“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.
What about thunder? Scientists understand the general mechanics of it, but not the exact process. Energy from a lightning strike heats the air immediately around it to around 18,000 degrees Fahrenheit. This extreme temperature increase causes the air to expand rapidly and make the thundering boom we hear.
The sharp crack is heard when you are extremely close to a lightning strike. The further you are away from a lightning strike, the more of a rumble you hear. That’s because high-frequency pitches are absorbed by the surrounding environment (trees, structures, etc).
Dayeh and other scientists used a large array of microphones in hopes of better understanding the exact mechanism behind thunder.
To do this, the scientists set up 15 microphones lined up about 310 feet away from a rocket pad. The microphones were spaced one meter apart.
“Thunder and lightning are fascinating, wild, and unpredictable,” said Dayeh. “Because of their erratic nature, the phenomena are best studied using triggered events.”
Scientists used a small rocket to trigger a lightning strike. The rocket has a grounded copper wire trailing it as it soars into thunderclouds. Remember Benjamin Franklin’s kite experiment? It’s the same principle.
The copper wire gives a predictable path for lightning and allows scientists to focus their instruments on one location.
The photo below shows a long-exposure photograph on the left. See the green glow? That’s the copper wire burning as lightning strikes it. The purplish streaks are “nine subsequent return strokes.”
The images on the right show the unique acoustic signatures of each return stroke. The curves you see are the sound speed propagation effects.
This next image shows the acoustically imaged profiles of the lightning strike. Note how the tilt in the lightning strike on the left also shows up in the acoustic image.
Dayeh wasn’t sure if his experiment worked at first. “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.”
It wasn’t until he looked at the different sound frequency bands that he noticed the images were clearer at higher frequencies.
The Southwest Research Institute says, “future experiments could allow scientists to study the probable acoustic signatures of current pulses, step leader branches, and discharge channel zigzags independently.”