wildland acres were burned by 88,458 fires in 2002.
In California alone, from
1997-2001, an average of 392 wildland fires were caused by arson,
burning 21,072 acres and causing over $3.2 million in damage
annually. During that period in California, in fires were a cause
determination could be made, arson was a more common wildfire cause
--California Department of Forestry &
"Investigators agree that
human activities, not lightning, are responsible for nine out of 10
wildfires...About three-quarters of the human-caused fires result
Valley Daily Bulletin, 1 November 2003
In the Northern Rockies, over 50%
of fires in 2003 (1,970 incidents) were determined to have a human
Incident Information Center
"The typical rate of solving
wildfire arsons is less than 10 percent a year."
Valley Daily Bulletin, 1 November 2003
These vignettes illustrate the severity of the
wildfire issue, and more specifically, the wildfire arson issue.
Vigorous investigation of wildland fires is the key to catching
arsonists, and can be critical in identifying serial arsonists before
they move on to bigger targets. Moreover, identification of fire
causes can assist fire protection professionals in designing
safeguards that prevent fires, thereby reducing property and habitat
loss over the long run.
Investigating a wildfire can seem a daunting task.
Wildfires are driven by variable environmental conditions: fuel load,
wind, weather, topography. Fire suppression activities, such as
backfires and fire lines, can influence the natural progression of
the fire and affect fire patterns the investigator will have to
interpret. In many cases, the fire has destroyed a large area,
possibly obliterating the evidence of its cause. After the fire is
suppressed, any remaining evidence can be subjected to, and degraded
by, weather conditions before the investigator has the opportunity to
preserve and collect the sample. The possible causes of wildland
fires are varied and unpredictable, from lightning to arson to
obscure events like a kite mishap. However, if you understand the
unique aspects of wildland fire fuels, behavior, and causes and apply
the same systematic investigative techniques as you do in a structure
fire, you are better prepared to determine wildfire origin, cause,
Wildfires are not structure fires outdoors. The
factors influencing fire development are different, fire suppression
tactics are different, and fire investigation nuances are different.
This two-part series will explore the specifics of wildland fire
fuels, behavior, and investigation. Part One covers wildland fire
fuels, spread, and possible causes. Part Two covers the investigation
THE BASICS OF WILDLAND FIRE
Pictured above: Convective heat
spreads fire from ground fuels up a tree into aerial fuels during the
Winter Valley Fire (Craig, CO). Notice how the wind pushes the flame.
Photo credit: Kari Brown. Courtesy of the Northern
Rockies Incident Information Center.
Wildland fires spread in two phases. First,
convected heat causes the fire to spread from low vegetation such as
grasses, underbrush and leaf litter (ground fuels) to higher
vegetation (aerial fuels) such as tree branches, often via the
mid-sized vegetation. In this phase, the fire grows vertically (see
Fig. 2). As the fire increases in intensity and size,
involving fuels at all levels, radiant heat becomes the primary
method of spread at both the aerial and ground fuel levels, and the
fire grows laterally.
How fuels ignite and fire spreads is heavily
- vegetation type, availability and density
- geography and topography (type of land,
slope of the land, presence of land features such as streams,
valleys, and hills)
- climate and weather (temperature,
humidity, amount of rain)
- fire suppression tactics
All of these factors interact, producing a
sometimes complex web of forces that shape the fire spread. First,
let's examine the role of vegetation.
Understanding Wildland Fire Fuels
In structure fires, the most common fuels are
construction materials, furnishings and personal belongings. In a
wildland environment, these structure fire fuels only come into play
if the fire reaches an inhabited area. In wildfires, the most common
fuels are wood and vegetation, both live and decomposing.
The composition of the fuel, including moisture
content, mineral content, and oil content is a factor in fire
ignition and spread. Effects vary because there are many species of
vegetation, all with different chemical and biological compositions.
In addition, the effects of topography, weather, and the fire itself
must be considered before fire behavior can be accurately described.
For example, Kirk's Fire Investigation notes that grass burns
differently depending on factors like moisture content and blade
height. Dry grass flashes quickly and burns out. Green grass dries
out as the fire passes over, but may not ignite. However, that same
grass, now dried by the fire, provides fuel for reburn. There is also
ignition and burning variability within a class of fuel. All wood
does not burn equally. For example, because of the high resin
content, pine burns faster and more fiercely than harder woods like
NFPA 921: Guide for Fire and Explosion Investigations
(2001 Edition) defines two classes of wildland materials for the
purposes of flammability analysis: ground fuels and aerial fuels.
