The electrical energy generated by thunderstorms can make our hair stand up, and even create invisible sparkles on trees. Now, we have photographic proof of this long-theorized electrical event for the first time.Â
During a thunderstorm in North Carolina in June 2024, scientists spotted weak electrical discharges called coronae on the tips of leaves. Based on this discovery, it’s possible that thunderstorms may paint entire tree canopies with an ethereal blue glow that is too faint for human eyes to see. The coronae can also burn the very tips of leaves. Given how often the leaf tips may burn, the storms could be harming the tree canopy and may have even shaped tree evolution to limit the damage. The findings are detailed in a study recently published in the journal Geophysical Research Letters.Â
“These things actually happen; we’ve seen them; we know they exist now,” Patrick McFarland, a study co-author and meteorologist at The Pennsylvania State University, explained in a statement. “To finally have concrete evidence [of] that…is what I think is the most fun.”
Searching for the blue glow
For nearly 100 years, scientists have speculated that weak electrical discharges from thunderstorms find their way onto plants, but have never observed or measured them in a real storm. Instead, they could only guess based on anomalies observed in the electric field in forests.Â
Lab experiments over the past 50 years also demonstrated how they could form in the wild. In theory, the charge of a thunderstorm above induces an opposite charge in the ground below. That ground charge is then attracted to the charge above it, and travels toward the highest point it can reach. In a forest, the highest point is the tips of leaves on trees. Once there, the electricity discharges through the treetops, forming coronae.
“In the laboratory, if you turn off all the lights, close the door and block the windows, you can just barely see the coronae. They look like a blue glow,” McFarland said, recalling when his team recreated the phenomenon indoors by putting grounded tree leaves underneath charged metal plates.
Similar experiments using potted trees also showed a striking relationship between plants and electrical discharge. The coronae’s ultra-violet (UV) radiation increased at the same rate with the electrical current measured in the trees. This raises the possibility that those UV emissions may be a way to gauge that current and any resulting damage. According to McFarland, studies dating back to the 1960s, showed that electrical current flows in trees broke down cell membranes and destroyed the chloroplasts that the trees use to photosynthesize—or make food from the sun.
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Hot wheels
To catch coronae during a real thunderstorm required an approach familiar to anyone who has seen the movie Twister. The team souped-up a 2013 Toyota Sienna minivan with a weather station, electric field detector, laser rangefinder, and roof-mounted periscope that could direct light right into an ultraviolet camera. This periscope and camera allowed the team to detect the coronae in the field solely based on their UV emissions. Low and invisible ambient light under stormy skies typically drowns out the light that coronae emit in the visible spectrum our eyes can see, so their camera and periscope system made that lower light more visible.
“We had to take out one of the seats and put in these vibration-dampening pads so our instruments wouldn’t bounce up and down as we drove,” McFarland said. “The most fun part was taking a jigsaw and cutting a twelve-inch hole in the roof. Totally killed the resale value, but that’s fine.”
When the weather van was ready, the storm chasing began. The team watched a video feed safely inside the car, and put the camera on three branches of a sweetgum tree in Pembroke, North Carolina.
“We sit there and stare at this video while the thunderstorm’s raging overhead,” McFarland said. “You’re looking for the faintest signals on a video feed of nothing…It’s really difficult to tell in real time if you’re seeing anything.”
A big buzz
When they analyzed the video, they saw 41 coronae on leaf tips in the span of 90 minutes. The tip-off that the phenomenon occurred was clusters of UV signals that tracked with the branches as they swayed in the wind. These glows lasted up to three seconds, often moving from leaf to leaf. The coronae occurred and behaved similarly on a nearby loblolly pine and on trees during four other storms that the team chased between Florida and Pennsylvania, despite differences in tree species and in the storms’ strength.Â
The team believes that this could mean coronae arise in abundance, radiating from tens to hundreds of leaves on every treetop during a single thunderstorm. McFarland says that someone with superhuman vision could, “see this swath of glow on the top of every tree under the thunderstorm. It’d probably look like a pretty cool light show, as if thousands of UV-flashing fireflies descended on the treetops.”
If this is the case, the coronae could leave leaf tips visibly burnt within seconds and even damage the cuticle—the waxy covering that protects leaves from UV damage and dehydration. While a single corona doesn’t appear to do much harm to the leaves, the team speculates that repeated coronae across the tree canopy from multiple storms could damage the leaves. In the future, the team hopes to work with forest ecologists and botanists to investigate these shocking phenomena further.
“That’s really where I’d like to go next, to figure out what impacts this has on the tree itself and on the forest as a whole,” he said.
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