The Rare UV Sparks Seen in Thunderstorms

When you observe thunderstorms for an extended period of time, there’s something unnerving about them. Not the lightning, which is a clear, dramatic, and almost expected part. It’s the things that are quieter. The way leaves tremble in response to something invisible, and how treetops sway in a charged wind. Scientists had been speculating for decades that those trees might be doing more than simply bending during the storm. Finally, there is proof that they are. Glowing is what they’re doing.

Not in a way that would be apparent to anyone standing beneath the canopy. Not even near. However, the tops of trees flicker with faint electrical discharges—coronae—spark-like bursts that behave more like restless fireflies than like lightning when captured by ultraviolet-sensitive cameras. It’s difficult to ignore how odd that sounds. Whole forests, silently glimmering in unseen light, while all we can see below is rain.

On paper, at least, the physics seems simple. The terrain below is compressed by the strong negative charge that thunderclouds accumulate. In an attempt to find equilibrium, the ground reacts by pushing positive charge upward. Tall, pointed, and asymmetrical trees turn into organic conduits. The tips of leaves, particularly the thin edges that catch the wind, are where charge concentrates. There, molecules snap back to stability, releasing ultraviolet light and energizing the surrounding air, which briefly glows. That’s the reason. However, the experience is altered when viewing the video.

Researchers parked a modified minivan with a cut-open roof and softly humming instruments in Pembroke, North Carolina, during a humid summer storm, and aimed their UV camera at a sweetgum tree. They captured what appears in processed footage to be anxious sparks hopping from leaf to leaf for approximately ninety minutes. 859 distinct flashes. 41 groups. Some lingered for seconds, as if uncertain whether to vanish, while others lasted less than a blink.

The most fascinating aspect seems to be the randomness. These emissions are not constant. They dart. They pause. They repeat on the same leaf, disappear, and then reappear in a different location. Observing them is more like witnessing behavior—almost like seeing something alive—than it is like studying physics. Whether that impression is deceptive is still up for debate.

For many years, scientists have been aware of corona discharges in controlled settings. They have observed the same dim blue glow in dark laboratories with high-voltage plates and thoughtfully placed branches. However, those circumstances always seemed manufactured, almost staged. Things are often complicated by nature. Variables increase in number. Wind gets in the way. Light pollution begins to appear. Data becomes disorganized. Because of this, the road trip component of this study is more important than it may appear.

While chasing storms in a modified 2013 Toyota Sienna while traveling from Florida to Pennsylvania, the research team was not only gathering data but also managing unpredictability. Although Florida frequently experienced storms, there weren’t always the appropriate trees or viewpoints. It took some time to set up. Calibration was required for the equipment. Storms moved more quickly than anticipated. Cutting a hole in a van roof to demonstrate the existence of a faint glow has an almost stubborn quality. However, everything came together when the ideal storm eventually developed.

The findings imply that these coronae are simply hidden rather than extremely rare. Many more discharges might have gone undetected because the UV camera used in the study only recorded a small range of wavelengths. It’s likely that entire canopies would appear alive with motion during storms, flickering constantly, if wider detection were feasible. Uncomfortable questions are raised by that concept.

These emissions are not innocuous decorations. According to lab research, they can cause damage to leaf tissue in a matter of seconds by burning tips and upsetting cellular structures. Thunderstorms may be subtly stressing forests in ways we haven’t fully considered if comparable effects happen in the wild, which is conceivable. Not in a big way. Not in a disastrous way. But steadily. Although no one is quite sure how, there is a sense that forests may have adapted.

For decades or even centuries, trees withstand frequent storms. We might anticipate obvious patterns of damage if coronae constituted a serious threat. However, forests continue to be robust, dense, and frequently flourish in areas that are prone to storms. Leaf structures may have developed to more safely disperse charge. It’s possible that the harm is negligible, cumulative, or compensated for by other biological benefits. Or maybe we haven’t been paying enough attention.

The ghostly blue glows that sailors once reported on ship masts are reminiscent of Saint Elmo’s fire. The phenomenon seemed enigmatic, even paranormal, at the time. It is now understood, categorized, and explained. However, it’s difficult to avoid experiencing a similar uneasiness when standing beneath a forest during a storm and knowing that the treetops are silently sparking above.

In actuality, this light display has probably existed for as long as storms and forests have coexisted. We simply lacked the resources—or perhaps the patience—to recognize it.

It makes an odd impression to watch this happen, even with data and reconstructed images. Not quite awe. Something more subdued. A reminder that layers of activity are still taking place just beyond our perception, even in places we believe we understand, like trees, storms, and the air. And there might be a lot more left.