Ground fuels "include all flammable materials lying on or
immedately above the ground or in the ground" (NFPA 921:
Guide for Fire and Explosion Investigations 2001 Edition, section
23.2.2, page 921-183). Examples of ground fuels are duff, peat soils,
tree roots, leaf litter, grass, low brush, and dead wood. 921
includes a detailed discussion of the flammability potential of each
of these fuels. In general, dead leaves and coniferous litter,
especially dry pine needles and fine dead wood (diameter of less than
2 in.), can play the greatest role in fire spread because they are
easily dried out and loosely arranged, allowing free flow of air
around them. See Figure 3 for an example of ground fire
behavior. Fine, dead wood ignites easily and is often the kindling
for larger fuels, like downed logs and large tree limbs. Not all
ground fuels necessarily accelerate fire spread. For example, low
brush might actually retard fire spread because it can hold moisture
in at the ground level, keeping leaf litter wet and therefore less
susceptible to ignition.
Aerial fuels "include all green and dead
materials located in the upper forest canopy" (NFPA 921:
Guide for Fire and Explosion Investigations 2001 Edition, section
23.2.3, page 921-184). These materials include tree branches and
crowns, dead trees (snags), tree moss, and high brush. In these
fuels, flammability is generally increased by the presence of dead
branches, coniferous needles, dry stumps and snags. Please refer to
NFPA 921, section 23.2.1-126.96.36.199 for specific information on
each type of wildland fire fuel and how it commonly plays a role in
The "bridge" between ground fuels and
aerial fuels is often mid-sized brush, saplings, partially-downed
tree branches, and small trees (see Fig. 4). These materials
catch fire due to their proximity to flaming ground fuels, then
spread that fire upward to mature trees and outward to other
The Influence of Wind
Wind plays a major role in fire spread and can
change over the life of the fire. Wind can:
- determine or influence the direction of fire spread
- accelerate the flame front onto new fuels
- accelerate evaporation of moisture and
dry out fuels in advance of the fire
- carry embers and flaming material aloft
and deposit them in unburned areas, possibly igniting spot fires
The direction and intensity of wind is influenced
by global wind patterns, differences in atmospheric pressure, solar
convection1, topography, and the
fire itself. Entrainment of air into the rising fire plume actually
creates wind that can further feed the fire's spread (see Fig.
5). At its most intense, fire winds can develop into a fire
storm, where indrafts into the convection column can create
The Influence of Geography
Geography, especially as it interacts with wind,
can affect fire development.
- Ground level depression geographic
features, such as valleys, can laterally confine the fire.
Confinement concentrates the heat in a smaller airspace, increasing
combustion and fire spread potential.
Cleared land geographic features, such as
wide rivers, cultivated land, or clearings, can be natural fire
breaks. They can be a barrier to fire spread because the fire cannot
leap the fuel-deficient span. However, this effect is not absolute;
wind can lift burning particles over these natural fire breaks onto a
new fuel load.
Pictured above: Extreme fire
behavior in a dense, mixed conifer stand. Fires of this size and intensity
can create their own wind, and develop into a fire storm. Courtesy of
Rockies Incident Information Center.
- The topography of slope, both angle and
orientation (to or away from the sun), can significantly affect fire
spread. Slope increases flame contact with uphill fuels, thereby more
rapidly heating them and increasing combustion potential. Therefore,
the fire burns more intensely and quickly. Uphill winds accelerate
this process, as do slopes that face the drying warmth of the sun.
The Influence of Weather
Weather, most notably temperature, humidity and
rainfall, can affect both the conditions that contribute to fire
ignition and the spread of the fire. Higher temperatures can cause
fuels to dry out more quickly, and make them more susceptible to
ignition. High humidity and/or rainfall can retard combustion and
fire spread by keeping fuels moist. Conversely, dry air evaporates
moisture from fuels, making them more susceptible to combustion.
The Influence of Wildfire Suppression Tactics
Possible Wildland Fire Causes
suppression tactics can also affect fire spread and the
interpretation of fire indicators. Methods of wildfire suppression
- Fire lines, which
are manmade barriers to fire spread, such as trenches and expanses of
cleared vegetation (see Fig. 6).
- Air drops, which
are the deployment of water or fire retardant from aircraft onto the
fire and/or onto uninvolved areas to hinder fire development (see
- Firing out or
backfires, which is controlled burning of the fuel between the
control line and the fire head to stop the advance of the fire by
depriving it of combustible fuel (see Fig. 8).
- Class A Foam,
which may be applied to slow burning fuels to extinguish them and/or
applied to unburned fuels as a protective barrier against ignition.
During the suppression of
a wildland fire, there may be opportunities to make tactical choices
that help preserve fire patterns and evidence. If possible, fire
service personnel should:
- Limit the
application of water to areas that have already burned. These areas
may contain important clues to fire spread and cause and these
indicators may be obliterated by copious amounts of water.
- Limit dragging
hose through burned areas. Dragging can obliterate patterns and
destroy fragile evidence.
