Most experts agree that waiting until you've heard a tornado's signature jet engine roar is a recipe for disaster. With only five to 13 minutes of advanced warning possible using current technology, your best bet is to seek shelter at the first hint of danger. By the time you hear the churning vortex of a twister, it's often too late.
Unbeknownst to many, however, is that tornadoes and the storms that spawn them generate something called infrasound. These sound waves have frequencies below 20 hertz (cycles per second) or beyond the lower limit of human audibility. Infrasound stations have been created around the world to monitor both man-made events (such as nuclear explosions and sonic booms) and natural events (such as earthquakes, volcanic eruptions and avalanches).
Tornadic thunderstorms emit infrasound up to two hours before "tornadogenesis" when the tornado forms its signature shape. Figuring out how to collect and decipher these sounds could buy communities precious time to prepare.
Brian Elbing, assistant professor of mechanical and aerospace engineering at Oklahoma State University, believes infrasound warning systems could revolutionize tornado prediction services.
"By monitoring tornadoes from hundreds of miles away, we'll be able to decrease false alarm rates and possibly even increase warning times," Elbing said ahead of a presentation on the idea at the 175th Meeting of the Acoustical Society of America. "It also means storm chasers won't need to get so close."
A different kind of microphone to pick up a different sound
An infrasound array monitoring station in Qaanaaq, Greenland. Ebling and his team have created a similar setup for listening to tornado infrasound. (Photo: The Official CTBTO Photostream/flickr)
Elbing and his team have been collecting and decoding infrasounds generated over a tornado's lifecycle using a specialized microphone array at Oklahoma State University. Arranged in a triangle and spaced some 200 feet apart, these infrasound sensors look more like spider webs than traditional microphones.
"First, these are larger for greater sensitivity to lower frequencies," Elbing said in a statement. "Second, we need to get rid of wind noise. ... We seal the microphone inside a container with four openings. A soaker hose — just like the ones used in gardens — is attached to each of these openings and stretched out in opposite directions."
In an interview with Wired, Elbing said normal wind entering the hose openings is incoherent and random. The infrasound generated by an approaching storm capable of spawning tornadoes presents a pressure wave that's much more uniform. "Those waves that go into the hose and go into the microphone, those will add together instead of canceling each other out," he added.
While the system is still in its early stages, there are reasons to believe it will one day be a useful tool for tornado prediction. On May 11, 2017, the infrasound station at Oklahoma State University picked up a tornado signal 12 miles away and 10 minutes before a twister descended from the sky. Its estimated size was 150 feet in diameter, a number later confirmed by the U.S. National Oceanic and Atmospheric Administration.
The ultimate goal for Ebling and his team is to create a system that detects a possible tornado forming and then leverages drones equipped with sensors to perform follow-up observations. Used in conjunction with current prediction technology, infrasound would enable meteorologists to create advanced warnings with a greater degree of certainty.
"Since infrasound is an independent data source, combining it with existing methods should help reduce false alarms," said Ebling. "Today, 75 percent of tornado warnings are false alarms and tend to be ignored."