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Dry Lightning Sparked Some of California’s Most Destructive Fires

Bone-dry conditions and rain-free lightning. What could go wrong?

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Firefighters monitor flames as they approach a residence in the valley area of Vacaville, northern California during the LNU Lightning Complex fire on August 19, 2020.
Firefighters monitor flames as they approach a residence in the valley area of Vacaville, northern California during the LNU Lightning Complex fire on August 19, 2020.
Photo: JOSH EDELSON/AFP (Getty Images)

A combination of dry lightning and drought conditions have caused some of the worst wildfires in California’s history, a new study finds.

The study published this week in Environmental Research: Climate found that dry lightning, which researchers describe as lightning that occurs with “less than 2.5mm of rainfall” or no rainfall, is responsible for huge wildfires in California. The lightning striking over especially dry vegetation, with no rain to slow down the spread of the flames, has set off some of the most destructive fires in the state’s recorded history. This includes the “Fire Siege of 1987,” which burned more than half a million acres in California that year.

According to Dmitri Kalashnikov, a doctoral student at Washington State University and the lead author of the study, lightning is connected to other large wildfires, like the 2020 August Complex fire that inspired the study. It was the largest wildfire in the state’s history and California’s first ‘gigafire’ after it burned over 1 million acres. Lightning struck the ground with no precipitation and during a drought in California, which helped researchers make the connection between the lightning and the fires.

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“It just took one massive outbreak like we saw in the middle of August that year... California was experiencing record hot and record dry conditions at that time,” Kalashnikov told Earther. “So we got this massive wildfire disaster.”

Kalashnikov and colleagues looked at precipitation records from the late 1980s to 2020 and daily lightning counts data from the National Lightning Detection Network. They found that 46% of lightning that struck the ground from May through October, which are especially dry months for the state, was dry lightning. They also found that the dry lightning is caused by a combination of dry air in the lower atmosphere and moisture and instability in the higher part of the atmosphere. Dry lightning does sometimes include rain in the higher part of the atmosphere, but it never makes it to the ground because the air in California can be so dry that the water evaporates before it hits the ground.

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The team was able to identify the large dry lightning fires by looking at the lack of precipitation that followed reports of lightning. According to Kalashnikov, an especially surprising finding about wildfires and dry lightning is how infrequent the lightning storms are in some years, and yet a few lightning strikes in especially dry areas can still lead to huge fires. One of the largest challenges associated with dry lightning fires often occurs because lightning can strike in various locations in quick succession. This often means that officials have to deploy more resources and people to contain the multiple fires. By contrast, fires started by human activity—like a gender reveal gone wrong—tend to begin at a single location and spread from there, Kalashnikov explained.

Kalashnikov called the 2020 year a “relatively quiet” year for lightning, to emphasize that even low lightning risk can still create enormous fires. Though researchers are still learning about dry lightning, connecting drought patterns to dry lightning can become part of the state’s wildfire preparation strategy.

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“It doesn’t even really matter what future trends are in dry lightning. In California, we know that it’s getting hotter. It’s getting drier, and projections call for continued warming and drying,” he said. “So any dry lightning, even if dry lighting does not increase or even if dry lighting decreases in the future—it’s going to be hitting drier vegetation under hotter conditions.”