- Park firefighting
vehicles away from burned areas. Vehicles and the associated foot
traffic may trample evidence and indicators. Be especially sensitive
to this possibility at roadside fires, where, if an arsonist set the
fire, there may be tire tracks, footprints, and trace evidence at the
above: Aerial fire suppression during the Burgdorf Fire (Payette
National Forest, Idaho). Photo courtesy of the National Interagency Fire
above: Firefighters dig a fire line during the Burgdorf Junction Fire
(Burgdorf, ID). Photo credit: Karen Wattenmaker. Photo courtesy of
the National Interagency Fire
Wildland fire causes are varied and the most
common ones differ substantially from the most common structure fire
causes. Major wildland fire causes (as ennumerated by NFPA
921) and potential indicators of that cause are as follows.
Incendiary Fire. Wildland fires set
intentionally often begin in accessible areas because they are easily
reached, but often lightly traveled--and therefore the firesetter is
less likely to be discovered. The method of ignition varies and may
be immediate or an improvised delay device. Juveniles may
intentionally or accidentally set a fire using matches, a lighter, or
other device. Incendiary fires can be indicated by ignition source
remnants, ignitable liquid residue, evidence of human presence (such
as footprints or tireprints), multiple points of ignition, trailers,
and remains of delay devices. The method of ignition is limited only
by the imagination of the arsonist, but Kirk's Fire
Investigation reports that the most common time-delay device is a
bundle of matches or matchbook surrounding a burning cigarette.
The remains of ignition materials, such as
cigarettes, can be very fragile. Before collecting these items, be
sure to thoroughly photograph and document them, in case their
fragile condition causes them to disintegrate upon collection. If a
cigarette is involved, be sure to collect it for class
characteristics identification, fingerprint analysis, and possible
Timothy G. Huff, former FBI profiler specializing
in arson and bombing cases, feels that the fire investigator must be
aware of the phenomenon of escalation with serial arsonists. "A
lot of serial arsonists start out with small vegetation fires in
ditch banks or vacant lots. Then, to escalate the danger and
excitement, they move on to abandoned sheds or vehicles, then on
further to targets of higher risk and greater 'reward.' This isn't a
constant; there are plenty of examples of serial arsonists who revert
back to a smaller fire during a pattern of escalation, but the
investigator needs to understand the phenomenon and be cognizant of
the fact that a seemingly insignificant fire can be the start of a
firesetting career." Huff estimates that the percentage of fires
in a given jurisdiction that are the result of arson can range as
high as 30% or higher, depending on whether there is an active serial
arsonist. But, no jurisdiction is untouched by wildland arson fire.
"In my experience, if your jurisdiction has no arsons, the fires
aren't being investigated well enough," asserts Huff.
Lightning. When lighting strikes, it can spark a fire.
Lightning often strikes trees, power lines and transmission towers,
and rocky peaks (see Fig. 9). Lightning can also strike open
ground. Lightning can splinter or explode the item it strikes and can
also leave a glassy residue, called fulgurites, as the heat melts
sand on the ground or on vegetation. Lightning strikes must be
confirmed by the weather service or a lightning detection service. Be
aware that a fire might not start immediately after a lightning
strike. The fire can smolder for some period of time before becoming
a full wildfire.
Spontaneous Heating. There are fuels
that can self-heat to temperatures sufficient for ignition. These
fuels include hay, grain dust, wood chips, and manure. Spontaneous
heating to ignition temperature occurs when heat from exothermic
chemical or biological processes does not dissipate, usually because
of restricted airflow. This often happens in large piles of the
self-heating fuel, or in hot conditions that increase the temperature
of the material. Heating is accelerated on warm, humid days. Unburned
amounts of the spontaneously-heated mixture may remain after the fire
if flame did not reach the bottom of the pile and/or if there was not
sufficient oxygen flow through the pile for complete burning.
Campfires. Campfires, especially if
left unattended or improperly extinguished, can spread to adjoining
fuels and start a wildfire. Clues that a campfire was present can
include rock circles, dug pits with large amounts of ash, and garbage
from human activity. Because campfires are started by human activity,
witnesses can be vital sources of information on the incipient
Smoking Materials. Discarded smoking
materials can ignite a wildfire, however the conditions must be
conducive to ignition in the time the smoking material is still
burning before it consumes itself and dies out. Even though smoking
materials burn at a very high temperature, if that heat does not come
into close, confined contact with a dry, fine fuel, ignition will
probably not occur. The filter or butt of the smoking material may
still be present after the fire. Smoking materials are discarded by
people, therefore witnesses are important sources of information.
Outdoor Debris Burning. In many
locations, outdoor debris burning is permitted. Where it is not
permitted, persons may still illegally burn refuse. Especially if
conditions are dry, outdoor burning can get out of control and spread
to vegetation in the surrounding area. Witnesses are a good source of
information about outdoor burning because intentional outdoor burns
are started, and sometimes monitored, by people. In addition, some of
the materials being burned may remain after the fire, a container
(such as an oil drum) that items were burned in may remain, and
accelerant residue from an ignitable liquid used to start the burn
Electricity, Oil, and Gas Machinery.
