UV camera snaps treetops glowing as thunderstorm passed overhead

Thunderstorms create large amounts of electricity that we see as lightning. Under these storms, scientists believed that electricity would flow through trees, giving them a dull ultraviolet glow, and affect the surrounding environment. These discharges are called coronae. However, no one measured these ‘glows’, predicted nearly a century ago, until recently.

In a new study published in Geophysical Research Letters, researchers from Pennsylvania State University reported using a new mobile instrument called the Corona Observing Telescope System (COTS) to provide the first direct observations of coronae in the wild, thus opening a new door to study how forests and thunderstorms interact.

COTS had a special camera that was sensitive only to a narrow range of ultraviolet light. Because the earth’s ozone layer blocks this specific wavelength of sunlight, the camera could detect the ultraviolet radiation from electrical discharges without being blinded by daytime light or reflected sun.

The team mounted COTS in a research vehicle fit with a periscope, allowing them to track storm clouds and observe the tops of tall trees from a distance. The researchers also used an electric field mill to measure the intensity of the storm’s electrification and a weather station to record conditions like rainfall and humidity.

This way, the team reported that during a thunderstorm in North Carolina in the U.S., it observed coronae on a sweetgum tree and a loblolly pine. Their findings provide the first direct evidence and quantification of coronae in the wild. The team also wrote that the ultraviolet glow is not fixed to one spot but hops sporadically from leaf to leaf and branch to branch. In some instances, the glow followed a branch even as it swayed in the wind.

These discharges typically lasted between a fraction of a second and a few seconds. By comparing the field observations with laboratory experiments on smaller trees, the team established a direct link between the brightness of the ultraviolet light and the amount of electrical current flowing through the tree.

The team also found that a typical corona discharge emits approximately one hundred billion photons, a level of light corresponding to an electrical current of about one microampere flowing through an individual tree branch. While 1 μA is a small amount of electricity — 10,000-times lower than the current in an LED — the researchers noted that these discharges occurred across entire forest canopies as the storm passed overhead, summing up to a sizeable current. They also expanded their observations to include four other storms ranging from Florida to Pennsylvania, suggesting these “swaths of scintillating corona glow” are a common and widespread phenomenon during peak thunderstorm activity.

The findings, the researchers wrote in their paper, inform our understanding of the atmosphere. They pointed out that the coronae produced large amounts of the hydroxyl radical (the ion OH^–), which acted like a detergent for the air by removing hydrocarbons and altering the forest’s air quality. The voltage surges associated with these glows can also cause small but permanent damage to trees by burning the fine tips of leaves.

The paper also read that the electrical charge released by millions of glowing trees could even influence the electrification of the clouds above them.