‘Dry lightning’ sparked California’s most destructive wildfires

Nearly half of the lightning strikes in northern California over the past three decades have occurred on days with little rain, sparking some of the most destructive wildfires in the state’s history.


8 August 2022

Mandatory Credit: Photo by Noah Berger/AP/Shutterstock (13057572c) Firetruck drives along California Highway 96 as the McKinney Fire burns in Klamath National Forest, Calif Western Wildfires, Klamath National Forest, United States - July 30, 2022

A wildfire burning in Klamath National Forest, California in 2022

Noah Berger/AP/Shutterstock

In central and northern California, nearly half of the lightning strikes over the past 34 years have occurred on days with very little rain. Some of the days with the most widespread “dry lightning” in that period corresponded to three of the most destructive wildfires in California history.

A dry thunderstorm develops just as usual, with warm updrafts carrying moisture to higher altitudes where it forms clouds and lightning. But if the thunderclouds form on top of a layer of warm, dry air, rain may not be as successful as lightning. “You need to have a warmer, drier, lower atmosphere,” says Dmitri Kalashnikov at Washington State University Vancouver. Dry lightning poses a particular risk to wildfires because there is no rain to put out any fires that strike.

Kalashnikov and his colleagues examined the meteorological conditions behind dry lightning in northern and central California, where lightning started nearly 30 percent of more than 5,000 fires recorded since 1987.

Using records of lightning strikes and precipitation in the region, including the fire-prone central and northern coast and the Sierra Nevada forest, the researchers found that 46 percent of lightning strikes occurred on days when less than 2.5 millimeters of rain – dry enough to be considered “dry lightning”. The strikes were recorded between 1987 and 2020 by a network of ground-based sensors that detect radio waves emitted by lightning strikes.

They then looked at the meteorological conditions on days with dry lightning, as well as the locations of dry lightning strikes to identify patterns specific to California. The study found that dry lighting occurred more frequently between July and August, although it was most widespread in terms of geographic area from June to September, when wildfire risk was highest. ​​​​The researchers found that dry lightning occurred as late as October, says Kalashnikov.

The more detailed view of the meteorology behind dry lightning in California could help forecasters create early warnings for lightning-triggered fires in the region, says Mike Flannigan at the University of Alberta in Canada. He says the same approach to studying dry lightning could be used in other places where the risk is increasing, including Australia, Siberia and Canada.

Fires are becoming more severe and more frequent as climate change results in drier vegetation, and warming may cause lightning to become more frequent.

Most fires in the United States are started by humans, but lightning can start more destructive fires. Cluster lightning can ignite many points at once, often in remote locations where it takes longer for anyone to notice the fire, Flannigan says.

California has a particularly fiery history with dry lightning – during the “Fire Outlook of 1987”, thousands of wildfires ignited by dry lightning spread widely over half a million acres. Wildfires started by dry lightning in 2020 burned nearly 2.5 million acres.

Journal reference: Environmental Research: ClimateDOI: 10.1088/2752-5295/ac84a0

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