Lightning 'Superbolts' Form over Oceans from November to February
University of Washington – September 9,
2019 -- The lightning season in the Southeastern U.S. is almost finished for
this year, but the peak season for the most powerful strokes of lightning won't
begin until November, according to a newly published global survey of these
rare events.
A University of Washington study maps
the location and timing of "superbolts" -- bolts that release
electrical energy of more than 1 million Joules, or a thousand times more
energy than the average lightning bolt, in the very low frequency range in
which lightning is most active. Results show that superbolts tend to hit the
Earth in a fundamentally different pattern from regular lightning, for reasons
that are not yet fully understood.
]The study was published Sept. 9 in the Journal
of Geophysical Research: Atmospheres, a journal of the American Geophysical
Union.
"It's very unexpected and unusual
where and when the very big strokes occur," said lead author Robert
Holzworth, a UW professor of Earth and space sciences who has been tracking
lightning for almost two decades.
Holzworth manages the World Wide
Lightning Location Network, a UW-managed research consortium that operates
about 100 lightning detection stations around the world, from Antarctica to
northern Finland. By seeing precisely when lightning reaches three or more different
stations, the network can compare the readings to determine a lightning bolt's
size and location.
The network has operated since the early
2000s. For the new study, the researchers looked at 2 billion lightning strokes
recorded between 2010 and 2018. Some 8,000 events -- four millionths of a
percent, or one in 250,000 strokes -- were confirmed superbolts.
"Until the last couple of years, we
didn't have enough data to do this kind of study," Holzworth said.
The authors compared their network's
data against lightning observations from the Maryland-based company Earth
Networks and from the New Zealand MetService.
The new paper shows that superbolts are
most common in the Mediterranean Sea, the northeast Atlantic and over the
Andes, with lesser hotspots east of Japan, in the tropical oceans and off the
tip of South Africa. Unlike regular lightning, the superbolts tend to strike
over water.
Explore a visualization of the data at https://public.tableau.com/profile/uw.news#!/vizhome/Superbolts/Dashboard1.
"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."
"The average stroke energy over
water is greater than the average stroke energy over land -- we knew
that," Holzworth said. "But that's for the typical energy levels. We
were not expecting this dramatic difference."
The time of year for superbolts also
doesn't follow the rules for typical lightning. Regular lightning hits in the
summertime -- the three major so-called "lightning chimneys" for
regular bolts coincide with summer thunderstorms over the Americas, sub-Saharan
Africa and Southeast Asia. But superbolts, which are more common in the
Northern Hemisphere, strike both hemispheres between the months of November and
February.
The reason for the pattern is still
mysterious. Some years have many more superbolts than others: late 2013 was an
all-time high, and late 2014 was the next highest, with other years having far
fewer events.
"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."
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