Tree Electrical Sparks Flicker Across Forest Canopies During Storms

Lightning is the most spectacular electrical force in a thunderstorm, yet it may not be the only one reaching the forest canopy.

Scientists have now, for the first time, watched tree leaves emit faint electrical sparks during active storms, indicating that whole canopies can release minute currents into the air.

Instead of one lightning strike, forests may be subjected to thousands of delicate electrical flickers: almost imperceptible discharges capable of changing leaf surfaces and influencing nearby air chemistry.

Researchers used an ultraviolet telescope mounted in a van to follow bursts of corona discharge jumping between leaf tips while storms moved overhead.

At Penn State, meteorologist Patrick McFarland documented recurring flashes in several storms and across different tree species, indicating that this understated exchange between the sky and canopy could be more widespread than previously believed.

Where the electrical spark appears

A summer thunderstorm provided the most detailed view so far. As dark clouds passed above, the ultraviolet camera stayed trained on three sweetgum branches for 90 minutes.

Over that period, the team logged 41 individual corona bursts, faint electrical leaks emerging from pointed leaf tips.

Several flashes continued for as long as three seconds, while the glow often jumped between leaf edges as the storm’s overhead charge changed.

“These things actually happen. We’ve seen them. We know they exist now,” said McFarland.

Charging the forest canopy

Within a storm cloud, positive and negative charges separate at high altitude, while the land beneath starts to accumulate an opposing charge.

This accumulated charge travels through a tree’s roots and trunk towards its uppermost points, gathering at leaf tips where the surrounding air starts conducting electricity.

As charge escapes from sharp edges, it creates a corona, short for corona discharge, producing a faint glow much weaker than lightning.

Although this discharge remains cool and localised rather than splintering wood, it can nevertheless alter leaf surfaces.

Ultraviolet reveals hidden currents

Corona’s visible glow is too faint to be seen clearly against storm clouds in the open air. The researchers therefore set their camera to ultraviolet light, a wavelength band the human eye cannot see, so it could record flashes that would otherwise remain unnoticed.

Laboratory calibration converted ultraviolet brightness into electrical current and showed that a single tree branch released roughly a millionth of an amp.

This was an important step because the current, rather than the glow itself, establishes how much energy reaches a leaf.

That energy is focused at the pointed ends of leaves. Laboratory experiments showed corona discharges scorching tissue and seemingly damaging the cuticle, the waxy layer that enables leaves to retain water.

One burst might cause only a minute blemish, but storms repeated over time could progressively spread such damage across numerous leaves.

How trees react in the longer term is still uncertain. In the absence of thorough field assessments after storms, scientists do not yet know which species recover rapidly or which could lose photosynthetic capacity over weeks or even seasons.

Electrical sparks reshape the air

In addition to scorching leaf tips, corona may alter the air that forests take in during storms.

When electrical charge escapes from trees’ sharp edges, it can trigger the production of reactive chemicals. A 2022 study found that corona discharges can increase concentrations of the hydroxyl radical, a powerful but short-lived molecule that helps remove pollutants from the atmosphere.

As these chemical reactions intensify, they can swiftly break down natural gases emitted by leaves and raise local ozone and particle levels.

The impact is likely confined around the treetops, because quickly moving storms and rain rapidly disperse and wash away the burst.

Nevertheless, every flicker amounts to a small pulse of atmospheric chemistry, recurring repeatedly as thunderclouds move overhead.

Tree electrical spark is hard to measure

Storm winds continually move leaves, rotating and tipping their points either towards or away from the sky. Corona is most likely to flare when a leaf faces directly upwards, and slight changes in angle can alter the intensity with which electrical charge gathers.

Damp surfaces may also channel current towards different edges, making the glow jump unpredictably from one tip to another. Pine needles, naturally pointed, displayed comparable flickering alongside broadleaf trees such as sweetgum.

This movement makes the effect challenging to record. The ultraviolet telescope could capture only a limited section of canopy at once, and leaves overlapping each other occasionally obscured the view.

Some weak discharges probably fell beneath the camera’s detection limit, so undercounting was almost unavoidable when estimating activity throughout entire forests.

Nevertheless, comparable signals were recorded during four further storm chases between Florida and Pennsylvania, covering several tree species.

“It’d probably look like a pretty cool light show, as if thousands of UV-flashing fireflies descended on the treetops,” McFarland said.

Mapping sparks across forests

Having confirmed the phenomenon in the field, scientists can progress beyond proof of concept and investigate how frequently corona occurs in different forests, as well as how leaves react in the following days.

McFarland hopes to work with ecologists to combine ultraviolet recordings with detailed leaf inspections that track surface damage and dehydration after storms.

Installing air sensors close to treetops could show the strength of each chemical burst, while storm measurements may connect corona events with particular cloud conditions.

Forest-wide effects will remain only approximate until researchers chart the glow across complete stands of trees and repeat the measurements during many storms.

Even so, the research reshapes scientists’ understanding of storm-forest interactions. It shows that trees and weather are linked by more than lightning, and that a previously concealed electrical process can now be measured directly to support tree-health research and local air models.