Everyone knows what lightning sounds like, but now for the first time, scientists at the Southwest Research Institute in San Antonio, Texas have turned the accompanying thunderclap into an image.

For many centuries, humanity believed that lightning was caused by the collision of clouds in the sky. In a way that's sort of true -- if you think of it as the collision of the elements inside clouds. When clouds are at a certain altitude, some of the water particles inside freeze. This leads to a mix of frozen ice crystals and a heavier mix of water and slushy ice called graupel.

These elements rub against each other in the wind, and the ice develops a positive charge while the graupel develops a negative charge. The lighter ice will then rise to the top of the cloud, and the heavier graupel will sink to the bottom, separating the positive and negative charges. When both positive and negative charges accumulate in sufficient amounts, they spark, causing lightning.

It is this lightning that causes the sound of thunder, a sort of sonic boom caused by the thermal expansion of plasma in the lightning channel. The increase in pressure and temperature causes the air around the lightning to expand at a rate faster than the speed of sound, causing a shockwave which extends about 10 metres (30 feet) around the lightning.

After that distance, it slows enough to become ordinary sound. In close proximity, thunder is as loud as standing in front of speakers at a rock concert, or standing within 60 metres (200 feet) of a jet engine during takeoff -- 120 decibels, loud enough to cause permanent hearing loss, while the shockwave can cause property damage.

Image Courtesy of University of Florida, Florida Institute of Technology, and Southwest Research Institute

It is this incredible sound that researchers at the Southwest Research Institute in Antonio, Texas, have captured visually.

"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."

Dr Dayeh's team built a custom rig to artificially induce lightning and record its sound.

A small rocket trailing a grounded copper wire is launched into thunderclouds, creating a conductive channel for lightning to travel down. This allows the researchers to focus their instruments and perform repeat experiments.

Image Courtesy of University of Florida, Florida Institute of Technology, and Southwest Research Institute

An array of 15 microphones, each spaced one metre (3.3 feet) apart, was then placed 95 metres (about 310 feet) from the rocket launch pad. Post-signal processing and directional amplification of the data captured by the microphone array was then used to create a visual representation of the sound produced by the lightning travelling down the copper wire.

Initially, the team thought the experiment had failed.

"The initial constructed images looked like a colourful 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," Dr Dayeh said.

However, at higher frequencies, the sound cleaned up, revealing a distinct thunder signature. Data such as this could assist in learning more about how lightning produces thunder, as well as how lightning discharges energy.