Thunderstorms can make trees twinkle with an electric glow

Thunderstorms bring a hidden glow to forests. The same forces that trigger lightning also make treetops sparkle like Christmas-tree toppers (even if this is barely visible to the human eye).

For nearly a century, scientists have discussed a blue glow known as Saint Elmo’s fire. Thunderstorms create electric fields in the sky. Electric currents can flow from them to some objects below, such as ship masts, triggering them to glow. More recently, researchers wondered if lightning might do something similar to treetops.

Initially triggered in the lab, these glows have now been spotted in nature.

Explainer: Understanding electricity

The idea of whether these glimmers might form at treetops first arose a few years ago, says Patrick McFarland. He’s a meteorologist at Pennsylvania State University in State College. He and William Brune were eating lunch at a picnic table under a tree. McFarland remembers Brune, an atmospheric chemist at the school, gazing up and musing: “Hmm, I wonder if those trees glow under thunderstorms.”

“That afternoon, we grabbed a branch off of a tree,” McFarland recalls. In the lab, they placed a high-voltage plate above the branch. This created a negative charge in the air around the branch. Then they attached the branch to a positively charged electrical plate (to simulate the ground). This formed an electric field. “And sure enough,” McFarland says, “we saw it glow.”

It was just barely visible. But it radiated a faint blue hue as well as some invisible ultraviolet (UV) light. Such glows are part of what’s called a corona. It develops when a strong electric field causes normally neutral air molecules to break and create ions (charged atoms or molecules). The air can now conduct electricity — and glow.

Seeing this in the lab made the researchers even more curious. McFarland says they mused: “Do we see these glows under thunderstorms as well?”

Patrick McFarland explains what drove him to study the coronas on trees and other structures that get triggered by thunderstorms: his interest in how those discharges trigger the production of an air-cleaning chemical.

Searching for the right storm

To find out, the group outfitted a 2013 Toyota Sienna van in the summer of 2024. They included all the instruments they would need to find a thunderstorm. They also added a special camera to record the distinct UV light that would be emitted by a corona. Then they hit the road in that van, McFarland says. They “drove it down to Florida for about a month.”

Why Florida? This state records the most thunderstorms. With sea breezes that come in from both the Atlantic Ocean and the Gulf of Mexico, he notes, “You get thunderstorms almost every single day.”

Still, finding a storm was not all it took to find coronas, he notes. “You have to find a public place to set up with trees that seem relatively tall.” You also have to get the instrument ready and aim it at a tree — all during a storm. “So it’s really, really challenging.”

Serendipity led them to their best corona-maker. A North Carolina storm “just so happened to form when we were driving back to Pennsylvania,” McFarland says. It was in the coastal plains town of Pembroke. Here, for 90 minutes they recorded video of a sweetgum tree and a loblolly pine.

In all, they spotted 41 coronas. None lasted more than three seconds. And the flashes didn’t stick to one spot. They danced and darted like twinkle lights, jumping between leaves and along branches swaying in the wind. 

The scientists shared their findings in the Feb. 28 Geophysical Research Letters.

The North Carolina storm provided most of their sightings. But they saw coronas in other storms in Florida and Pennsylvania. Each time, the sparkling proved short-lived.

“These glows seem to be really, really widespread,” McFarland now concludes. “There may be many, many more … that we just don’t have the sensitivity to see.”