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Lightning is far more than just a skyborne phenomenon: Remarkably, it can also form at ground level and shoot upwards. This upside-down lightning is the subject of a paper published in the Journal of Atmospheric and Solar-Terrestrial Physics, in which the strange behavior of these inverted bolts is revealed.
Despite the fact that there are roughly 40-50 lightning strikes somewhere around the world every second, they are surprisingly poorly understood. That said, the basic underlying physics is agreed upon. As a thunderstorm cloud forms and drifts over the Earth’s surface, it builds up a negative electrical charge within itself; simultaneously, an equal, positive charge accumulates beneath it on the ground. Lightning strikes are nature’s way of sorting out this electrical charge imbalance. A channel of air filled with electrically excited particles (ions) forms, and the strike occurs.
Upside-down lightning strikes are yet another curious twist in the tale. These strikes, which appear to form frequently at the tops of tall towers, can burst through the clouds and reach atmospheric heights of up to 90 kilometers (56 miles).
In a normal strike, the negative ions from the cloud rush downwards and meet the positive ions rushing upwards. In an upside-down strike, the positive ions rush upwards far quicker, forming an electrical “circuit” with the clouds and the upper atmosphere.
Curiously, some of these strikes appear not to be associated with preceding downward strikes. Trying to determine which of these strikes appeared independently (self-triggered) as opposed to occurring just after a downward strike (other-triggered) was one of the key objectives of this new study.
Unlike most tall structures equipped with lightning rods, wind turbines are particularly vulnerable to lightning strikes. They are sometimes found on the crests of mountaintops, and although this optimizes their ability to capture wind, it also leaves them incredibly vulnerable to damaging strikes.
“Ground-to-cloud lightning strikes have been observed since the 1930s, but it is only recently, with the growing use of wind turbines, that it has become a real concern,” said Aleksandr Smorgonskiy, a doctoral student at École Polytechnique Fédérale de Lausanne and lead author of the study, in a statement.
Using nearly 15 years of data on lightning strikes striking two European mountaintop wind turbines, Smorgonskiy hoped to determine how lightning strikes form and impact them. To his surprise, he noted that there were 100 times more upside-down strikes occurring than regular ones. Not only that, but by checking each individual strike against the Europe-wide lightning detection system EUCLID, it was revealed that over 80 percent of them were self-triggered.
Air temperature is known to have a variable effect on the formation of lightning. In this study, higher temperatures in the summer around one of the towers produced more of both types of upside-down strikes than in the winter; conversely, the other tower experienced many other-triggered strikes in the winter, whereas it had more self-triggered strikes in the summer.
Taken together, the study suggests that these upside-down strikes take place much more frequently than originally thought, and that they can occur without any preceding downward strike. “These strikes are quite dangerous,” Smorgonskiy told IFLScience. “Tourists congregate around these sorts of sites; without any lightning grounding, they could be incredibly vulnerable. As for the wind turbine blades, at the moment, there is no optimal solution to protect them.”
Ultimately, the explanation for why air temperature has a different effect on the two different towers, and the reason why most upside-down strikes occur independently, both remain electrifying mysteries.
