Lightning

Scientists Fire Lasers at the Sky to Control Lightning

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Laser beams could be used to deflect lightning strikes from vulnerable places such as airports and wind farm

Scientists Fire Lasers at the Sky to Control Lightning
A new “laser lightning rod” in action.

Lightning strikes about 40 million times in the U.S. each year. This natural phenomenon is terrifyingly random, and we rely mainly on lightning rods—a nearly 300-year-old technology—to deal with it. But researchers are finally working on a more 21st-century solution: laser beams.

Pioneered by Benjamin Franklin, the lightning rod works well to defend a building. But it only has a limited ability to protect larger swaths of land or sprawling facilities such as wind farms, airports and rocket launch pads. So a team of scientists has tried using a high-powered laser to guide lightning strikes atop a mountain in Switzerland. This “laser lightning rod” technique that could one day deflect strikes from important large-scale infrastructure. The results of the researchers’ new study were published this week in Nature Photonics.

“What they’ve done is very impressive,” says Jerry Moloney, an optical scientist at the University of Arizona, who was one of the early pioneers of this laser application but was not involved in the study. It’s “a very, very sophisticated setup.”

Lightning occurs when friction among water droplets creates a static electric charge within clouds, usually during storms. This electricity builds before being discharged in a giant spark, which can travel between the cloud and the ground, either upward or downward, following the path of least resistance. Regular lightning rods are made of conductive metal, and they provide a preferential point for the lightning to strike and then safely channel the charge around a building and into the ground. But metal is not the only way to attract lightning away from more vulnerable targets.

Laser lightning rod
The laser lightning rod fires 1,000 times a second.

In the new experiment, a high-powered laser turns a column of air into an electrical conductor. When the laser is fired, the air molecules in the beam’s path are stripped of their electrons in a process called ionization. This transforms the air, which is normally insulating, into an attractive point for the lightning to hit—effectively creating a giant, temporary and controllable lightning rod in the sky above the area to be protected. Scientists had dreamed of building laser lightning rods for decades, but previous experiments had largely failed. Lasers that were available at the time could only pulse around 10 times per second, explains Aurélien Houard, a physicist at the École Polytechnique in France and first author of the study. That rate is too slow to keep an air column ionized. The new laser can fire 1,000 times per second, with each pulse lasting one trillionth of a second.

“You can burn stone if you want with this laser,” says Houard. The laser has an average power of one kilowatt (roughly the amount of electricity required to operate a large oven or refrigerator), says the paper’s senior author Jean-Pierre Wolf, a physicist at the University of Geneva.

The researchers tested their laser’s ability to draw lightning atop Säntis, a prominent peak in the Swiss Alps that was chosen because lightning often hits a telecommunication tower at its summit. There, during the summer of 2021, the team observed 16 lightning strikes—four of which occurred while the laser was powered on. And in all four cases, sensors—either a high-speed camera or a high-frequency electromagnetic wave detector—captured the lightning following the beam’s path. The results are preliminary for now, and the authors hope to fine-tune the technique with more data from future studies.

“The next step will be closer to the real-world applications,” Wolf says, “basically redoing this experiment, say, close to a launching pad or close to an airport.”

Lightning strikes at airports are an “ongoing issue,” and they not only delay flights but can also injure or kill employees and travelers, says Irene Miller, an assistant professor of aviation at Southern Illinois University, who was not involved in the new study. Most airports currently rely on early-warning systems to prevent planes from taxiing or landing when the risk of a strike is high.

It remains unclear how laser lightning rod technology might be adapted to this setting because even tiny lasers aimed at the sky are notoriously dangerous to pilots. During their recent mountaintop experiment, the researchers worked with aviation authorities to designate a no-fly zone around Säntis. One way to address such concerns could be to adjust the laser’s wavelength and power, and the study authors hope to explore that idea in future projects. For now, though, Benjamin Franklin’s innovation will have to do.

Lightning will increase by 50 percent with global warming, research says

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Today’s climate models predict a 50 percent increase in lightning strikes across the United States during this century as a result of warming temperatures associated with climate change.

