For the first time in scientific history, atmospheric scientists have captured visual evidence of faint ultraviolet illuminations emanating from arboreal canopies during electrical storms.
A long-standing hypothesis among researchers posited the existence of this imperceptible phenomenon, theorized to arise from the electrical charge of an approaching storm inducing an electrical current within the trees situated beneath.
This emanation, known as a corona discharge, is produced by a concentration of electrical charge at the tips of leaves. Previously, this effect had only been replicated under laboratory conditions and inferred from peculiar alterations in the electrical fields of forest environments during tempestuous weather.
Recognizing the imperative to observe and confirm this occurrence, a research contingent spearheaded by Patrick McFarland, a meteorologist at Pennsylvania State University, embarked on an expedition to document this elusive evidence.
“These events are indeed real; we have witnessed them; their existence is now definitively established,” stated McFarland.
Thunderstorms are characterized by immense electrical volatility. Gigantic cumulonimbus formations contain dynamic mixtures of ice crystals and dust particles that facilitate charge separation, functioning akin to a colossal electrostatic generator.
When the potential difference between these segregated charges reaches critical levels, electrical discharges, commonly recognized as lightning, manifest either high in the atmosphere between clouds or between the clouds and the Earth’s surface.
However, this electrodynamic exchange between the Earth and the heavens is not invariably characterized by overt displays. On occasion, an electrical charge imbalance can ascend the nearest available tree, with its moisture-laden trunk and branches providing a conductive pathway.
Inhibited from further propagation by an insulating stratum of air, the electrical charge accumulates at the tree’s foliage, from which it emits a subtle corona of ultraviolet radiation.
McFarland and his collaborators first observed this corona phenomenon by replicating it in a controlled setting. Small spruce and maple saplings were positioned in non-conductive containers beneath charged metallic plates, designed to simulate the passage of charged storm clouds overhead. Subsequently, all artificial illumination was extinguished.
“Within a laboratory environment, if all lights are turned off, the door is closed, and windows are obscured, the coronae are faintly discernible. They manifest as a pale blue luminescence,” articulated McFarland.
Subsequently, the research group endeavoured to detect these nearly imperceptible flashes in their natural milieu by equipping a 2013 Toyota Sienna with a meteorological sensing suite, an electric field sensor, a laser-based distance measurer, and a roof-mounted periscope to channel light into an ultraviolet imaging device.
The resultant visual recording initially appears unremarkable: sweetgum leaves (Liquidambar styraciflua) being buffeted by the winds of a fierce storm traversing North Carolina.
Nevertheless, the instrumentation employed by the team possessed sufficient sensitivity to register clusters of ultraviolet signals distributed across the branches, documenting 41 distinct luminous episodes, each spanning durations from 0.1 to 3 seconds.
These emissions exhibited intermittent behavior, “migrating between leaf surfaces and occasionally reappearing on the same leaf,” as the researchers elucidated. This pattern aligns with observations previously made in laboratory simulations of storm-induced electrical effects.
Comparable effects were documented in loblolly pine trees (Pinus taeda) and sweetgums, across various locations along the eastern seaboard of the United States.
With a perception akin to superhuman vision, McFarland suggested, “I surmise that one would discern this expansive luminescence crowning every tree beneath a thunderstorm.”
“It would likely present a captivating visual spectacle, reminiscent of countless fireflies equipped with UV-flashing capabilities descending upon the treetops.”
Each recorded corona discharge released approximately 100 billion photons at a spectral wavelength of roughly 260 nanometers per video frame.
“Consistent findings across four subsequent storm intercepts, extending from Florida to Pennsylvania, foster a conceptualization of vast expanses of shimmering coronal radiance as thunderstorms traverse forested regions,” observed McFarland and his colleagues.
“Such pervasive coronal activity carries ramifications for the degradation of volatile organic compounds emitted by trees, subtle damage to tree foliage, and potentially limited contributions to thunderstorm electrification.”
The precise influence of this comparatively substantial electrical current on arboreal life globally remains undetermined.
For instance, recurrent exposure to these electrical surges could result in the necrosis of a tree’s uppermost branches, mirroring the process observed when a tree develops an upward-propagating leader in preparation for a cloud-to-ground lightning strike.
“The consequences of these coronae on atmospheric chemistry, forest ecosystems, biological resilience and evolutionary trajectories, and the dynamics of thunderstorm electrification necessitate a comprehensive re-evaluation and deeper understanding, particularly given the observed escalation in thunderstorm frequency, and consequently, coronal occurrences, within a warming global climate,” the team concluded.
