Record-Breaking Lightning Flashes Help Change Definition Of Lightning Events

Published: Wednesday, September 21, 2016 - 5:10pm
Updated: Monday, September 26, 2016 - 3:37pm
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(Photo courtesy of World Meteorological Organization)
A single lightning flash crossed nearly 199.5 miles of Oklahoma sky.
(Photo courtesy of World Meteorological Organization)
A lightning event over southern France persists for a record 7.74 seconds.

Two record-breaking lightning flashes have changed experts’ ideas of what is possible for such events.

The first, a cloud-to-cloud lightning flash on June 20, 2007, set a world distance record when it shot across nearly 200 miles of Oklahoma sky. Just over five years later, another flash blazed over southern France for 7.74 seconds, shifting notions of how long a lightning event can last.

The new data prompted a World Meteorological Organization (WMO) committee to recommend revising the definition of lightning discharges. The panel was assembled by Randy Cerveny, an Arizona State University geographical sciences professor, who is also rapporteur of weather and climate extremes for the WMO.

The findings were reported in the Sept. 15 edition of Bulletin of the American Meteorological Society.

According to Cerveny, the remarkable discoveries were made possible by instrument arrays put in place over the past few decades.

“Twenty, 30 years ago, we couldn’t have done this kind of a study — we wouldn’t have been able to say how long a lightning strike actually is — but now our technology has improved to the point that, for certain circumstances, we can get a very precise determination of both its horizontal distance and the length of time that the lightning flash exists," said Cerveny.

The Lightning Mapping Array detects the static generated by lightning events and uses a 3-D model to triangulate each flash’s position. From there, researchers can calculate factors such as duration and distance travelled, although Cerveny said nailing down such details is far from simple.

“We have to be able to differentiate one strike from another, or make sure that, when one strike is ending, it’s not continuing with another strike,” he said. “It’s very, very detailed and precise work.”

The lightning flashes in question are not necessarily the most extreme ever to occur — they’re just the most remarkable ones that researchers have caught in their net so far.

But they were enough to prompt the WMO committee to vote unanimously to amend the definition of lightning discharge, as defined by the American Meteorological Society’s Glossary of Meteorology. Previously, the definition said a lightning discharge took place within one second. That phrase has since been replaced by the word "continuously.”

Cerveny said the results also underline the need for caution when dealing with lightning.

“From a general public standpoint, these results really point out the need that, when lightning is occurring, you need to take cover — that lightning can travel extremely long distances and that, if there’s any lightning in the vicinity, you are really not safe," he said.

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