MONTREAL, Canada — With thunder, there’s always lots to hear. Now there’s also something to see. For the first time, scientists have precisely mapped the loud clap radiating from a lightning strike. This picture of thunder’s origins could reveal the energies involved in powering some of nature’s flashiest light shows.
Lightning strikes when an electric current flows from a negatively charged cloud to the ground. This rapidly heats and expands the surrounding air, creating sonic shock waves. We hear this as thunder.
|SEEING THUNDER Scientists shot a long copper wire into a cloud using a small rocket. This generated a bolt of lightning. The current followed the wire to the ground. This allowed researchers to record the sound waves of the resulting thunder. The intense heating of the copper wire caused the green flashes.UNIV. OF FLORIDA, FLORIDA INSTITUTE OF TECHNOLOGY, SRI|
Scientists have a basic understanding of the origins of thunder. Still, experts have lacked a detailed picture of the physics powering the loud cracks and low rumbles.
Maher Dayeh works at the Southwest Research Institute in San Antonio, Texas. As a heliophysicist, he studies the sun and its effects on the solar system, including Earth. He and his colleagues also study lightning — by making their own. These experts trigger the bolts by firing a small rocket into an electrically charged cloud. Trailing behind the rocket is a long, Kevlar-coated copper wire. The lightning travels along that wire to the ground.
For their new experiment, the scientists used 15 sensitive microphones laid out 95 meters (312 feet) from the strike zone. The team then precisely recorded the incoming sound waves. Those from higher elevations took longer to reach the microphones. That allowed the scientists to map the
acoustic (sound) signature of the lightning strike. That map revealed the strike with “surprising detail,” Dayeh says. He presented his team’s findings here on May 5 at a meeting of the American Geophysical Union and other organizations.
How loud a thunderclap sounds will depend on the peak electric current flowing through the lightning, the researchers found. Explains Dayeh, this discovery could one day allow scientists to use thunder to sound out the amount of energy powering a lightning strike.