Power transmission lines are a common source of ignition of
wildfires. The ways in which power lines can start fires include:
- Electrical transformer malfunction or
explosion, dropping flaming, sparking, or hot material onto fuels.
Damage to the electrical equipment over the area of origin is often
- Overhead power lines coming in contact
with trees. This ignition will often leave a brand where the power
line and the tree made contact.
- Animals short-circuiting the power line,
then falling to the ground and spreading flame to fuels.
- Fallen wires from wind or storm damage
spark and ignite fuels. Damage to the electrical equipment over the
area of origin is often present.
- Arcing between conductors brought into
accidental contact, often by high winds and/or tree limbs.
- Trees falling on power lines and
grounding them. Damage to the electrical equipment over the area of
origin is often present.
Oil and gas drilling also involves flammable and electrical
materials that can start a fire. Cooperation of the utility can help
determine if a malfunction or other event occurred.
Equipment Failure. Machinery and vehicles
in wildland areas are subject to electrical and mechanical failure
that can spark a fire. Equipment also often requires ignitable
liquids, such as gasoline, that can start a fire if ignited by a
spark or other flame source (see Fig. 10). A suspected
equipment failure cause should be examined by an expert and the cause
of failure and chain of events, including first material ignited,
must be established. Any appliance or equipment failure should be
reported to the U.S. Consumer Product
Pictured above: The Ranger Hills Fire
(TX) was ignited on August 29, 2000 when this overheating Suburban
leaked flammable fluids which flowed downhill, also igniting a
semi-tractor trailer. The burning vehicles then ignited the adjacent
vegetation. The long-term drought and dense vegetation led to extreme
burning conditions within minutes. The Ranger Hills Fire produced a
smoke column approximately 2000 feet tall in only 30 minutes. Photo
courtesy of the National
Interagency Fire Center.
Railroad. Trains can emit sparks,
heat, and hot materials that can ignite nearby fuels. Possible
sources of flame and/or heat include exhaust fumes, hot brake metal,
and overheated wheel bearings. Railroad crews cutting, grinding, and
welding track are the source of some railroad fires. An area of
origin near railroad tracks should be checked out with the railroad
to see if a train, train equipment, or the activities of train
personnel might have been the source.
Fireworks. Fireworks can ignite dry
vegetation with sparks and hot debris. After the fire, part of the
firework, its packaging, or a crater from its explosion may remain.
Fireworks are set off by humans, therefore witnesses are important
sources of information.
Controlled Burn. A controlled burn
set for land management purposes can get out of hand and grow into an
uncontrolled wildfire. Professionals should be available to provide
information on this possibility.
Natural Disaster. Lava and superheated ash
from volcanic activity can spark fires (see Fig. 11). When
volcanic material heated to thousands of degrees comes in contact
with fuel, the fuel can combust. Volcanic activity in the United
States is tracked and can be verified by the U.S. Geological Survey
Volcanic events are usually well documented.
Pictured above: An accretionary lava
ball from Kilauea comes to rest and smolders on the grass after
rolling off the top of an 'a'a flow in Royal Gardens subdivision
(Hawai'i). Accretionary lava balls form as viscous lava is molded
around a core of already solidified lava. Photo credit: J.D. Griggs,
7/2/83. Photo courtesy of the U.S. Geological Survey Hawaiian
Focusing of Sunlight. Glass
fragments with lens properties and concave reflective metal objects
can focus light rays into a small area, concentrating the heat from
the sun. This concentrated heat can reach sufficient temperature to
ignite the fuel it illuminates. Remnants of the focusing object may
remain after the fire.
This information covers the basics of wildland fire dynamics and
causes. Next month, Part Two of this article will examine the
investigation of wildfires.
The author gratefully acknowledges the
contributions of the following professionals to this article:
Timothy G. Huff
Bob Duval, NFPA
Jim Smalley, NFPA
Robert A. Corry
Timothy G. Huff: Biography
Timothy G. Huff is retired from the California
Department of Forestry and Fire Protection and the Federal Bureau of
Investigation. Huff for 10 years specialized in arson and bombing
profiling for the FBI, assigned to Quantico, VA at the National
Center for the Analysis of Violent Crime. Prior to that, he was a
30-year veteran of the California Department of Forestry and Fire
Protection and retired at Chief Law Enforcement Officer. He now is a
law enforcement consultant in California.
1 Solar convection is the daily process of air heating and
cooling, which produces diurnal winds. During the day, air heats up
and rises, creating winds that blow upslope. At night, daytime heat
dissipates, and the cooler air sinks, creating downslope winds.