In the summer of 2024, a team of researchers went storm-chasing in a Toyota Sienna minivan in search of tiny, faint sparks lighting up the tips of leaves. As a thunderstorm raged overhead, the researchers pointed their camera at three branches of a sweetgum tree in Pembroke, North Carolina. The footage later revealed the first documentation of coronae in the wild: brief, colorful glows that hop from one leaf to the next.
Although coronae have long been theorized about, the electrical discharges had never been observed zapping the tops of trees during a thunderstorm. That was until scientists from Pennsylvania State University set out to capture coronae on the branches of several tree species along the East Coast.
In a recent study published in the Geophysical Research Letters, scientists recorded the first observation of coronae on trees during thunderstorms. “These things actually happen; we’ve seen them; we know they exist now,” Patrick McFarland, a Penn State meteorologist and lead author of the study, said in a statement. “To finally have concrete evidence [of] that…is what I think is the most fun.”
Treetop light show
For almost a century, scientists have speculated that weak electrical charges light up treetops during thunderstorms. Experiments performed in laboratories demonstrated how coronae could form in the wild when the charge of a thunderstorm overhead induces an opposite charge in the ground below. The charge in the ground is attracted to the one above and tries to travel to the highest point it can reach. In the case of trees, that’s the tips of leaves.
“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.
McFarland and his team set out to record the real thing during a thunderstorm. The team outfitted its minivan with a weather station, electric field detector, and laser rangefinder. A roof-mounted periscope also directed light to an ultraviolet camera, allowing the scientists to detect the coronae through their ultraviolet emissions. “The most fun part was taking a jigsaw and cutting a twelve-inch hole in the roof,” McFarland said. “Totally killed the resale value, but that’s fine.”
Camera rolling
The scientists analyzed the footage they had captured of trees during a raging thunderstorm, detecting 41 coronae on leaf tips in the span of 90 minutes. Each glow lasted for approximately three seconds, often hopping from one leaf to the next.
The team chased four other storms between Florida and Pennsylvania that summer and detected coronae on a nearby loblolly pine and other trees. Despite the differences in tree species and the strength of the storm overhead, the electrical discharges behaved similarly.
The study suggests coronae are far more common than expected, possibly lighting up tens to hundreds of leaves on treetops during any given thunderstorm. Unfortunately, the faint discharges are not visible to the naked eye. If we were able to see it, “it’d probably look like a pretty cool light show, as if thousands of UV-flashing fireflies descended on the treetops,” McFarland said.