Pyro-forensics: Investigating the Cause and Origin of Wildland Fires

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This case study is part of a collection of pages developed by students in the 2012 course, "Megafires: rare events or the new norm," in the Department of Earth Sciences, Montana State University. Learn more about this project.

Pyro-forensics is the science of determining the cause and origin of wildland fires. It is valuable to understand how and why a wildfire started in order to assign responsibility, mitigate future adverse impacts, and inform management decisions and educational campaigns.

Introduction

After a wildfire, citizens and management officials alike are left with many questions. Where did the wildfire start? Was the ignition human or natural, accidental or intentional? What can the local community take away from this information? It is the work of fire investigators and pyro-forensic specialists to answer these and many more questions. Pyro-forensics at its root represents an attempt to determine where a fire started and what started it. A multitude of fire directional indicators can be used to map out the origin, and evidence found at the origin can assist in determining the cause of ignition. Finally, the work of pyro-forensic specialists has come to play an important role in the legal system with several high-profile cases. Read on to more fully explore these topics!

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Figure 1.2 Burned forest in Yellowstone National Park

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Figure 1.1 Finding the fire origin by going back and forth between transition zones.

Provenance: Courtesy of Wildfire Origin & Cause Handbook
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Fire Cause Categories

Fires can be categorized by their cause or ignition source, which can be either natural or human-caused. Studying the ignition source of the fire can also determine whether or not a human-caused fire was started intentionally or accidentally. The most common type of natural ignition source is lightning, while human triggered fires can be caused by accidental misuse of fire (smoking, lighting campfires, etc.) or by intentional ignition (arson). The following categories can be used to classify fire ignition sources:

Natural Ignition Causes:

1. Lightning: This includes ignitions caused by natural lightning events. One of the most common indications of a lightning ignition is a lightning scar which may be visible on vegetation around the fire source. See figure 2.1

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Figure 2.1 Example of a tree lightning scar

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2. Spontaneous Combustion: There are some fuels that can spontaneously ignite when certain environmental conditions are met. During periods of hot weather with high humidity, certain kinds of organic materials, including manure, grain, hay and piles of decomposing vegetation provide the most ideal situation for spontaneous combustion to occur.

Human Ignition Causes:

1. Campfires: This includes any fire constructed for cooking, warmth, or any other purpose. These fires are often related to the activities of campers and all others who recreate in the outdoors. Campfires can be distinguished by pits of ash, wood, and the use of rock to surround the fire. Signs of recent camping activity, including food scraps and discarded camping equipment can also give indication of possible campfire activity.

2. Smoking: This includes fires caused by the misuse of smoking devices including cigarettes, cigars, pipes, lighters, etc. There are a number of conditions that must be met for a cigarette to ignite a fire, including a relative humidity of under 22% and a placement position among fine, dry fuels. The presence of wind causes cigarettes and other smoking devices to burn at a faster rate, leading to a higher probability of ignition.

3. Debris Burning: This category includes fires caused by the burning of yard waste, slash, garbage and other fuels. This can occur at either private residences or at work sites where large amounts of biomass is burned to clear large areas of land. Wildland fires occur when wind carries hot ash to other locations from these controlled burn areas. There is also the possibility of a holdover fire, where fire occurs at a land clearing site well after the original fire was burned. See figure 2.2

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Figure 2.2 Example of a debris pile that may burn out of control and spread to other locations

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Figure 2.3 Fireworks can cause fire in areas of abundant fuel

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4. Arson: These fires are caused by intentional ignition in an attempt to cause harm or damage to an area. Arson-caused fires are often set in the cover of darkness in high-travel areas where affects will be greatest. There are a number of ignition devices used by arsonists, including lighters, fireworks, homemade time-delayed devices, and fire carried from other locations.

5. Miscellaneous: There are a number of ignition sources that cannot be placed into the above categories. These ignition sources include the negligent use of fireworks, sparks caused by the use of machinery and other industrial equipment, and fires related to buildings and structures. See figure 2.3

Fire Indicators

Fire indicators allow for investigators to reconstruct a fire's spread outward from the origin, which is located by tracing directional indicators backwards to a point source. There are 14 classes of fire indicators.

1. Protection: unexposed side of fuel is shielded from burning or heat damage.

2. Grass stem: advancing fires consume all but the base of the stem, backing fires burn the base of the stem and leave the rest of the unburned stem flat lying, with the seed head pointing in the direction the fire came from.

3. Freezing: leaves and small stems are "frozen" by extreme heat in the direction of prevailing winds created by the fire. See Figure 3.1

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Figure 3.3 Angle of char for an advancing fire. Note how angle is greater than slope.

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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Figure 3.2. Angle of char for a backing fire. Note how angle is parallel to slope.

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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4. Degree of damage: higher degree of damage (deeper charring, more loss of material, more white ash) on side of fuel exposed to advancing fire.

5. Depth of char: side of fuel with deepest charring and most scaling was facing oncoming fire.

6. Angle of char: backing fire leaves char angle equal to slope (See Figure 3.2), advancing fire leaves char angle steeper than slope. See Figure 3.3

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Figure 3.1 Branches frozen in direction of winds at time of fire.