Reporting in the Nov. 14 issue of the journal Science, University of California, Berkeley, climate scientist David Romps and his colleagues look at predictions of and cloud buoyancy in 11 different climate models and conclude that their combined effect will generate more frequent electrical discharges to the ground.

“With warming, thunderstorms become more explosive,” said Romps, an assistant professor of earth and planetary science and a faculty scientist at Lawrence Berkeley National Laboratory. “This has to do with water vapor, which is the fuel for explosive deep convection in the atmosphere. Warming causes there to be more water vapor in the atmosphere, and if you have more fuel lying around, when you get ignition, it can go big time.”

More lightning strikes mean more human injuries; estimates of people struck each year range from the hundreds to nearly a thousand, with scores of deaths. But another significant impact of increased lightning strikes would be more wildfires, since half of all fires – and often the hardest to fight – are ignited by lightning, Romps said. More lightning also would likely generate more nitrogen oxides in the atmosphere, which exert a strong control on atmospheric chemistry.

While some studies have shown changes in lightning associated with seasonal or year-to-year variations in temperature, there have been no reliable analyses to indicate what the future may hold. Romps and graduate student Jacob Seeley hypothesized that two atmospheric properties – precipitation and cloud buoyancy – together might be a predictor of lightning, and looked at observations during 2011 to see if there was a correlation.

“Lightning is caused by charge separation within clouds, and to maximize charge separation, you have to loft more water vapor and heavy ice particles into the atmosphere,” he said. “We already know that the faster the updrafts, the more lightning, and the more precipitation, the more lightning.”

Precipitation – the total amount of water hitting the ground in the form of rain, snow, hail or other forms – is basically a measure of how convective the atmosphere is, he said, and convection generates lightning. The ascent speeds of those convective clouds are determined by a factor called CAPE – convective available potential energy – which is measured by balloon-borne instruments, called radiosondes, released around the U.S. twice a day.

“CAPE is a measure of how potentially explosive the atmosphere is, that is, how buoyant a parcel of air would be if you got it convecting, if you got it to punch through overlying air into the free troposphere,” Romps said. “We hypothesized that the product of precipitation and CAPE would predict lightning.”

Using U.S. Weather Service data on precipitation, radiosonde measurements of CAPE and lightning- strike counts from the National Lightning Detection Network at the University of Albany, State University of New York (UAlbany), they concluded that 77 percent of the variations in lightning strikes could be predicted from knowing just these two parameters.

‘Blown away’

“We were blown away by how incredibly well that worked to predict lightning strikes,” he said.

They then looked at 11 different climate models that predict precipitation and CAPE through this century and are archived in the most recent Coupled Model Intercomparison Project (CMIP5). CMIP was established as a resource for climate modelers, providing a standard protocol for studying the output of coupled atmosphere-ocean general circulation models so that these models can be compared and validated.

“With CMIP5, we now have for the first time the CAPE and precipitation data to calculate these time series,” Romps said.

On average, the models predicted an 11 percent increase in CAPE in the U.S. per degree Celsius rise in global average temperature by the end of the 21st century. Because the models predict little average precipitation increase nationwide over this period, the product of CAPE and precipitation gives about a 12 percent rise in cloud-to-ground per degree in the contiguous U.S., or a roughly 50 percent increase by 2100 if Earth sees the expected 4-degree Celsius increase (7 degrees Fahrenheit) in temperature. This assumes carbon dioxide emissions keep rising consistent with business as usual.

Exactly why CAPE increases as the climate warms is still an area of active research, Romps said, though it is clear that it has to do with the fundamental physics of water. Warm air typically contains more water vapor than cold air; in fact, the amount of water vapor that air can “hold” increases exponentially with temperature. Since is the fuel for thunderstorms, lightning rates can depend very sensitively on temperature.

In the future, Romps plans to look at the distribution of lightning-strike increases around the U.S. and also explore what lightning data can tell climatologists about atmospheric convection.