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How Fire Works
The Basics |
| Fire
can only exist when three critical elements are
combined: oxygen, fuel, and heat. Fire will
not be able to burn if one of these crucial elements is
not present. The Earth's atmosphere is
comprised of 21% oxygen, so there is usual no lack of
this element. Fuel is any combustible material.
Oregon has a lot of fuel:
41 million acres (>64,000 sq. miles) of
forest and rangeland, significant agricultural
production in the Willamette Valley and north central
Oregon, and over 500 communities at risk in high fire
prone areas. Heat is primarily the result of
weather, drought and heat waves, which dry out
vegetation. In the United States in the year 2000,
85%
of US wildland fires were due to human causes, the
remaining 15% were caused by lightning.
Arson
made up 26% of the human caused fires, and smoking
accounted for 4% of fires.
The fire triangle is an easy way to
remember the relationship between these elements.
Remove one side of the
triangle and there will be no fire.
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The Stages of
Fire
Before a fire starts, a
preheating stage occurs
where water is expelled
from plants and wood by
nearby flames, drought,
or a long hot summer
day. Wood must get
very hot before it will
combust. At 615°F
the cellulose in wood
breaks down and releases
flammable gases.
The actual burning of a
log involves many
processes of energy
release. Heat
moves inward through the
wood by the process of
conduction, it
decomposes the lignin
and cellulose of the
wood into flammable
gases which fuel the
flames, this is called
pyrolysis. Heat is
released as gases and
hot air through
convection, diffusion
transfers particles to
cooler areas, and
radiation transfers heat
as electromagnetic waves
by flames.
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The
Spread
of Fire |
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Fire
spreads
differently
for
varying
reasons,
but for
the most
part
fuel,
wind,
topography,
and
behavior
within
the fire
itself
are the
major
factors
which
have the
most
effect
as to
the
severity
and
extend a
fire
will
spread.
Fuel, as
was
mentioned,
is
anything
that
will
burn; in
the wild
fuel is
mostly
natural
vegetation.
Chemical
compositions
and oils
inside
plants
can have
varying
ranges
of
flammability,
which
can
cause
some
fires to
burn
more
severely.
Winds
are a
major
factor
in the
spread
of fire,
and
usually high
winds
accompany
the
fires
that
produce
the most
damage.
Winds
bring
fresh
oxygen
which
push
flames
forward
and
preheat
other
areas
where
the fire
can burn
and
spread to very
easily.
Topography
is a
factor
mostly
due to
slopes
in hilly
and
mountainous
areas;
fire
burns
faster
up
slope.
Topography
also
supports
little
microclimates
that can
develop in
valleys
and
other
little
places
that
contain
different
plant
communities
that can
possibly
burn
with
different
intensities.
The fire
itself
will
also
have an
affect
where it
will
spread.
Fires
produce
unstable
air and
can
change
directions
very
fast.
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A major
factor
affecting
how
travel
fires is
the
presence
and
amount
of
"litter"
or
"slash"
on the
forest
floor.
Litter
or slash
is
organic
matter
on the
ground
beneath
trees
and
forest
left
after
logging
or wind
storms;
it can
also be
the
shrubs
and
grasses
on the
forest
floor.
These
smaller
shrubs,
trees,
and
grasses
can also be
called
the understory.
When a
fire is
present
the
understory
and
litter
will act
as
"ladder
fuels,"
that
will aid the
fire
to
climb to
the tops
and
crowns
of the
trees,
created
major
disastrous
fires.
| Types of Fires
| Ground Fire - Litter is usually scarce and fire will pass through the forest quickly, little harm is done to trees. Surface Fire - Litter is thick fire will burn hot and slow and cause damage or death to trees.
Crown Fires - Litter is thick and there is a substantial understory. Fire will move from ground to treetops because of these ladder fuels. These are the most dangerous and destructive class of wildland fire.
Ladder Fuels
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