For the first time, researchers have observed and measured weak electric charges, called crowns, in trees during thunderstorms. A new study describes these almost invisible flashes that appeared similarly on the branches of several tree species along the east coast of the United States during the summer of 2024, suggesting that thunderstorms could color entire tree canopies with a faint blue glow, too weak to be perceived by the human eye.
Crowns also glow at the tips of leaves. Given their omnipresence in forests during thunderstorms, researchers have hypothesized that these crowns could damage the canopy, potentially influencing the evolution of trees to limit this type of damage.
“These phenomena actually occur; we have seen them; we now know they exist,” said Patrick McFarland, meteorologist at Penn State University and lead author of the study. “Finally having concrete evidence, in my opinion, is the most exciting thing.”
The study is published in Geophysical Research Letters, the AGU journal dedicated to high-impact, innovative articles rapidly published on major advances in Earth sciences.
Finding faint glows between leaves
Scientists have speculated on the existence of weak electric charges in plants during thunderstorms for almost a century, but until now, they had never observed or measured them in nature, inferring their presence only from forest electrical field anomalies during stormy episodes. Laboratory experiments over the past half-century had at least shown how they could naturally form: a storm charge aloft induces an opposite charge in the ground. This ground charge, drawn by the charge above, moves toward the highest point it can reach—namely the tips of leaves in tree canopies—through which electricity discharges, forming crowns.
In the lab, if you turn off all the lights, close the door, and block the windows, crowns are barely visible. They look like a blue glow,
These weak electric charges can cause ultraviolet flashes over large forested areas during storms, which could affect the canopy’s health.
Similar experiments with potted trees in the laboratory also revealed a surprising relation: the UV radiation from the crowns was proportional to the electrical current measured by the team in the trees. This opens up the possibility that these UV emissions could provide a way to measure this current and any potential damage it causes. Studies conducted in the 1960s, according to McFarland, showed that current flows in trees decomposed cell membranes and damaged chloroplasts used for photosynthesis.
One man, one plan, and one minivan to track storms
Documenting crowns under actual storms required a different approach: in this case, a 2013 Toyota Sienna equipped with a weather station, an electric field detector, a laser thermometer, and a periscope installed on the roof that directs light to an ultraviolet camera. This last device allowed the team to detect crowns in the field through their UV emissions because even the faint ambient light under stormy skies masks the light they emit in the visible spectrum.
“We had to remove one of the seats and install anti-vibration pads so the instruments wouldn’t move during driving,” McFarland recounted. “The most fun was cutting a 30-centimeter hole in the roof with a jigsaw. That completely ruined the resale value, but it doesn’t matter.”
