Undersea Microphone Helps Measure Hurricanes

In order for scientists to measure the strength of a hurricane, they typically must rely on the tricky maneuver of flying an airplane through the storm. But a discovery from the field of underwater acoustics means it's possible to measure a hurricane's strength just by listening to the sounds it makes — under the sea.

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Tracking a hurricane's position is pretty easy now that the sky is full of weather satellites. To measure a hurricane's strength, though, one would still need to fly a special plane right through the storm. That's dangerous and it's expensive, so scientists have come up with a very different approach, as NPR's Jon Hamilton reports.

JON HAMILTON: You can learn a lot by listening to a hurricane. Its high winds produce a signature sound as they whip across the earth's surface. The sound is so powerful, you can even hear it underwater. Military sensors have been picking up that sound since World War II, but no one did much with it until a couple of MIT professors ran into each other at a meeting.

Professor NICK MAKRIS (Mechanical and ocean engineering, Massachusetts Institute of Technology): It was the first time Kerry and I met and, you know, he works in the atmosphere. I work in the ocean.

HAMILTON: Nick Makris studies underwater acoustics. The Kerry he's talking about is Kerry Emanuel, a big time hurricane expert.

Makris said their conversation led to a place where their fields converge - the surface of the ocean and what it sounds like during a hurricane. Emanuel asked a question.

Prof. MAKRIS: Do you think that you could use the noise measured underwater to determine the destructive power of a hurricane? I told him, well, yes, I actually think you can, because, in fact, we've known since at least World War II that there's a very good correlation between the intensity of underwater sound and wind speed.

HAMILTON: Great idea, but how do you test it? If you're at MIT, you give that problem to a graduate student. Makris picked Joshua Wilson. Wilson - who is now Dr. Wilson - began looking for an undersea microphone that might have recorded a hurricane, and he found one. It was suspended in the middle of the Atlantic half a mile down.

The National Oceanic and Atmospheric Administration, or NOAA, was using sensors like this one to listen for underwater earthquakes.

Prof. MAKRIS: And it turned out that a hurricane - Hurricane Gert - did pass right over those sensors.

HAMILTON: Back in 1999. The actual recording is a low, low rumble.

Prof. MAKRIS: Very low frequency. We're talking about frequencies that are barely audible for humans, the frequencies of thunder and cannons or a bass guitar, the low notes on a bass guitar.

HAMILTON: But they're easy to hear when they're shifted up a couple of octaves. The recordings were an amazing find, but did that sound correspond to the power of hurricane? Once again, help came from NOAA. The agency had flown a plane through Hurricane Gert on the very same day that it made the underwater recordings. Makris and Wilson combined all the data, and viola.

Prof. MAKRIS: What we found was the power of the wind was directly proportional to the power of the underwater sound.

HAMILTON: In fact, the sound was just as good at measuring a hurricane's wind speed as an airplane. Makris says underwater microphones are a lot cheaper than airplanes, and it's easier to drop them into the path of an oncoming hurricane.

Scientists like Makris and Wilson owe a lot to the military, which created the field that's now known as ocean acoustics. George Frisk at Florida Atlantic University says the first acoustic oceanographers were just guys in a submarine.

Dr. GEORGE FRISK (Ocean Engineering, Florida Atlantic University): Some of the best sonar officers are able to identify the sources of sound just by listening to them.

HAMILTON: In war time, they used that skill to identify ships and to estimate their speed. Frisk says the military set up the first network of underwater microphones to listen for enemy submarines. Now, he says, these microphones are monitoring everything in the ocean, from shrimp and ships to the weather above.

Jon Hamilton, NPR News.

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