1. Basic Fire Behavior Walt Thomson
Assistant Director of Field Conservation Services
The Nature Conservancy
Florida Chapter

2. Objectives: * Identify Fire Behavior Terms * Explain Fire Triangle
* Phases of Combustion
* 3 Elements of Heat Transfer
*Discuss ROS and Flame Length
* Identify the Parts of a Fire
* Describe Fire Environment Elements (3)
* Basic Fuel Description - 3D
* Identify Basic Fuel Types
* Intensity vs Severity

3. Fire Triangle OXYGEN HEAT FUEL
Fire needs all three to sustain combustion.
What are we removing when we construct a fire line?
What are we removing when we put water on a fire?
Both dirt and foam also help remove the oxygen as well as heat from a fire
FUEL

4. Heat Transfer RADIATION CONVECTION CONDUCTION
Radiation: heat generated from burning object affecting adjacient object.
Convection: Hot gases rising from fire. Dries out fuels and also can carry hot embers to start spot fires.
Conduction: Heat energy transferred within an object; metal rod in a camp fire. Wildland fuels are NOT a good conductor of heat
CONDUCTION

5. Branches preheated by convection and radiation
Brush preheated by radiation
Tree trunk preheated by
radiation
Log preheated by
Conduction and radiation
Litter or duff preheated by
Conduction and radiation

8. Phases of Combustion
Pre-Ignition
Flaming
Smoldering
Glowing

9. Rate of Spread Forward Rate of Spread (*Measured in Chains per Hour)
Head Fire: Burning with the wind or up-slope
Flank Fire: 90 deg. to the wind or slope
Backing Fire: Against the wind or down-slope

11. Types of Fire in 3D space Surface Fires: Ground Fires: Crown Fires:
Passive:
Active:
Independent:

12. Types of Crown Fires

13. Intensity vs. Severity
What is the Difference???

14. Fire Intensity is... Measures the dimensions of the fire itself.
Examples:
Flame Length
Peak Temperature
HUA
Reaction Intensity
FLI
Residence Time

15. Fire Severity is...
A measure of the effect of the fire intensity on parts of the ecosystem
Examples:
Percent of fuel cover consumed.
Leaf or needle Scorch
Char Height
Bark char depth

16. Intensity/Severity - Relationships
Low Intensity - Low Severity
Low Intensity - High Severity
High Intensity - Low Severity
High Intensity - High Severity

17. FUELS WEATHER TOPOGRAPHY Three Principal Environmental Elements
Affecting Wildland Fire Behavior
FUELS
WEATHER
TOPOGRAPHY

18. FUEL Surface fuels: Ground fuels: small conifers mineral soil
downed woody litter
litter duff
shrubs
grass
forbs.

19. FUELS Fuel Type Fuel Moisture Size and Shape Fuel Loading
Horizontal Continuity
Vertical Arrangement
Will discuss each of these fuel factors in following slides

20. Fuel Particle Properties Affecting Fuel Moisture Content
Composition
Surface Coating
Living or dead
Amount of Decomposition

21. Live Fuel Moisture “Live Fuels are Different”
WHY????
Live Fuels
Can Either Be a Heat Source of Sink:
WHY???

22. Categories of Fuels Light fuels: grass, leaves, shrubs Heavy fuels:
Limbs, logs, stumps
Light fuels adapt quickly to ambient envoronemnt..IE can dry out and burn quickly after moisture event. Takes only small amount of moisture to dampen or heat to dry out
Heavy fuels Take longer to adapt to environment. Takes either a lot of moisture to get wet and long period of warm & dry to dry out.

23. Fuel Model Categories Grass Dominated Shrub Dominated
Primary carrier of fire is grass
Shrub Dominated
Primary carrier is brush or litter
Timber Litter Dominated
Primary carrier is litter beneath the stand
Logging Slash
Primary carrier is logging slash

24. WEATHER Temperature Measure of warmth or coldness of a substance
Fuel and ground temperature due to direct radiation from the sun
Higher ground and fuel temperature enables fuel to ignite/burn easier
Fuel temperatures can vary as much as 50o due to shading

25. WEATHER Relative Humidity
Ratio of amount of moisture in the air to amount of moisture it could hold at a certain temperature and pressure, expressed in %
As RH increases, fuel moisture increases
Grasses and other light fuels respond quickly to RH
Logs and other heavy fuels respond slowly to RH

26. WEATHER Precipitation Increases fuel moisture Amount Duration
Light fuels affected quickly
Large fuels affected more by duration than by amount – time to absorb moisture before it runs off

27. WEATHER Wind - Increases supply of oxygen
Drives convective heat into adjacent fuels
Influences spread direction and spotting
Carries moist air away replacing it with drier air
Dries Fuels
Raises fuel moisture if the air contains moisture

28. Topography Aspect - Direction a slope faces Slope - Steepness
Position of Fire - Top, middle, or bottom of slope
Shape of Country - Narrow canyons & box canyons
Elevation - Relates to curing of fuels, precipitation, length of fire season, etc.
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29. Mountains Cause Channeling of Wind
Saddle