Voice From the Vortex

Physicist Al Bedard detected some unusual noises while testing sensitive microphones for a new avalanche warning system. The subaudible sounds came not from avalanches but from somewhere in the sky. We knew we were getting sounds from severe weather, but we didn’t know what the origin was, says Bedard, who works at the National Oceanic and Atmospheric Administration in Boulder, Colorado. Using radar, he spotted some tornadoes, tracked them, and found that their location seemed to correlate with the sounds. Bedard theorized that a tornado’s vortex expands and contracts, as if it’s breathing, generating subsonic pressure waves. Other researchers had proposed that tornadoes might breathe like this, but Bedard was the first to associate such a dynamic with sound. To study twister acoustics in the laboratory, Bedard created a vortex like the one shown here by using water, a stirring arm, and food coloring to follow the flow patterns. Because of the different scale, the bottled tornado’s sounds were audible and quite annoying, says Bedard. The sounds are in the range Bedard predicted for his little tornado, although he hasn’t yet proved that the tornado’s expansion and contraction generate them. This finding may lead to a better early-warning system for tornadoes, says Bedard. Unlike the 40-mile limit of tornado-detecting radar, the range of the new instruments could extend hundreds of miles.