Since ancient times, sailors have feared sudden lightning strikes that could set ships alight and doom crews. Today that danger is minimal, but the lightning itself could be even more common. Recent research indicates that oceangoing vessels might, in fact, be making lightning more likely.
The mechanisms for this lightning enhancement are still debated. One theory holds that aerosols emitted in ships’ exhaust alter how storms develop, leading to more lightning. Alternatively, the ships themselves—tall metal objects on a watery plain—could be drawing in strikes.
In a new study published in Earth and Space Science and a forthcoming presentation at AGU’s Annual Meeting 2023 in San Francisco, scientists take a fresh look at the question with the help of additional data pairing the locations of lightning strikes and the locations of ships.
Though the data don’t show direct strikes to ships, they do show a distinct prevalence of lightning strikes very close to shipping lanes versus slightly farther away, said study author Michael Peterson, a scientist at Los Alamos National Laboratory. His takeaway: Ocean vessels are attracting lightning strikes and likely even causing them in some cases.
Electrifying Encounters
The idea that shipping traffic might be increasing the frequency of lightning strikes was first put forth in a 2017 paper that used data from a network of ground-based sensors called the World Wide Lightning Location Network (WWLLN). The authors found that lightning density was up to 2 times greater in some areas of the South China Sea and Indian Ocean near shipping lanes than in regions without ship traffic.
They suggested that aerosol particles from ship exhaust might be changing convection inside thunderstorms, increasing electrification and causing more lightning bolts to strike.
Peterson said he thinks there also may be another, simpler explanation: Ships are big metal poles.
He looked at the same WWLLN data as the authors of the 2017 paper did and added in data on shipping emissions from the Emissions Database for Global Atmospheric Research (EDGAR), as well as transponder data that track the locations of ships around the world.
“Understanding the origin of lightning enhancement over ship tracks is important and unresolved,” wrote Yakun Liu, a postdoctoral researcher at the Massachusetts Institute of Technology who studies lightning, in an email. “Lightning generation over ship tracks is an excellent natural experiment to infer the physics involved,” said Liu, who wasn’t involved in the new research.
By putting together ship transponder data with data from WWLLN recording the locations of individual lightning strikes, Peterson found that lightning was 15 times more likely to occur near a ship than 2 kilometers (1.2 miles) away and 66 times more likely than 25 kilometers (15.5 miles) away.
The magnitude of the increase in strikes was surprising, Peterson said. “That ended up being something that we didn’t expect to find, to be honest,” he said.
To Peterson, it’s a clear sign that ships are precipitating lightning strikes. If strikes were being caused by aerosols from ship exhaust, they’d likely happen over a more dispersed area, he said, not just in the very near vicinity of the ships themselves.
Location Matters
Ship transponder data are expensive, so Peterson was able to look at data only from in and near U.S. coastal waters. With the EDGAR data, he was able to a get a worldwide view of shipping lanes. Ship emission aerosols lead to the formation of low-lying clouds, much like airplane contrails, that appear behind ships as they move. They are therefore a proxy for shipping traffic, though they don’t show individual ship locations.
“You introduce a tall metal object like that in such an environment where you already have strong electric fields—that could potentially incite additional lightning flashes.”
That data set also showed that lightning strikes were more common very close to shipping lanes compared to areas nearby with less shipping traffic—but only in the Bay of Bengal and the South China Sea. In other regions, such as the Red Sea, Mediterranean, and U.S. East Coast, there was no increase in lightning near shipping lanes.
Peterson said he thinks the Bay of Bengal and South China Sea might host clouds that are highly electrified but don’t produce many lightning strikes. Ships could provide the extra push to tip these clouds into generating sizzling bolts of electricity.
“You introduce a tall metal object like that in such an environment where you already have strong electric fields—that could potentially incite additional lightning flashes,” Peterson said.
Meanwhile, strikes in other places, such as near the U.S. East Coast, might also be precipitated by ships, but not frequently enough to reveal patterns in WWLLN data.
In all, 36% of the vessels with transponder data that Peterson looked at in his study had experienced a near or potentially direct lightning strike over the course of 3 years. Some ships had seen dozens or even hundreds of close strikes, like those that sail through stormy waters in the Gulf of Mexico.
“Clearly, there is not a simple or direct answer.”
Though Peterson’s work indicates metal ships likely incite lightning strikes, he points out that aerosols probably play a role as well. It’s likely that both aerosols from marine diesel and the presence of ships themselves are enhancing lightning near shipping lanes, though their relative contributions are still unclear.
“Clearly, there is not a simple or direct answer to the question of how lightning discharges are invigorated over ship tracks,” Liu said.
Better data on lightning activity near ships would help clear the matter up and, Liu said, would also be important to the shipping industry. Though ships are designed to channel lightning to the water without harming people, equipment for steering, navigation, and more might be damaged by strikes, he said. Knowing where a strike may be more likely and what, if anything, could be done to reduce the risk could help keep ships, and their human passengers, safe.
—Nathaniel Scharping (@nathanielscharp), Science Writer