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Lecture

GOPH 375 Lecture Notes - Lightning, Berkeley Hills, 2003 Okanagan Mountain Park Fire


Department
Geophysics
Course Code
GOPH 375
Professor
All

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Chapter 14: Fires
Canada mostly tree fires caused by people, but lightening caused fires burn more trees
Figure 14.1/14.3 number of fires / area burned across period of time
o Not necessarily correlated, one big fire can cause more damage than a bunch of little
ones
Fire needs (triangle): Fuel, oxygen, and heat. Fuel is the limiting variable because oxygen (air)
and heat (sun) are usually readily available. Any combustible material is fuel (trees, houses, slash
[logging debris]). To fight a fire, remove any one of these 3
Backfires a fire lit by firefighters in front of an approaching wildfire, killing fuel for the wildfire
Ladder fuels allow small ground fires to be carried to the tops of trees, creating large wildfires,
by the varying heights of vegetation.
Fire is oxygen + hydrogen + carbon, photosynthesis in reverse plant material is heated beyond
its ignition point, and reaction gives off heat in a fire. Solar energy stored by plants during their
growth is released when they are burned
Wildfires recycle nutrients from plant materials back into the environment & regenerate plant
communities decomposition also does this if the climate is warm and moist
Two types of fires: figure 14.8
o Ground/surface fires most dangerous
o Crown fires at the tops of trees; ladder effect, need something to get fire from the
ground to the tops of the trees
Figure 14.10 3 Stages of Combustion All stages occur simultaneously in a wildfire
o Pre-heating water expelled from plants/wood/fuels by nearby flames, drought or sun
Pyrolysis: thermal degradation of wood, gives off flammable gasses
o Flaming combustion pyrolized surface of the wood burns fast and hot, stage of
greatest energy release. Heat is transferred in radiation, conduction, and diffusion.
Pyrolysis gases fuel the surface flames, and preheat the insides of a log
o Glowing combustion fire consumes solid wood instead of pyrolized gases; active
flames disappear, but the word surface still glows. Wood burns more slowly, and at
lower temperatures. Slow oxidation of charred remainders
Fire Styles: glowing combustion moving slowly along the ground, flaming combustion wall of fire, or
crown fire moving through tree tops. The spread of fire depends on:
Fuel: chemical composition of plants, organic debris. Eucalyptus burns easily
Weather: winds bring a continuous supply of oxygen, distribute heat, push flames forward, and
bend them towards preheated fuels. Without wind, a vertical column of convected heat is
created and fire moves slowly. Column can interact with upper level winds that could increase
fire. Strong winds pick up embers called firebrands and drop them in unburned areas, starting
new blazes called spot fires
Topography: winds blowing over rugged topography experience turbulence. Fire spread fastest
up slopes because the heat that rises preheats the vegetation above it
Fire behavior: rising columns of hot unstable air can spin off fire whirls; wind sucked into the
base of the fire brings oxygen.
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