Senior Science Disasters identify some of the conditions that can combine to trigger a bushfire including dry weather high temperatures and flammable vegetation Conditions of bushfires T hree ID: 529777
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Slide1
Part 3: Bushfires
Senior Science
DisastersSlide2
identify some of the conditions that can combine to trigger a bushfire
including dry weather, high temperatures and flammable vegetationSlide3
Conditions of bushfires
T
hree
things that are necessary for a fire to
burn:Dry fuelHeatOxygenConditions that can combine to trigger a bushfire include:dry flammable vegetation (dead leaves, bark and twigs)high temperatures (and a fire that has already ignited)dry weather (low humidity air).Slide4
Fire weather is most dangerous when hot and dry winds blow from the inland, particularly if coming from a westerly or a north-westerly direction. The worst conditions occur when deep low-pressure systems near Tasmania bring strong, dry, westerly winds to the coast. This occurred in both 1994 and 2001. Other factors affecting the danger and behavior of bushfires include:
quantity
, type, condition and
distribution of fuelair temperature and sunlightrelative humiditywind direction and velocityrainfalltopographySlide5
Describe the effect of the slope of the land and intensity of the wind on the speed of the bushfireSlide6
Spread of bushfires
Spotlight
Pg
315
summariseSlide7
Identify and describe some of the energy transfers and transformations associated with bushfiresSlide8
energy transfers
The main energy released in bushfire is heat.
Heat energy can be transferred in three main ways
:
conduction where vibration of particles in solids and liquids is transferred slowly from particle to particleconvection in which heated liquid or gas moves in what are called convection currentsradiation in which electromagnetic radiation (mostly infra-red) travels at the speed of light through gas or a vacuum.Slide9
1. conduction
Conduction causes major damage to the trunks of trees from ground
level to
the first branches and this may cause collapse of the trees after
the bushfire has passed. Conduction of heat through the ground and as a danger to bushfire fighters is negligible.Slide10
2. convection
Convection currents move upwards. They can be very dangerous
to
firefighters
as the heat moves up a slope. Large amounts of heat can be transferred to the firefighters and the fire can move rapidly.Slide11
3. radiation
About 25% of the energy release in a bushfire is released as radiation
and this
mostly moves horizontally. Radiation is very effective in drying
out and preheating fuel ahead of the fire front. Firefighters must have protection against radiation.Slide12
Energy transformations
In a bushfire chemical energy from reaction of the fuel
with oxygen
is changed to heat energy, light energy and kinetic (
movement) energy of particles ejected by the fire.Australian bushfire fuels release about 20 000 kJ per kg but a significant amount of this heat is absorbed into drying the vegetation so that the net release of heat energy is about 15 000 kJ per kg. Grass and crop fuels release about half as much energy as trees.Fuel + oxygen
carbon dioxide + water + heatSlide13
Discuss the reduction of fuel by controlled burns and backburns
in reducing the risk of bushfiresSlide14
Textbook
Pg. 318-319
Controlling Bushfires