Lightning research is a burgeoning field that spans not only meteorology and atmospheric science but also public policy and personal safety. As in all scientific fields, however, mysteries sometimes arise: In May and June of this year, a network of lightning detectors recorded distinctly lower than average lightning counts across the continental United States. The cause of this downturn isn’t well understood, but a ridge of high pressure in the atmosphere might have played a role, the researchers suggest. It’s also possible that the decline is linked to decreased levels of pollution associated with the ongoing COVID-19 pandemic, other scientists propose.
A Midyear Checkup
Meteorologist Chris Vagasky and his colleagues mined data from the National Lightning Detection Network (NLDN), which uses roughly 120 sensors to monitor lightning over the continental United States. (Vagasky’s employer, Vaisala, runs the NLDN.) The researchers compiled data from January through July 2020 to take a midyear look at lightning statistics.
“We’ve passed the peak of lightning signals in the United States, so we wanted to see where we were at this year compared to previous years,” said Vagasky.
The scientists found significantly lower than average lightning counts in May and June. During those months, the NLDN recorded just over 51 million instances of in-cloud and cloud-to-ground lightning. That’s a 32% decrease compared with the roughly 76 million lightning events recorded on average for May and June from 2015 to 2019. That difference is far more than the expected interannual variability, which is on the order of 5%–10%, said Vagasky. “We were kind of surprised.”
These results appeared in a blog post that Vagasky published last month.
“It’s an intriguing finding,” said Joel Thornton, an atmospheric scientist at the University of Washington who was not involved in the research. But it’s still not well understood why this downturn occurred and whether it’s a rare phenomenon, he said. “What we’re lacking right now is some context.”
A Ridge of High Pressure
Vagasky and his colleagues have some ideas. Using data from the National Centers for Environmental Prediction, they measured a ridge of high pressure over the Southern Plains, eastern Colorado, and the Gulf Coast region in May and June.
“We saw that there was an anomalously strong area of high pressure over the main region where you’d get severe weather,” said Vagasky. This high pressure would have prevented air from rising, a necessary ingredient for a thunderstorm. (Upward movement allows air to cool and condense into clouds, which is where supercooled water, ice crystals, and graupel collide and transfer electrons. That sets up the charge separation necessary for lightning.)
So far, other lightning networks haven’t reported similar results. Robert Holzworth, an atmospheric and space physicist at the University of Washington and the director of the World Wide Lightning Location Network (WWLLN), analyzed unpublished WWLLN data and found a statistically insignificant decrease in lightning counts worldwide in 2020 compared with the year prior.
“The overall decrease in 2020 compared to 2019 is just 0.4%, or an order of magnitude smaller than the expected daily variation,” he said. However, it’d be worth analyzing WWLLN data just over the continental United States to make a direct comparison with Vagasky’s findings, Holzworth acknowledged.
A (Potential) Virus Link
The idea that a localized region might have experienced a decrease in lightning in the spring of 2020 isn’t completely out of the blue, said Holzworth. Researchers have speculated that the COVID-19 pandemic might have inadvertently triggered a downturn in lightning, he said. That’s because with the economy on hold, there’s less air pollution and therefore likely fewer aerosols. The presence of aerosols has been linked to enhanced lightning activity, at least over the ocean.
Vagasky and his colleagues are continuing to collect data, and they’ve seen that the lightning counts for July and August have been closer to average. They hope to have more answers by the end of the year. “We’ll prepare our annual lightning report in December,” said Vagasky. “We’ll be able to compare to a much broader data set in the coming months.”
—Katherine Kornei (@KatherineKornei), Science Writer
Eos would like to thank Robert Holzworth, who generously provided the analysis of unpublished WWLLN data.