Researchers from the Southwest Research Institute in Texas launched a rocket with a trailing copper wire into a storm, and were able to make lightning happen right where they thought it would. With this predictability, they could train arrays of microphones at the spot and capture the sound waves of thunder visually, in unprecedented detail.
Science has come a long way since 1752, but when it comes to inducing lightning not that much has changed since Ben Franklin. Only instead of kites, we now shoot rockets into storms, which feels like progress.
Maher A Dayeh, a research scientist in the SwRI Space Science and Engineering Division, told me he gets the Ben Franklin comparison a lot. I asked him over email why he studied the "acoustic signature" of thunder.
"Among other reasons, to better understand the origins of thunder," Dayeh said. "For instance, to understand which processes of the lightning discharge contribute to the generation of thunder."
Although we experience lightning as a single, really fast phenomenon, it's actually a process. Electrostatic charges build in churning storm clouds, creating branching veins of electricity that reach to the ground, called "step leaders," that open the channel where instantaneous return strokes form the lightning flash. We still don't know which of these creates an acoustic signal, but Dayeh believes they now have a way to do so.
He and a team of researchers went to Florida, the most lightning-stricken state. They built arrays of fifteen microphones that were spaced one meter apart, and set them up 95 meters from the rocket launch pad. Then, boom.
Here's what induced lightning and thunder looks like:
"The red indicates the highest acoustic pressure associated with the shock wave arrival at the array's 'ear,'" Dayeh said. "The surrounding green and blue are from ground reflections and background noise."
How we experience the sound of thunder mostly depends on distance from the storm, Dayeh told me. When you're close it sounds like a loud crack and from a distance it sounds like a rumble as the acoustic signatures of the different parts of the lightning overlap.
"The farther you go, it gets more complex as propagation effects get more into play," he said. "That said, this is a topic to be investigated."
The rocket experiment was more of a proof of concept for conducting those investigations. The researchers are presenting their findings at the joint meeting of American and Canadian geophysical societies in Montreal this week.
My own personal lightning-thunder research has mostly revolved around counting the seconds between the two, which, I'm pleased to report, has been confirmed by a real-life scientist as a "very reliable" method. Dayeh told me that given a sound speed of about 343 meters per second, three seconds between lightning and thunder indicates "a kilometer or so."
But if you need it closer—and only if you need it closer—consider the rocket method.