In the lab, light and electrical charge are shed from the tips of spruce needles, induced by electrical plates. These discharges have recently been seen outdoors, induced by a thunderstorm.
William Brune
Thunderstorms may bring more than rain and gloom. The same forces that cause thunder and lightning also make treetops sparkle in ultraviolet light, like a Christmas tree topper invisible to the human eye.
For almost a century, scientists have discussed a phenomenon called Saint Elmo’s fire, where electrical discharges elicit a bluish glow from pointy objects such as ship masts during thunderstorms. More recently, researchers have wondered if thunderstorms might draw weak electrical discharges from the treetops. These discharges have been detected in the lab, and now, they’ve been spotted in nature.
The tips of wild trees shed electrical charge along with a blue and ultraviolet glow in response to an opposite charge in the atmosphere during a thunderstorm, scientists report in the Feb. 28 Geophysical Research Letters.
The question of whether these discharges might form at the tops of trees came up a few years ago at lunch, says Patrick McFarland, a meteorologist at Penn State. McFarland’s advisor, William Brune, “just sort of leaned back from the picnic table that we were sitting at, and he looked up at the top of the tree right above us, and just kind of postulated, you know, ‘Hmm, I wonder if those trees glow under thunderstorms,’” McFarland says.
“That afternoon, we grabbed a branch off of a tree, we took it into our lab, we put a high-voltage plate on top of it,” McFarland says. The team placed the high-voltage plate above the branch, creating a negative charge in the air around it, and attached the branch to a positively charged electrical plate to simulate the ground. “And sure enough, we saw it glow.”
The glow, part of an electrical discharge called a corona, was just barely visible, radiating faint blue as well as invisible UV light. Detecting this in the lab made the researchers even more curious, McFarland says: “Do we see these glows under thunderstorms as well?”
To answer that question, the group outfitted a 2013 Toyota Sienna van with all the instruments they would need to find a thunderstorm, plus a camera that could spot the distinct UV light emitted by a corona. In the summer of 2024, it was time to hit the road.
“We built that van, and we drove it down to Florida for about a month,” McFarland says. Florida has the most thunderstorms in the United States because of the sea breezes that come in from both the Atlantic Ocean and the Gulf of Mexico. “You get thunderstorms almost every single day.”
Though thunderstorms were frequent, finding a storm was just one factor when searching for the coronae, McFarland says. “Then you have to find a public place to set up with trees that seem relatively tall. And then you have to get all set up and, you know, get the instrument turned on and pointed at the tree. So it’s really, really challenging.”
Serendipity led them to their best thunderstorm. “This storm in North Carolina actually just so happened to form when we were driving back to Pennsylvania,” McFarland says. The team found a spot in the coastal plains town of Pembroke, N.C. They set up their equipment and recorded video of a sweetgum tree and a loblolly pine for 90 minutes.
In the video, the team identified 41 coronae, none lasting 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.
While the study focused mainly on data collected during the North Carolina storm, the team notes that coronae appeared in thunderstorms in Florida and Pennsylvania and had that same transient sparkling appearance. “These glows seem to be really, really widespread,” McFarland says. “There may be many, many more coronae that are occurring that we just don’t have the sensitivity to see.”
