Category: Weather

Hi I’m Holly Strand of Utah State University’s College of Natural Resources.

It’s fire season in UT. The hill slopes have turned a parched yellow-brown and the trees look thirsty and flammable. As of Aug. 13, there were 7 fires burning across the state.

One of the first questions that arises with any wildfire is “What started it?”

And I wonder: “How in the world would you figure this out given the destruction that a fire leaves in its wake?”

The first step toward identifying a cause involves finding the exact spot where the fire started. To do this, investigators look for witnesses. And having information on wind direction for the duration of the fire helps a lot. But even in the absence of these, the fire itself leaves clues regarding the direction of movement. And if you know the direction of movement, you can trace the path backwards to the ignition site.

For instance, on a tree or post, the side exposed to the oncoming fire will show deeper charring, more loss of wood and more white ash than the unexposed side.

However the leeward side of a tree may have the highest char mark. That’s because as strong winds blows the fire past a tree, the flames are drawn into the eddy zone on the leeward side and extend higher up the trunk. Still, the deeper char will be on the side facing the advancing flame. So to get to the area of the fire origin, you’d want to follow direction indicated by the most damaged tree face.

When green leaves of shrubs or trees are scorched, they tend to become soft and pliable and bend in the direction of the prevailing wind. After the fire passes they become fixed in this position as they cool, still pointing in the direction of the wind. So the opposite direction of the pointing leaves will take you closer to the fire origin.

Another thing that generally helps fire investigators is the fact that all fires need time to achieve their maximum spread rate/intensity. A newly ignited fire may take 30 min or more to ramp up. As a result even with high intensity fires, the area of initial ignition will show relatively less damage; upper foliage and branches may even remain intact.

Once the area of origin is identified, investigators look for the human or natural source of the blaze. Footprints, tire marks or evidence of a campfire are noted with interest. Nearby power lines, railroad tracks or electric fences may have provided the initial spark. Investigators often end up on their hands and knees searching for things such as cigarette parts, ignitable liquid residue; bullets or empty shell casings.

If lightening is a suspected source investigators look for strike marks or splintered wood fragments. Lightening can also leave a glassy residue, called a fulgurite, when the strike melts sand on the ground or on vegetation.

Thanks to Wesley Page of USU’s Department of Wildland Resources for sharing his wildfire expertise.

For sources and more information on investigating the cause of wildfires go to www.wildaboututah.org

For Wild About Utah and USU’s College of Natural Resources, I’m Holly Strand.

Credits:

Images: Hyrum Fire, Courtesy & Copyright 2013 Holly Strand
Courtesy Wikimedia, John Elson, Licensed under GNU Documentation License V1.2
Also images from Wildfire Origin & Cause Determination Handbook, Courtesy National Wildfire Coordinating Group(NCGW.gov)
Text: Holly Strand

Sources & Additional Reading

Cheney, Phil and Andrew Sullivan. Grassfires: Fuel, weather and fire behavior. 2008. CSIRO. https://www.amazon.com/Grassfires-Fuel-Weather-Fire-Behaviour/dp/0643093834

Wildfire Origin & Cause Determination Handbook. 2005. A publication of the National Wildfire Coordinating Group Fire Investigation Working Team NWCG Handbook 1. PMS 412-1. May 2005. https://www.nwcg.gov/pms/pubs/nfes1874/nfes1874.pdf

Investigating Wildfires: Part One. Interfire online. https://web.archive.org/web/20240406114120/https://www.interfire.org/features/wildfires.asp
(https://www.interfire.org/features/wildfires.asp accessed August 14, 2013 — Updated Apr 17, 2025 and pointed to Waybackmachine archived content)

Investigating Wildfires: Part Two. Interfire online. https://web.archive.org/web/20240406114120/https://www.interfire.org/features/wildfires2.asp
(https://www.interfire.org/features/wildfires2.asp accessed August 14, 2013 — Updated Apr 17, 2025 and pointed to Waybackmachine archived content)

Map of current large active wildland fires in Utah.
(accessed August 14, 2013) https://www.utahfireinfo.gov/

Live Worldwide Network for Lightning and Thunderstorms in Real Time, Blitzortung, https://en.blitzortung.org/live_lightning_maps.php?map=30 [URL inactive as of 1 Aug 2020}]

Wildfire Investigation, Wildland fire investigation: common wildfire causes, National Interagency Fire Center, US Department of the Interior, https://www.nifc.gov/fire-information/fire-prevention-education-mitigation/wildfire-investigation