Flow

About 1,000 tornadoes strike the United States each year, causing billions of dollars in damage and killing about 60 people on average. Tracking data show that they’re becoming increasingly common in the southeast, and less frequent in “Tornado Alley,” which stretches across the Great Plains. Scientists lack a clear understanding of how tornadoes form, but a more urgent challenge is to develop more accurate prediction and warning systems. It requires a fine balance: Without warnings, people can’t shelter, but if they experience too many false alarms, they’ll become inured.

One way to improve tornado prediction tools might be to listen better, according to mechanical engineer Brian Elbing at Oklahoma State University in Stillwater, in the heart of Tornado Alley. He doesn’t mean any sounds audible to human ears, though. As long ago as the 1960s, researchers reported evidence that tornadoes emit signature sounds at frequencies that fall outside the range of human hearing. People can hear down to about 20 Hertz—which sounds like a low rumble—but a tornado’s song likely falls somewhere between 1 and 10 Hertz.

Brandon White, a graduate student in Elbing’s lab, discussed their recent analyses of the infrasound signature of tornadoes at the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics.

Elbing said these infrasound signatures had seemed like a promising avenue of research, at least until radar emerged as a frontrunner technology for warning systems. Acoustic-based approaches took a back seat for decades. “Now we’ve made a lot of advances with radar systems and monitoring, but there are still limitations. Radar requires line of sight measurements.” But line of sight can be tricky in hilly places like the Southeast, where the majority of tornado deaths occur.

Maybe it’s time to revisit those acoustic approaches, said Elbing. In 2017, his research group recorded infrasound bursts from a supercell that produced a small tornado near Perkins, Oklahoma. When they analyzed the data, they found that the vibrations began before the tornado formed.

Researchers still know little about the fluid dynamics of tornadoes. “To date there have been eight trusted measurements of pressure inside a tornado, and no classical theory predicts them,” said Elbing. He doesn’t know how the sound is produced, either, but knowing the cause isn’t required for an alarm system. The idea of an acoustics-based system is straightforward.

“If I dropped a glass behind you and it shattered, you don’t need to turn around to know what happened,” said Elbing. “That sound gives you a good sense of your immediate environment.” Infrasound vibrations can travel over long distances quickly, and through different media. “We could detect tornadoes from 100 miles away.”

Members of Elbing’s research group also described a sensor array for detecting tornadoes via acoustics and presented findings from studies on how infrasound vibrations travel through the