Why does it matter whether a wildfire was started by lightning or by people?
It matters because lightning is the dominant natural ignition source for wildland fire, and a lightning determination generally removes human liability from the matter entirely. That makes the choice between a lightning cause and a human cause consequential. The determination is complicated by a behavior of lightning-ignited fires that is easy to overlook: a lightning-ignited fire need not appear on the day of the lightning strike.
What is holdover in a lightning-caused wildfire?
Holdover is the persistence of a lightning-ignited fire without visible flame before it spreads: a lightning strike can ignite a smoldering fire in duff, in punky wood, in a snag or in accumulated organic material, where the fire persists without visible flame. Given sufficient fuel depth and dryness, a holdover fire can survive rain and cool conditions and continue burning slowly for days, occasionally longer, before conditions allow it to transition to flaming combustion and spread.
The practical consequence of holdover is that the lightning strike date and the wildfire discovery date can differ substantially. A wildfire investigation that searches lightning strike data for the day of discovery and finds nothing has not excluded lightning; it has searched the wrong window.
What can lightning strike detection data show about a wildfire’s cause?
Lightning strike detection data can show whether a recorded cloud-to-ground strike occurred near a wildfire’s origin area, which supports a lightning hypothesis without proving it. Lightning location networks record cloud-to-ground strikes continuously, with time, position, polarity and estimated peak current, at accuracies useful for correlating against an origin area. Lightning strike data is archived and obtainable for historical periods.
The limitations of lightning strike detection data are worth stating alongside it. A recorded strike location has an error radius that varies with network geometry and strike characteristics, and the detection efficiency of lightning location networks is not perfect. So a recorded lightning strike near the origin area supports the lightning hypothesis without proving it, and the absence of a recorded strike weakens the lightning hypothesis without excluding it — particularly for a lower-current event.
What physical evidence does a lightning strike leave behind?
Where a lightning strike ignited a wildfire, the struck object frequently survives in identifiable form. Trees struck by lightning show characteristic spiral or vertical scarring where the current followed the cambium, with bark explosively removed along the path and wood splintering that is distinct from mechanical damage.
Lightning current also affects the ground at the base of a struck tree — fulgurites in sandy soil in some conditions, disturbed soil, and damage to roots. Where such a tree is found within the wildfire’s origin area, and its condition is consistent with a lightning strike in the relevant window, the physical evidence and the recorded lightning strike data corroborate one another. That combination is considerably stronger than either the physical evidence or the recorded strike data alone.
What evidence do human-caused wildfire ignitions leave?
Human-caused wildfire ignitions leave evidence specific to each cause: equipment use leaves metal fragments, exhaust carbon and disturbed ground; vehicle ignitions leave components, catalytic converter debris or evidence of a dragging part along a roadway; escaped debris burns leave a burn pile with a structure of its own; and smoking materials and campfires leave characteristic small-scale evidence at a definable point. A wildfire ignition from equipment use is also usually tied to an identifiable operation with records and personnel.
The origin area search is what distinguishes these human ignition sources in a wildfire investigation, which is why gridding and metal detection are routine rather than optional. Recovery of a competent human ignition source within the specific origin area is usually decisive.
What indicates that a wildfire was set deliberately?
Deliberate wildfire ignition has its own set of incendiary indicators: multiple separate origins that cannot be explained by spotting, delay or incendiary devices and their remains, ignition patterns along a route consistent with a moving vehicle, and ignitions timed to conditions favoring rapid spread.
Spotting is the confusion to guard against when assessing incendiary indicators in a wildfire. An established wildfire throws embers that start new fires ahead of the front, and the resulting multiple ignition points are an ordinary consequence of fire behavior rather than evidence of deliberate ignition. Distinguishing spot fires from separate deliberate origins relies on the spatial relationship, the timing and the wind reconstruction.
How does weather reconstruction help determine a wildfire’s cause?
Weather and fuel moisture reconstruction helps determine a wildfire’s cause by establishing whether conditions permitted holdover, whether fuels were receptive to a particular ignition mechanism on a particular day, and whether the transition from smoldering to spreading is consistent with the reported timeline. The same weather and fuel moisture reconstruction also supports the wildfire’s origin determination, so it serves both the origin question and the cause question.
Where a lightning holdover hypothesis is advanced, the weather and fuel moisture reconstruction is the substance of it. A claim that a lightning strike some days earlier produced the wildfire needs the intervening conditions to support persistence — and where they do not, the holdover hypothesis is testable and fails.
How should a wildfire investigator sequence the origin determination and the lightning and human-cause hypotheses?
The wildfire origin area is defined first, without reference to cause, precisely so that the search is not shaped by an expectation of what it will find; in practice the lightning and human-cause hypotheses are worked in parallel rather than in series. The origin area is then searched systematically for any competent ignition source, natural or human, and the lightning strike record for a window extending well before discovery is obtained regardless of what the ground search produces.
Working in that order matters because a lightning determination and a human-cause determination are both vulnerable to the same failure: an investigator who forms a cause hypothesis early tends to define the origin area in a way that accommodates it. Separating origin definition from the cause hypothesis is a straightforward guard against that failure, and it is visible in the investigation documentation when it has been done.
How are lightning and human-cause wildfire determinations challenged?
Challenges to a wildfire cause determination follow the structure of the evidence. A lightning determination is attacked on the accuracy of the strike correlation, on the absence of a struck object, or on whether holdover was plausible under the intervening weather. A human-cause determination is attacked on whether the origin area was correctly identified, whether the search was thorough enough to exclude a natural source, and whether the recovered item was actually a competent ignition source in those fuels.
In both directions the answer is the same: a wildfire cause determination that rests on the physical evidence, the recorded data and the weather reconstruction agreeing is durable. A wildfire cause determination that rests on a single line of evidence, or on the failure to find an alternative, is not.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.