An electrical signal generated by lightning activity might foreshadow a tornado touchdown (Credit: Justin Hobson / Wikimedia Commons)

Tornadoes are being betrayed by their lightning in a way that could help save lives, according to researchers who made an accidental discovery.

Last summer, geophysicist Benjamin Barnum of Johns Hopkins University’s Applied Physics Laboratory in Maryland detected a previously unknown electrical signal generated by lightning activity that foreshadowed a tornado touchdown. This electrical signal could potentially be used with existing tornado prediction methods to improve the early warning system. Barnum presented a poster of the discovery Tuesday at the American Geophysical Union’s Fall Meeting.

On June 1, 2012, a strong thunderstorm blew over the Washington, D.C., area, producing several tornadoes. That evening, Barnum and other scientists were running tests on lightning detectors when a tornado funnel formed just 10 miles away.

“It was a fortuitous bit of luck that this tornado touched down so close to where we were running tests,” said Barnum.

About 15 minutes before the tornado touched down, Barnum’s detectors measured a huge increase in lightning activity in the direction where the tornado was developing. Similar storms without tornadoes normally produce 300 to 600 in-cloud lightning strikes per minute, but the detectors were reporting 1,037 strikes per minute in the forming tornado’s vicinity. After the tornado touched down, the lightning strike count rapidly dropped.

In a typical thunder cloud, there is a layer of positive electrical charge at the top of the cloud and a layer of negative charge at the bottom. When a tornado starts to form, the cloud is compressed and the two charged layers are forced closer together. This compaction causes rapid discharges in the form of in-cloud lightning, Barnum said.

While scientists had already known that lightning strikes can accompany developing tornadoes, one of Barnum’s detectors measured a surprising change in the nearby electric field not usually associated with the typical increased lightning activity. As the tornado formed and the lightning strikes became more frequent, the electric field’s strength began jumping up and down about once every second. As soon as the tornado touched down and lightning rate dropped off, the electric field stabilized and returned to normal.

Though further research is needed, devices that detect these electric field changes could potentially be deployed alongside existing tornado warning systems, Barnum said.

“This could be one more tool helping to give locations and early warnings of tornadoes,” he said. “If you put these things in an array alongside existing detectors, you could potentially help more people know that tornado activity is imminent.”

–Thomas Sumner is a science communication graduate student at UC Santa Cruz