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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Figure 3.4 Leaves curl inward toward oncoming fire.

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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7. Spalling: shallow chips in the surface of a rock generally face advancing fire.

8. Curling: green vegetation curls inward toward direction fire is coming from; usually occurs with slower moving backing and lateral fires. See Figure 3.4

9. Sooting: soot more heavily deposited on side of object facing oncoming fire.

10. Staining: stains on non-combustible objects appear on side facing flames.

11. White ash: more white ash on sides of fuels facing oncoming fire. See Figure 3.5

12. Cupping: limbs and twigs on the origin side of the fuel will exhibit a rounded off cup, limbs and twigs on the non-origin side will exhibit a sharply pointed top of the cup

13. V-patterns: the fire origin is located at the point of the V or the cup of the U of burnt vegetation. See Figure 3.6

14. Die-out patterns: areas where the fire decreases intensity are marked by fingers or islands of unburned to partially burned fuels.

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Figure 3.5 Left image shows exposed side, right image shows shielded side.

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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Figure 3.6 Fire origin at point of V

Provenance: Courtesy of Wildfire Origin & Cause Determination Handbook
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Legal Applications of Pyro-Forensics

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Figure 4.1 London Fire Investigation Unit

Provenance: Courtesy of http://en.wikipedia.org/wiki/Fire_investigation
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"Fire does not destroy evidence—it creates it." - Manuel Vasquez, Texas fire marshal

Fire investigators are often involved in determining if a fire was purposely set, and if so, identifying the responsible party. Arson, the crime of intentionally and maliciously setting fire to structures or wildlands, is always considered a possible fire cause by investigators. Consequently, the site of a fire origin is initially treated as a crime scene until it can be definitively determined that the fire set naturally or accidentally. The following links detail case studies in which pyro-forensics has played a key role in the legal process.

• Arson: the Motivation and Fire Investigation -- http://www.npr.org/2012/07/12/156677234/arson-the-motivation-and-fire-investigation
• Colorado Fires Arrest -- http://www.npr.org/player/v2/mediaPlayer.html?action=1&t=1&islist=false&id=1145135&m=145135
• Arson Forensics Sets Old Fire Myths Ablaze -- http://www.npr.org/2011/11/19/142546979/arson-forensics-sets-old-fire-myths-ablaze
• Trial by Fire -- http://www.newyorker.com/reporting/2009/09/07/090907fa_fact_grann?currentPage=1
• Man held 42 years in deadly Tucson Pioneer Hotel fire to be released -- http://azstarnet.com/news/local/man-held-years-in-deadly-tucson-pioneer-hotel-fire-to/article_984a194e-7c21-53d6-946e-ae1a1a53d303.html

How can I learn more?

The following sections provide fun and in-depth resources for further inquiry into the exciting science of pyro-forensics!

Learning Activity

• How well do you understand pyro-forensics after reading this webpage? Find out with a quick quiz! Simply follow the link, enter your name, and the quiz will appear. https://testmoz.com/149035/
• How well do you know your fire indicators? See if you can fill out this crossword puzzle. https://crosswordlabs.com/view/pyro-forensics-fire-indicators

Related Links

• For an in-depth look at the protocol for determining fire origin and cause, see the Wildfire Origin & Cause Determination Handbook at Wildfire Origin & Cause Determination Handbook. All of the images in the "Fire Cause Categories" and "Fire Indicators" sections were obtained from this handbook.
• For an introduction into the work of fire investigators, the origin and movement of fire, and the behavior of people caught in fires, see Nicholas Faith's book Blaze: the Forensics of Fire http://books.google.com/books?id=m9-lCwex5EQC&dq=pyro+forensics&source=gbs_navlinks_s
• For a comprehensive website with training resources for fire investigators, see interFire Online at http://www.interfire.org/
• A paper on the psychology of arson and pyromania: <a>http://www.jaapl.org/content/40/3/355.full.pdf</a>
• An interview with a wildfire investigator who discusses several unique practices in wildfire investigation: http://www.youtube.com/watch?v=6paDi98MD4w

References

The Arson Research Project, 2012, http://www.thearsonproject.org/index.html (accessed April 2013).

Factsheet: Wildfire Origin and Cause Determination, 2013, Landelijk informatieknooppunt natuurbranden: http://www.infopuntveiligheid.nl/Infopuntdocumenten/201302%20Factsheet%20Wildfire%20Origin%22and%20Cause%20determination.pdf (accessed April 2013)

InterFire Online, 2013, http://www.interfire.org/ (accessed April 2013)

Lovreglio, R., Leone, V., Gianquinto, P., and Notarnicola, A., 2010, Wildfire cause analysis: four case-studies in southern Italy: iForest Biogeosciences and Forestry, v. 3, p. 8-15.

National Wildfire Coordinating Group Fire Investigation Working Team, 2005, Wildfire Origin & Cause Determination Handbook, May 2005: PMS 412-1.

Authors

Jeff Gay, James Mauch, Eli Merrell

Montana State University ERTH 102: Megafires