Dry lightning – when strikes occur with little or no rainfall — has sparked some of California’s biggest and most destructive wildfires. The August Complex of 2020, for example, burned more than a million acres.

But a widely used threshold used to define dry lightning doesn’t fully capture wildfire ignition risk across the western United States, according to a new multi-institutional study. In parts of Northern and Central California, for example, lightning strikes can trigger conflagrations amid wetter weather.

“It's not as clear-cut as simply (a) one-size-fits-all precipitation amount,” said study author Dmitri Kalashnikov, a climate scientist at Washington State University-Vancouver.

Lightning-ignited wildfires accounted for nearly half of the acres burned in Northern California from 2001 through 2022, according to data from the National Interagency Fire Center.

Dry lightning occurs when the air beneath a thunderstorm is dry, and raindrops evaporate before reaching the Earth’s surface. Lightning flashes can also travel far from where they were generated, said Brian Vant-Hull, a research meteorologist at the City University of New York Remote Sensing Earth System Institute — even 15 miles from the core of a storm.

The result is the ignition of a fire, without rain to extinguish flames. Experts typically categorize lightning as “dry” when it’s accompanied by less than 2.5 millimeters of precipitation in a day, or about a tenth of an inch. While this threshold has been in use for decades, it’s not clear where it came from, Kalashnikov said.

In the study, the authors analyzed wildfires across the western United States between 2015 to 2020, matching about 3,700 with a lightning strike. The researchers saw stark differences among distinct ecological areas, especially when they distinguished between promptly detected blazes versus wildfires that were detected two to five days following a lightning strike.

“They’re extra sneaky,” Kalashnikov said of these fires, known as holdovers. “The fire might start out as more of a smolder and then a few days later when things dry out, the winds pick up maybe, it can then blow up to a major wildfire.”

While the amount of precipitation associated with promptly detected wildfires ranged from 0.07 to 0.18 inches, holdover wildfires occurred under wetter conditions, sparking despite 0.12 to 0.3 inches of rain.

Places that saw the highest proportion of holdovers were mountainous forest regions. In such locales — including the Sierra Nevada and Northern Coast ranges — dead plant material that’s gathered at the base of trees can decompose and create a layer resembling mulch, which can burn with smoldering combustion, glowing like a cigarette.

“That can stay in forest areas for days and days,” said Scott Stephens, a professor of fire science at the University of California.

Part of the reason is because tree branches can shelter holdover fires from rain — and from view.

“You have the canopy to actually shield the smoldering for a few days,” Kalashnikov said.

In previous work, Vant-Hull had also found regional differences in precipitation associated with dry lightning, and appreciated how the new study went a step further to explore how vegetation might explain the variation.

Different regions could use the distinct thresholds to better prepare and assess whether thunderstorms could ignite wildfires, Kalashnikov said.