Lightning is extremely rare in the Bay Area. But here’s why it can be so dangerous when it strikes (2024)

When a lightning bolt rips across the sky, it releases enough energy to heat the air to a scorching 50,000 degrees Fahrenheit — about five times hotter than the surface of the sun.

As this superheated air explodes outward, it produces a booming clap of thunder. The sizzling heat can also ignite trees and brush to produce destructive and deadly flames.

Some of California’s largest wildfires have been sparked by lightning. Two years ago, hundreds of bolts hit the ground along the Coast Range in Northern California, igniting what would become California’s first-ever gigafire – the August Complex – which burned more than 1 million acres across six different counties.

This month, strikes from thunderstorms ignited the Six Rivers Lightning Complexburning in Humboldt and Trinity counties. It’s the second-largest blaze to ignite this year.

While acreage isn’t all that matterswhen it comes to measuring a fire’s impact, lightning has an outsize effect in kicking off fires that spread unchecked in hard-to-reach areas.

And yet, in California, lightning is exceptionally rare.

“Coastal California is one of the areas on Earth with the least amount of lightning,” said Daniel Swain, a climate scientist at UCLA and The Nature Conservancy.

But rare thunderstorms in California have large effects.While lightning-initiated wildfires make up a relatively small percentage of fires in the state and nationwide, they account for a large percentage of the acres burned, said Christopher Schultz, a meteorologist with NASA’s Short-Term Prediction and Research Transition Center.

Lightning strikes ignited about 15% of all wildfires in Northern California from 2001 through 2021, according to data from the National Interagency Fire Center. But these blazes accounted for about 50% of the total area burned by wildfires in the region during that time. The numbers tell a similar story nationwide.

Thunderstorms and lightning typically strike across the continental U.S. during the summer, when the sun heats up air that hovers near the ground.

Rare lightning

Lightning strikes don’t touch down at the same rates across the country. They occur much less frequently along the West Coast in comparison with the rest of the U.S.

Counties in California, Oregon and Washington average some of the lowest number of lightning flashes per square mile each year nationally, according to data from the National Lightning Detection Network.

These numbers include cloud-to-ground strikes as well as in-cloud lightning, which occurs within or between clouds due to differences in charge. There are about three times as many in-cloud flashes as there are ones that reach the ground.

Seminole County in Florida averaged about 60 lightning flashes per square mile annually, from 2015 through 2020. That was almost 5,000 times more lightning than in San Francisco County during the same period. All of the Bay Area counties averaged less than one flash per square mile per year.

The reason why California sees some of the least lightning in the world? Our climate.

“You need three things: moisture, instability and a source of lift,” said Chris Vagasky, a meteorologist with Vaisala, an environmental measurement company that owns and operates the National Lightning Detection Network.

In Florida — where some counties have the highest concentration of lightning in the U.S. — the Gulf of Mexico and Gulf Stream blanket land with moist, warm air.

Along the West Coast, it’s a different story. The cool marine layer keeps air from rising.

“If you give the marine layer a kick upward, it just falls back right to where it started,” Swain said.

On top of that, because California sits between the tropics and the Arctic, atmospheric circulation patterns cause air to gradually — but continually — descend above our heads. This presents yet another obstacle for a looming thundercloud to develop.

“It's putting a lid on any clouds,” Swain said.

But certain conditions do create lightning-producing clouds. Most California thunderstorms occur in inland areas, especially in the southeastern corner of the state. This region is often enveloped by the annual monsoonal moisture that impacts southwestern states every summer. Still, only four California counties averaged more than one flash per square mile per year, from 2015 through 2020.

However, though more rare, lightning also flashes in the Bay Area. From 1988 through 2017, about 14,000 cloud-to-ground lightning strikes landed within the region’s nine counties, according to data from the National Weather Service and Vaisala’s National Lightning Detection Network.

This heat map shows the total number of strikes detected during each week in the nine Bay Area counties, across 30 years of data. While most of the U.S. has the most lightning in mid-summer, flashes in the Bay Area were concentrated during weeks in August and September. Warm temperatures during these months can promote storms and lightning in the area.

But still, it’s hard to pinpoint exactly the week and time of day that lightning will strike next in the region. Because there are relatively few flashes in the Bay Area, individual storms can bend the statistics. Many of the strikes that occurred at 6 a.m. during the week of September 3, for example, happened over the course of a single year.

Bolts to blazes

Consecutive years of record-breaking drought, and spring and summer months without rainstorms, means vegetation is parched come late summer and fall. All the ingredients are in place for wildfires to spread rapidly when lightning strikes.

Though thunderstorms are rare, lightning-initiated wildfires have burned millions of acres across California over the past two decades, with recent damage concentrated in Northern California.

The Bay Area has also been affected. In addition to the 2020 August Complex, which coalesced from 37 lightning-sparked fires, this lightning siege also ignited the SCU Lightning Complex, which burned about 400,000 acres in Contra Costa, Santa Clara, Alameda, San Joaquin and Stanislaus counties the same year.

“Asbestos forests” of the Santa Cruz Mountains — named for their historic fire resilience — were burned by high-severity fire the region hadn’t experienced for decades, ignited by hundreds of lightning bolts raining down during that same August storm. The CZU Complex fires burned nearly 1,500 structures to the ground in Santa Cruz and San Mateo counties.

These giant lightning-sparked blazes are erratic. Prior to 2020, the most recent lightning siege was in 2008, when thunderstorms in Central and Northern California produced over 6,000 lightning strikes in 26 counties, sparking more than 2,000 fires.

Data from the National Interagency Fire Center, which encompasses wildfire incidents reported by various fire agencies across the country, don’t indicate a clear upward trend or pattern in lightning-initiated fires in the state, though many more acres have been burned in Northern California.

Some climate models have shown that with global warming, increased storm energy could result in more lightning flashes. This, in turn, could potentially lead to more wildfires.

What is clear is that climate change has meant dried-out trees and parched brush, fuels that can be more easily sparked by lightning to produce severe wildfires.

“The land is primed,” Schultz said.

Dry lightning on the rise

Dry lightning — flashes that occur with little to no precipitation hitting the ground — presents a particularly dangerous scenario when it comes to wildfire ignition.

A recent study found that dry lightning accounts for nearly half of all cloud-to-ground lightning in Northern and Central California.

Though Florida may have frequent thunderstorms and lightning, accompanying rain keeps fires from igniting and growing. That’s not the case with dry lightning. A lightning-sparked fire can lie in wait, like a campfire that isn’t completely out.

“It’ll smolder for a day or two or even more,” said Washington State University Vancouver climate scientist Dmitri Kalashnikov, who worked on the study with Deepthi Singh. “All of a sudden, the weather turns — it becomes hotter, drier, windier — and the fire grows.”

The team found that dry lightning generally occurred in July and August at higher elevations. At lower elevations, like the Bay Area, dry lightning strikes peaked in September and October.

This timeline matches a few past lightning-sparked fires in the region, like the Skeggs Firethat burned 50 acres in San Mateo County in September 2017. It also aligns with the September lightning events depicted in the heat map of strikes in the Bay Area from 1988 through 2017.

The researchers identified signatures of atmospheric high pressure associated with dry lightning. These findings could provide new clues for how climate change impacts lightning rates, including outbreaks like the lightning sieges of 2008 and 2020.

“Now that we've identified the patterns, we can look in models to see how those large-scale patterns that result in dry lightning in the region are changing as well,” Singh said.