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Average lightning can hit hard. But then there are superbolts. Identified for the first time in the 1970s by satellites designed to monitor nuclear explosions, they can be thousands of times more numerous. energetic than normal lightning.
But you are even less likely to be struck by one: scientists have exploited data from the approximately 80 sensors in the global lightning network to study the locations of superbolts. They were surprised to discover that the most powerful lightning did not occur in known hotspots or in the times of the year when lightning was generally expected. Instead, they reported last week in the Journal of Geophysical Research: Atmospheres, The superbolts occur mainly on open water from November to February.
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Robert Holzworth, physicist of the atmosphere and space at the University of Washington, and his colleagues analyzed more than 1.7 billion lightning observations by radio frequency. Focus on only the most robust lightning detections strikes from 2010 to 2018, Dr. Holzworth and his team labeled 8,171 superbolts above one million joules. This is about 0.0005% of the lightning recorded by the Lightning Detection Network, which he directs.
"It's a tiny fraction," said Dr. Holzworth. The superbolts are so rare that even he, one of the greatest lightning experts in the world, has never witnessed this.
Dr. Holzworth and his team determined the location of each lightning strike by triangulation, as radio signals from lightning bounced off parts of the earth's atmosphere.
"They can travel almost anywhere in the world," said Dr. Holzworth. By accurately programming the arrival of radio waves on several network sensors, his team estimated the location of each attack a few kilometers away.
The superbolts seem to be a different breed of lightning than their smaller brothers.
First of all, they tend to occur over water – the superbolts are most prevalent in the east of the North Atlantic and the Mediterranean Sea. This was unexpected, as lightning usually tends to occur on land in America, sub-Saharan Africa, and Southeast Asia.
"These three regions seem to produce the vast majority of lightning in the world," said Dr. Holzworth. "This is not where we find the superbolts."
The superbolts also have a different seasonality than the less energetic locks: more than three quarters of them were recorded during the Northern Hemisphere winters from November to February. But lightning detection networks have shown that lightning usually peaks during warmer seasons. This discovery was "totally unexpected," said Dr. Holzworth.
What explains the oddball nature of superbolts? They can form mainly above the water because there are no places where lightning strikes. "There are not a lot of towers or trees," said Dr. Holzworth. The researchers therefore think that the charges accumulate until they are completely relaxed.
The prevalence of superbactyls from November to February may be due in part to warm ocean currents flowing in winter, Dr. Holzworth said. "When you are passing cold air over hot water, you can get a strong convection."
In the future, Dr. Holzworth and his collaborators want to study the destructive impact of land-based superbugs. Researchers have turned their sights on South America, the only place where there is a major superbactole activity on a continent, Dr. Holzworth said.
"We would like to see the beautiful Andes shoes," he said.
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