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Investigating Wildfires: Part One

By Cathleen Corbitt-Dipierro
Interactive Designer, interFIRE VR

Figure 1
Pictured above: The Valley Complex Fire (Derby, MT). Photo credit: Karen Wattenmaker. Courtesy of the Northern Rockies Incident Information Center.
Wild Fire Investigation
Nationally, 6,937,584 wildland acres were burned by 88,458 fires in 2002.
--National Fire News

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 than lightning.
--California Department of Forestry & Fire Protection

"Investigators agree that human activities, not lightning, are responsible for nine out of 10 wildfires...About three-quarters of the human-caused fires result from carelessness."
--Inland 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 cause.
--Northern Rockies Incident Information Center

"The typical rate of solving wildfire arsons is less than 10 percent a year."
--Inland 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, and responsibility.

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 of wildfires.

Figure 2
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.
Wild Fires  Causes and Influences

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 influenced by:

  • vegetation type, availability and density
  • wind
  • 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 oak.

Figure 3
Pictured above: Ground fire behavior in an open Ponderosa Pine stand. Photo credit: Kari Greer. Courtesy of the Northern Rockies Incident Information Center.
Wild Fires  Causes and Influences
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.

Figure 4
Pictured above: A close-up look at "bridge" fuels involved in the Valley Complex Fire (Derby, MT). Photo credit: Karen Wattenmaker. Courtesy of the Northern Rockies Incident Information Center.
Wild Fires  Causes and Influences
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- for specific information on each type of wildland fire fuel and how it commonly plays a role in fire spread.

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 materials.

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 tornado-like effects.

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.
  • Figure 5
    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 the Northern Rockies Incident Information Center.
    Investigating Wild Fires
    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.
  • 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

Fire suppression tactics can also affect fire spread and the interpretation of fire indicators. Methods of wildfire suppression include:
  • 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 Fig. 7).
  • 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 roadside.

Figure 7
Pictured above: Aerial fire suppression during the Burgdorf Fire (Payette National Forest, Idaho). Photo courtesy of the National Interagency Fire Center.
Wildfire suppression tactics for investigators

Figure 6
Pictured above: Firefighters dig a fire line during the Burgdorf Junction Fire (Burgdorf, ID). Photo credit: Karen Wattenmaker. Photo courtesy of the National Interagency Fire Center.
Wildfire suppression tactics for investigators

Figure 8
Pictured above: Idaho Panhandle Hotshots starting the burn-out at Fish Creek Fire (8/26/03). Photo credit: Michael Rieger. Courtesy of the Northern Rockies Incident Information Center.

Possible Wildland Fire Causes

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 DNA analysis.

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.

Figure 9
Pictured above: Lightning strikes a woodland ridge. Photo courtesy of the National Interagency Fire Center.
Wild Fires  Causes and Influences
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 fire.

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 may remain.

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 present.
  • 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.

Figure 10
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.
Wild Fires  Causes and Influences
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 Safety Commission.

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.

Figure 11
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 Volcano Observatory.
Wild Fires caused by natural disasters
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.

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
Joseph Toscano

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.

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