Strange rumbles heralding deadly tornadoes could one day predict when and where extreme windy weather will strike. Storms emit sounds before tornadoes form, but such signals at less than 20Hz are inaudible for humans. But the cause of these inaudible rumbles has also been a puzzle.
Now scientists have narrowed down the reasons for the sounds – an important factor in harnessing the knowledge to improve tornado detection.
Dr Brian Elbing, of Oklahoma State University, who is part of the team behind the research, said: “The three possibilities are core oscillations in the tornado, pressure relaxation, and latent heat effects.
“They are all possibilities because what we have seen is that the signal occurs before the tornado touches the ground, continues after it touches the ground, and then disappears some time after the tornado leaves the ground.”
The low-frequency sound produced by tornadoes has been detected for several decades, but a big problem has been a lack of understanding about their cause, and difficulties in unpicking them from a tornado and other aspects of the weather.
The subject has seen renewed interest in recent years, with Dr Elbing believing it could prove particularly useful for hilly tornado-prone areas surrounding Texas and North Carolina.
He said: “Infrasound doesn’t need line of sight like radar, so there is hope that this could significantly improve warnings in Dixie Alley where most deaths from tornadoes occur.”
The researchers used microphones capable of detecting low-frequency sounds sealed inside a dome with four openings at right angles to each other, each of which is attached to a hose.
Three of these domes were arranged in an equilateral triangle, 200ft (60m) away from each other.
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This setup allowed the researchers to filter out sounds from regular wind and calculate the tornado’s direction of travel.
The signal itself offers an idea of the tornado’s size, with a frequency of 1Hz indicating a very large tornado, while a 10Hz suggesting one far less powerful.
During their research, Dr Elbing and colleagues reported a case in Oklahoma in which they were able to pick up audio clues eight minutes before the twister formed, with a clear signal detected four minutes before it hit the ground.
This, they say, is important as the tornado was not detected by radar.
Dr Elbing believes low-frequency sounds have been detected up to two hours before a tornado forms
He said: “There is evidence that the amount of lead time before the tornado is dependent on how large the tornado is.
“This tornado we detected was very small, there was no warning issued for this tornado, which is why even a four-minute warning is a big deal.”
While the Oklahoma tornado was only 12 miles from the setup, Dr Elbing said once the sound signal was better understood, the technique could be used over even greater distances.
“If we know the acoustic signature of a tornado, it is realistic to expect to detect a tornado from over 100 miles,” he said.