AUDIE CORNISH, HOST:
Sometimes the sky turns an eerie shade of green and blue, it starts raining, the wind starts picking up, and the National Weather Service issues a tornado warning, but about three-quarters of the time, a tornado doesn't actually form. As Rachel Hubbard of member station KOSU reports, some new research based on sound could improve the accuracy of those warnings.
RACHEL HUBBARD, BYLINE: After a tornado passes through a community, it's almost inevitable to hear an interview like this one.
(SOUNDBITE OF ARCHIVED RECORDING)
UNIDENTIFIED PERSON: Sound like a freight train coming. I'll vouch for it. It's a freight train.
HUBBARD: People talk about how the storm sounded, but there's another sound, one that you can't hear that's just below what the human ear can detect. It starts as much as two hours before the storm. Brian Elbing teaches mechanical and aerospace engineering at Oklahoma State University.
BRIAN ELBING: The sound is specific to the tornado. It carries information about the size, the wind speed, the core pressure.
HUBBARD: Scientists have known for years that weather events, including earthquakes and volcanoes, make so-called inaudible sounds, but they haven't figured out how to use that information to predict big weather events. So last year, Elbing and a group of students put out some microphones on campus. And in May, they picked up a tornado.
ELBING: We could look at radar data of the larger storm system, and when the rotation there was really strong, it produced almost no infrasound. And then 10 minutes before the tornado itself formed is when you start seeing this big increase in the sound levels. And then it lasted the duration of the tornado.
HUBBARD: If you boost what the microphones recorded into the range of what a person can hear, it sounds like wind, kind of like an actual tornado.
(SOUNDBITE OF WIND BLOWING)
HUBBARD: With more testing, this sound could help predict the formation of a tornado. Rick Smith is the warning coordination meteorologist at the National Weather Service office in Norman, Okla. Right now, forecasters combine all sorts of data, from radar, atmospheric conditions and human observation, before they issue a tornado warning.
RICK SMITH: It's still a mystery, though. We don't know everything. So the science is certainly helping us issue better warnings, but the science is not to the point yet where we can definitively say, storm A definitely will have a tornado, storm B definitely won't.
HUBBARD: With the little amount of data scientists gathered last year, Elbing says his team is refining its hypothesis. For example, it showed that it was the tornado making the sound, not the larger storm. And they suspect by studying the amplitude of the sound, they might be able to predict how strong the tornado is in real time. He believes with just $50,000 in funding for equipment, the system could be operational across Oklahoma within five years. For NPR News, I'm Rachel Hubbard in Oklahoma City.
(SOUNDBITE OF LONG DISTANCE CALLING'S "IN THE CLOUDS") Transcript provided by NPR, Copyright NPR.