXYGEN ]  =  FIRE

• Removing Heat: Applying water or chemical fire retardant lowers the fuel temperature below its ignition point.
• Removing Fuel: Digging a mineral soil fireline with hand tools or using a bulldozer removes all burnable vegetation ahead of the fire front. Backfiring also burns out fuel before the main fire arrives.
• Removing Oxygen: Smothering embers with mineral dirt using a Pulaski shovel, or dropping retardant foam, starves the fire of air.

Requirement 5b Resources

Requirement 5c: Anatomy & Parts of a Wildfire

To complete this requirement, draw a clear diagram illustrating the key parts of a wildfire:

Key Parts of a Wildfire

• Point of Origin: The exact location where the fire started.
• Head: The fastest-moving edge of the fire, traveling with the prevailing wind or burning uphill.
• Flanks: The sides of the fire parallel to the main direction of travel (Left Flank and Right Flank).
• Rear / Heel: The slow-moving edge burning against the wind or downhill.
• Finger: A long, narrow strip of fire extending out from the main body.
• Pocket: An unburned area of land enclosed between the head and a finger.
• Island: An unburned patch of forest completely surrounded by fire inside the perimeter.
• Spot Fire: A new fire ignited outside the main fire perimeter by airborne embers.

Requirement 5c Resources

Next Steps

Now explore personal protective equipment (PPE) and wildland firefighter safety systems.