Lightning is the dominant natural ignition source for wildland fire, and a lightning determination generally removes human liability from the matter entirely. That makes it consequential, and it is complicated by a behaviour that is easy to overlook: a lightning-ignited fire need not appear on the day of the strike.
Holdover
A strike can ignite a smouldering fire in duff, in punky wood, in a snag or in accumulated organic material where it persists without visible flame. Given sufficient fuel depth and dryness it 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 is that the strike date and the fire discovery date can differ substantially. An investigation that searches strike data for the day of discovery and finds nothing has not excluded lightning; it has searched the wrong window.
Strike detection data
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. The data is archived and obtainable for historical periods.
Its limitations are worth stating with it. Location has an error radius that varies with network geometry and strike characteristics, and detection efficiency is not perfect. So a recorded strike near the origin area supports the hypothesis without proving it, and the absence of a recorded strike weakens it without excluding it — particularly for a lower-current event.
Physical evidence of a strike
Where a strike ignited a fire, the struck object frequently survives in identifiable form. Trees 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.
The current also affects the ground at the base — fulgurites in sandy soil in some conditions, disturbed soil, and damage to roots. Where such a tree is found within the origin area, and its condition is consistent with a strike in the relevant window, the physical and the recorded evidence corroborate one another. That combination is considerably stronger than either alone.
What human ignitions leave
Human causes each carry their own signatures. Equipment use leaves metal fragments, exhaust carbon and disturbed ground, and is usually tied to an identifiable operation with records and personnel. 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. Smoking materials and campfires leave characteristic small-scale evidence at a definable point.
The origin area search is what distinguishes these, 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.
Incendiary indicators
Deliberate ignition has its own set: 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 favouring rapid spread.
Spotting is the confusion to guard against. An established fire throws embers that start new fires ahead of the front, and the resulting multiple ignition points are an ordinary consequence of fire behaviour rather than evidence of deliberate ignition. Distinguishing them relies on the spatial relationship, the timing and the wind reconstruction.
Weather reconstruction serves both questions
The same weather and fuel moisture reconstruction that supports the origin determination also bears on the cause. It establishes whether conditions permitted holdover, whether fuels were receptive to a particular ignition mechanism on a particular day, and whether the transition from smouldering to spreading is consistent with the reported timeline.
Where a holdover hypothesis is advanced, this reconstruction is the substance of it. A claim that a strike some days earlier produced the fire needs the intervening conditions to support persistence — and where they do not, the hypothesis is testable and fails.
Sequence of elimination
In practice the two hypotheses are worked in parallel rather than in series. The 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. The area is then searched systematically for any competent ignition source, natural or human, and the strike record for a window extending well before discovery is obtained regardless of what the ground search produces.
Working in that order matters because both determinations are 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 the two steps is a straightforward guard against that, and it is visible in the documentation when it has been done.
Where the determination is contested
Challenges 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 determination that rests on the physical evidence, the recorded data and the weather reconstruction agreeing is durable. One that rests on a single line, 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.