⚡ Trees that launch lightning: first images of corona discharges

During thunderstorms, the tops of trees light up with a faint ultraviolet glow, invisible to the human eye. This phenomenon, known as a corona discharge, has been observed for the first time in real-world conditions by researchers at Penn State University.

These mini-lightning bolts could play an important role in atmospheric chemistry by generating compounds that clean the air of pollutants like methane. The team thus confirmed a decades-old hypothesis, paving the way for new research on the interactions between forests and thunderstorms.


This glow originates from the electrical imbalances of thunderstorms. Clouds carry a negative charge that attracts positive charges from the ground. These charges travel up through trees and concentrate at the tips of leaves, where the electric field becomes strong enough to ionize the air. The result is a glow, mainly in the ultraviolet, which is difficult to observe during the day due to sunlight. The researchers had to use a special instrument blocking solar UV to detect it.

The team traveled along the East Coast of the United States in a minivan equipped with an ultraviolet telescope. After three unsuccessful weeks in Florida, success came in North Carolina during a violent thunderstorm near the University of Pembroke. For nearly two hours, the instruments recorded 859 corona discharges on a sweetgum tree, and 93 on a nearby pine. Coronas were also observed on four other species, showing that the phenomenon is not rare.

Each discharge lasts from a fraction of a second to several seconds. The team was able to detect them using a special optical system that filters out solar ultraviolet light, allowing only signals from coronas, lightning, or fire to pass through. This device, coupled with electric field sensors and GPS location, made it possible to isolate the events. The researchers thus obtained the first direct proof of the existence of these discharges in nature.

Beyond simple observation, these discharges have important chemical consequences. The UV light they emit breaks water molecules apart to form hydroxyl radicals, powerful oxidizers. These radicals react with volatile organic compounds emitted by trees and with human-made pollutants like methane, transforming them into less harmful substances. Laboratory experiments had already shown this link, but confirmation in real-world conditions reinforces the idea that thunderstorms help clean the air via forests.


The researchers are now questioning the effect of these discharges on the trees themselves. Observations have shown minor leaf damage at the corona points. Have trees developed protective mechanisms? Could the hydroxyl radicals produced influence forest health? To answer these questions, the team is collaborating with ecologists and biologists.

According to doctoral student Patrick McFarland, lead author of the study, much remains to be understood about the role of coronas in atmospheric chemistry and forest dynamics. The next steps will involve quantifying the overall impact of these discharges on air quality and integrating this data into climate models.