Nearly half of Northern California’s lightning strikes over the past three decades occurred on days with little or no rain, sparking some of the most destructive wildfires in state history
August 8, 2022
In central and northern California, nearly half of lightning strikes over the past 34 years occurred on days with very little rain. Several of the days with the most widespread “dry lightning” during that period corresponded to three of the most destructive wildfires in California history.
A dry thunderstorm develops just like a normal one, with a warm updraft that carries moisture to higher elevations where it forms clouds and lightning. But if the thunderclouds form on top of a layer of warm, dry air, the rain may not reach the ground with the lightning. “You have to have a warmer, drier, lower atmosphere,” says Dmitri Kalashnikov of Washington State University Vancouver. Dry lightning poses a particular wildfire risk because there is no rain to extinguish fires started by the strikes.
Kalashnikov and his colleagues investigated the meteorological conditions behind dry lightning in northern and central California, where nearly 30 percent of the more than 5,000 recorded fires since 1987 were started by lightning.
Using lightning and precipitation records in the region, which includes the fire-prone Central and North Coast and the forested Sierra Nevada, the researchers found that 46 percent of lightning strikes occurred on days when there was 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 looked at the meteorological conditions on days with dry lightning, as well as the location of dry lightning strikes to identify patterns specific to California. The study found that dry lighting occurred most often between July and August, although it was most widespread in terms of geographic area from June to September, when wildfire risk is highest. The researchers found that dry lightning also occurred as late as October, says Kalashnikov.
The more detailed overview of the meteorology behind dry lightning in California can help forecasters create early warnings for lightning-caused fires in the region, says Mike Flannigan of the University of Alberta in Canada. He says the same approach to studying dry lightning could be used elsewhere where it poses an increasing risk, including Australia, Siberia and Canada.
Fires are becoming more severe and more frequent as climate change leads to drier vegetation, and lightning may also become more frequent with warming.
Most fires in the United States are started by humans, but lightning can start more destructive fires. Lightning clusters can ignite many points at the same time, often in remote locations where it takes longer for someone to notice the fire, says Flannigan.
California has a particularly fiery history of dry lightning – during the “Fire Siege of 1987,” thousands of wildfires ignited by widespread dry lightning burned more than half a million acres. Forest fires started by dry lightning in 2020 burned almost 2.5 million acres.
Journal reference: Environmental research: ClimateDOI: 10.1088/2752-5295/ac84a0
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