Lightning Does Strike Twice On The Same Path, And Now We Know Why

The Dutch component of the LOFAR array with images of lightning combined on top. Danielle Futselaar/Artsource.nl

The old saying “lightning never strikes twice in the same place” couldn't be more wrong, at least taken literally. At Lake Maracaibo, Venezuela there are 1.2 million lightning strikes a year, but even in more usual places it is common for there to be two or more lightning strikes that follow the same route to the ground within seconds of each other. It's taken a network of radio telescopes to explain this phenomenon, but now we know why.

Updrafts separate positive and negative particles in clouds, creating an electric field between. When these get strong enough the air turns to plasma, across which lightning can jump. A channel forms either within the cloud or between the cloud and the ground, producing lightning. It turns out, the negative charges inside a thundercloud are not discharged all in a single flash, but are partly stored alongside the main channel at various breaks, and can trigger a second strike.

It may not look like it, but lightning starts from an initiation point and travels in two directions at once, with a positive leader moving towards the negative charges to neutralize them, while a negative leader does the reverse. But the two behave differently, with negative leaders taking discrete steps while emitting intense radio pulses, while positive leaders move more smoothly and are radio-quiet.

This doesn't explain, however, why sometimes we see repeated bolts of lightning following the same channel when it would have been expected the first bolt would have discharged the surrounds.

To find this out, Australian National University Emeritus Professor Harvey Butcher was part of a team that turned the Dutch component of the Low Frequency Array (LOFAR), a vast network of radio telescopes spread across Europe, onto storm clouds. Butcher explained to IFLScience the array has a distinctive buffering system that allows it to capture sudden events like radio bursts in parts of the sky it is not focussed on. While designed for events beyond our planet, or even our galaxy, this feature meant LOFAR is also well suited to capture images of storms in our own atmosphere, and creating 3D models of what is occurring with a resolution of 2 meters (7 feet). They published their results in Nature.

What Butcher and fellow authors found were features they have dubbed "needles," 10-100 meters (30-330 feet) long and less than 5 meters (16 feet) wide, that branch out from the positive leader. Traveling at around 300 km/s (720,000 mph) they draw charge off the main channel. “The negative charges inside a thundercloud are not drained all in one flash but are, in part, stored alongside breaks in the main lightning channel,” Butcher said in a statement

The remaining negative charge can be discharged shortly afterward in a second trike, reusing the same channel, a fact that might have been very useful for Marty McFly to know.

Butcher told IFLScience he is hopeful the work will someday be useful in limiting the damage lightning can do, perhaps in improving the design of lightning conductors.

The development of lightning in slow motion. In real time, the duration is less than 0.2 seconds and towards the end, the discharge spans about 5 kilometers in all directions. The bright yellow flashes are the new radio pulses detected. The positively charged lightning channels are seen at the top of the lightning flash, grow upwards, and appear to twinkle because of the newly discovered needles.. The negatively charged channels grow downwards and are seen to grow continuously. Stijn Buitink (VUB, Brussels) and Brian Hare (University of Groningen). 

 

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