How lightning starts wildfires, and why those fires get big

01

How many wildfires does lightning start?[1][2]

Lightning is the main natural ignition source in the United States. Congressional Research Service figures put 89% of wildfires as human-caused between 2018 and 2022, leaving roughly 11% from lightning, but lightning fires burned 53% of the average acreage over the same period.

The gap between those two numbers is the whole story. Human ignitions cluster near roads, houses and infrastructure, which is also where they are spotted quickly and reached quickly. Lightning ignitions land in backcountry, often in steep terrain, sometimes many hours from the nearest crew.

Storms also start many fires at once. A single dry-lightning outbreak can produce hundreds of ignitions across a region in an afternoon, which overwhelms the suppression resources that would comfortably handle them one at a time. That is how individual storm systems end up associated with entire fire seasons.

02

Why does continuing current matter more than peak current?[2]

Because ignition is about delivered energy over time, not instantaneous power. A 200,000 ampere stroke lasting microseconds deposits less heat into fuel than a modest current sustained for tenths of a second. Strikes with long continuing current are the ones that reliably start fires.

Continuing current is a low-amplitude flow, typically in the hundreds of amperes, that persists after the return stroke for anywhere from tens to hundreds of milliseconds. It is thousands of times weaker than the peak, and it lasts thousands of times longer, so it moves far more total charge.

That is why positive strikes are over-represented in fire ignition. They carry continuing current more often and for longer than negative strikes, and they also strike further from the storm core, which means further from the rain that would otherwise put the fire out.

Research on which factor dominates ignition is not settled. Continuing current is one leading explanation, while other work emphasises fuel moisture and fuel type. The disagreement is about weighting rather than about whether continuing current matters.

03

What is dry lightning?

Dry lightning is a thunderstorm that produces strikes with little or no rain reaching the ground. It happens when the air below the cloud base is dry and deep enough that rain evaporates during its fall, leaving the ignition source with nothing to suppress it.

The result is the worst combination for fire. Strikes arrive in fuels that are already dry, and the storm's outflow winds fan whatever ignites. The gusts that reach the surface from an evaporating rain shaft can be strong and erratic, which is dangerous for anyone working nearby.

This is common in the interior West in summer, where afternoon storms build over terrain with a deep, dry layer underneath. It is also why fire agencies watch lightning detection data directly during those outbreaks: a strike location is a candidate ignition point, and the map is often the first indication that one exists.

04

What is a holdover fire?

A holdover is a lightning ignition that smoulders undetected for days before it grows enough to be visible. The strike puts fire into duff, a rotten log or the base of a snag, where it burns slowly with little smoke until weather changes and it spreads.

Holdovers are why lightning strike records get cross-referenced against fires that appear later. A fire discovered on a clear afternoon with no storm nearby may trace back to a strike logged four or five days earlier, and in deep organic soils the delay can be longer.

The practical consequence is that a lightning outbreak does not stop producing fires when the storm ends. Agencies fly detection patterns over strike clusters for days afterwards, targeting the coordinates the detection network recorded.