Most everyone knows about lightning and the havoc it wreaks on forests. They don’t know about the weak electrical glow called corona, which is thought to form on tree leaves under thunderstorms. Researchers at Penn State University used ultraviolet-sensitive equipment to directly observe and measure this electrical phenomenon in sweetgum, celery pine, and other tree species during thunderstorms in several U.S. states.
Coronae glow on the tip of a spruce needle caused by a charged metal plate in the laboratory. Image credit: William Bruun.
Lightning falling from clouds to the ground has been attracting attention for as long as thunderstorms have been occurring in Earth’s forests. Trunks split, wildfires start, and night briefly turns to day.
But scientists are now focusing on more subtle electrical phenomena that form on leaf tips during thunderstorms.
Unlike lightning, which heats the air to tens of thousands of degrees, corona is a weak electrical discharge whose temperature is only slightly higher than the surrounding air.
But these modest sparks can produce large amounts of hydroxyl, the main oxidant in the atmosphere, damaging tree foliage and causing charged particles in thunderstorm cloud bases.
“These things do happen, we’ve seen them, and we know they still exist,” said Pennsylvania State University meteorologist Dr. Patrick McFarland.
“To finally have that tangible evidence…I think that’s the most fun thing.”
“In the lab, if you turn off all the lights, close the doors and block the windows, you can barely see the corona. It looks like blue light,” he added.
For the study, Dr. McFarland and his colleagues developed a mobile instrument with multiple components that can measure arboreal crowns and the atmospheric properties that influence their formation.
The main component is a 25 cm diameter telescope that focuses UV radiation onto a solar-blind UV camera that is sensitive to wavelengths between 255 and 273 nm.
Scientists were able to observe corona on sweetgum and pine trees during thunderstorms in North Carolina.
“The coronavirus could also hop between leaves or follow branches swaying in the wind,” the researchers noted.
The authors made similar observations for other tree species during four other thunderstorms from Florida to Pennsylvania.
“Our observations show that the corona glows in bands of trees under thunderstorms,” the researchers said.
“These coronas can change air quality in forests, cause subtle damage to foliage, and cause overhead thunderstorms.”
of study Published in an online journal on February 12th Geophysical Research Letters.
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PJ McFarland others. 2026. Corona discharges glow on trees under a thunderstorm. Geophysical Research Letters 53 (4): e2025GL119591;doi: 10.1029/2025GL119591