Module 3: Fire Behaviour Flashcards
Define: Fire Behaviour
The manner in which a fire reacts to fuel, weather and topography.
What are the three reasons it is vital to understand the principles of fire behaviour?
- Personal Safety
- Suppression
- Use as a prescriptive management tool
Fire is the simultaneous release of what?
Heat, light and flame caused by the rapid oxidization of fuel
What 3 things does a forest fire produce?
- Carbon dioxide
- water vapour
- heat
3 factors required for fire to perpetuate
from Van Wagner circa 1983
- sufficient supply of continuous fine fuels
- fuels must be dry enough for open flame
- must be an ignition source
2 limiting factors in fire spread
Van Wagner circa 1983
- needs sufficient heat from fire to warm unburned fuels to ignition temperature
- enough fuel to support a solid flame
What are the 3 phases of combustion
- Preheating Phase: unburned fuels in front heated to critical temp of 350 degrees C
- Distillation Phase: at 350 degrees and up volatile gases form from the solid fuels and ignite in oxygen, producing solid flame.
- Charcoal Phase: not enough vapour to support open flame and residual solid fuels smoulder away
What is pyrolysis?
when solid fuels are heated beyond the critical temp of 350 and are broken down into a volotile vapour
4 zones of pyrolysis
- Ash
- Char
- Pyrolysis
- Unburnt Fuel
What are the 3 sides of the fire triangle
Oxygen, Heat, Fuel
How to suppress a fire via Oxygen, Heat or Fuel?
O2: cover the fuel with dirt
Heat: cool the fuels with water or chemicals
Fuels: separate the fuels from the fire line
what are the 4 types of heat transfer (think campfire)
- Convection: natural upward movement of hot air
- Radiation: heat moved in straight lines with the speed of light from a warmer to cooler surface (preheating)
- Conduction: movement of heat through a solid surface (wood is a poor conductor)
- Embers: heat movement form wind carrying embers ahead of fire
The 3 types of fires?
Crown Fire: fire involving the tree crowns
Surface Fire: fire involving surface fuels only (often associated with intermittent or active crown fires)
Ground Fire: fire involving ground fuels (lingering in old stumps or duff)
Define Smouldering
Barely spreading fire with no flames.
Define Creeping
Low flames spreading slowly
Define Running
Rapidly spreading surface fire with a well developed front
Torching/Candling
Single or small group of trees flaring up
Spotting
New fires being started ahead of the main fire
Crowning
Fire in the crown of the trees
Active Crown Fire
a running fire that also includes the crown fuels
Intermittent Crown Fire
an aggressive surface fire with intermittent crown fires
Independent Crown Fire
crown fire that spreads without a surface fire
Bays
located between Fingers, a marked indentation in the fire
Finger(s)
elongated burn area projecting outward from the main body
Flank
sides of the fire
Head
furthest forward portion of the fire
Back
opposite the Head having the lowest rate of spread and intensity
Islands
areas of unburned fuels within the body of the fire
Point of Origin
exact location where ignition began
3 sides of the Fire Behaviour Triangle
- Fuel (8)
- Weather (7)
- Topography (6)
Fuel points
- Fuel Type
- Moisture Content
- Quantity
- Size of Fuel
- Arrangement of Fuel
- Fuel Continuity
- Fuel Chemistry
- Seasonal Condition
Weather points
- Relative Humidity
- Wind
- Temperature
- Precipitation
- Solar Radiation
- Cloud Cover
- Atmospheric Stability
Topography points
- Slope
- Aspect
- Elevation
- Position of Slope
- Barriers to fire spread
- Shape of country
signs of a stable atmosphere
Stratiform clouds
Limited vertical motion of air
Cooler temperatures
Light Winds
signs of an unstable atmosphere
cumuliform clouds
increased vertical air movement
warmer temps
gusty winds
Describe Box Canyons, Narrow Canyons and Ridges effects on fire
Box Canyon: strong updraft could be created increasing the intensity and rate of spread
Narrow Canyon: radiant heat could warm fuels opposite of fire and allow easier spotting
Ridge: Winds flow like water around and over ridges and can create eddies on lee sides allowing fire to be pushed down hill
Common Denominators of Fire Behaviours in near misses/tragedies
- small or quiet sectors on larger fires
- relatively light fuels
- unexpected wind shifts, gusts or squalls
- upslope fire run: chimney, narrow valley or steep slope
Define: Fire Environment
The surrounding conditions, influences and modifying factors that determine fire behaviour at a given time and location.
What does White Smoke mean?
High fuel moisture and Low fire intensity
What does Grey Smoke mean?
Fuel is moist and Moderate fire intensity
What does Black Smoke mean?
Fuel is dry and High fire intensity
What does Copper-Bronze smoke mean?
Fuel is very dry and a High to Severe fire intensity
3 types of smoke columns
Drifting Low: Stable atmosphere, lower fire intensity
High Vertical Column: Atmosphere unstable, higher fire intensity
Sheared off column: Atmosphere unstable, high winds aloft
Fire Behaviour Rules of Thumb
- Temp > or = RH: crossover and control problems likely
- RH < or = 20%: extremely dry fine fuel
- FFMC > or = 90: fast moving surface fire
- 101 - FFMC = moisture content % of fine fuels (ex: 101 - 93 = 8% mc)
- ISI > or = 10: fuels burn quickly, Black Spruce likely to crown
- BUI > or = 60: med-heavy fuels cause problems in fire behaviour
- DC > or = 300: difficult to control and possible holdover fires
