One of the trademarks of summer are loud, angry storms that roll in with rumbles of thunder and bold flashes of lightning. Most lighting occurs during the summer with the National Weather Service estimating that lightning strikes the ground about 25 million times in the U.S. each year.
But according to new research, the biggest and strongest lightning bolts actually strike from November through February each year, which is winter in the Northern Hemisphere. These rare "superbolts" release 1,000 times more energy than the average lightning bolt.
"It's very unexpected and unusual where and when the very big strokes occur," lead author Robert Holzworth, a University of Washington professor of Earth and space sciences, said in a statement.
Holzworth runs the World Wide Lightning Location Network, a research consortium run by the university that operates about 100 lightning detection stations around the world. By recording exactly when lightning reaches three or more different stations, the network is able to determine a lightning bolt's size and location.
Sizing up superbolts
For the study, published in the Journal of Geophysical Research: Atmospheres, researchers mapped the location and timing of superbolts. They looked at 2 billion lightning strokes that were recorded between 2010 and 2018. About 8,000 — one in 250,000 strokes, or less than a thousandth of a percent — were superbolts.
They found that superbolts are most common in the Mediterranean Sea, the northeast Atlantic and over the Andes. Unlike regular lightning, superbolts most often hit over water.
"Ninety percent of lightning strikes occur over land," Holzworth said. "But superbolts happen mostly over the water going right up to the coast. In fact, in the northeast Atlantic Ocean you can see Spain and England's coasts nicely outlined in the maps of superbolt distribution."
Also surprising was that superbolts strike at a totally different time of year than traditional lightning. Researchers say the reason for this seasonal change is "mysterious."
"We think it could be related to sunspots or cosmic rays, but we're leaving that as stimulation for future research," Holzworth said. "For now, we are showing that this previously unknown pattern exists."