Lightning and thunder go together like peanut butter and jelly, but scientists are surprisingly fuzzy on the details. Lightning is easy enough to see (though difficult to predict), but thunder is not a visual medium. Sure, they know how it happens, but it’s not clear how the sound and shock waves travel. Now, scientists from the Southwest Research Institute have developed a clever way to “see” thunder, opening the doors for much more detailed research into this and other weather phenomena.
“Lightning strikes the Earth more than 4 million times a day, yet the physics behind this violent process remain poorly understood,” said Dr. Maher A. Dayeh, a research scientist in the SwRI Space Science and Engineering Division. “While we understand the general mechanics of thunder generation, it’s not particularly clear which physical processes of the lightning discharge contribute to the thunder we hear. A listener perceives thunder largely based upon the distance from lightning. From nearby, thunder has a sharp, cracking sound. From farther away, it has a longer-lasting, rumbling nature,” he said.
Lightning is more than just the momentary flash we see. It begins as charged particles gather in the clouds. They start to branch out, “feeling” for was to go to ground and discharge. Once the most efficient and direct route to the ground is determined, the clouds discharge ferociously and instantly, leading to the electric arc we associate with lightning.
Because lightning is so unpredictable (and because the sound of thunder is so location-based), Dayeh and his team decided to trigger their own lightning by firing a rocket attached to a grounded copper wire up into a thunderstorm. Because the wire reached directly to the ground and copper is such a good conductor of electricity, it provided an ideal environment for lightning.
In order to capture thunder’s acoustic signature, the team set up an array of microphones 95 meters from the rocket launch site, spaced one meter apart. To image the vertical profile of the bolt, he used post-signal processing techniques and directional amplification of the data signals captured by the microphone array.
“The initial constructed images looked like a colorful piece of modern art that you could hang over your fireplace. But you couldn’t see the detailed sound signature of lightning in the acoustic data,” he said.
Dayeh didn’t think it would work, but tinkering with the frequency eventually yielded what he was after: A visual image of thunder’s acoustic signature. After studying the main lightning discharge channel, future research may involve smaller lightning branches, or the effects of zigzags in the main discharge channel.