We think of lightning as something that comes out of the sky, but actually, it can start on the ground as well. Upward lightning strikes are a phenomenon which seems to occur where tall and slender structures are located on high ground, which makes them a particular concern for the growing number of wind turbines across the world.
That's why Aleksandr Smorgonskii, a doctoral student at the EMC Lab at Switzerland's EPFL university, decided to write his thesis on the phenomenon. His findings, recently published in a recent issue of the Journal of Atmospheric and Solar-Terrestrial Physics, provide some new insights about the mysterious atmospheric phenomenon.
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"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," Smorgonskii said in a a press release.
"In order to optimize wind capture, wind turbines are commonly built in the mountains - on crests no less," the researcher explained. "The combination of a tall structure and high altitude corresponds exactly to the conditions in which ground-to-cloud lightning forms. If the tower is made of metal, the blades are the weak point. Not only are they long and light, and thus fragile, but they are also made of non-conductive composite materials. As a result, they are often damaged, if not destroyed, by the extreme electrical charge."
Smorgonskii analyzed data on upward strikes at two radio transmission towers, one in Austria and one in Switzerland over a 15 year period. Some of the results were surprising. For example, he found that in in a typical year, these high structures can record up to 100 more upward strikes than conventional sky-to-Earth lightning.
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Additionally, he discovered that upward strikes don't occur during thunderstorms. Using the Europe-wide lightning-detection system Euclid, Smorgonskii determined, for a given upward strike, whether a downward strike had taken place within a roughly 30-mile radius less than one second before. He found that to be true in just 15 percent of cases.
"Weather conditions, especially the temperature and its distribution in the air, also appear to be an important factor in triggering these lightning strikes," said Smorgonskii.