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Listen to part of a lecture in a meteorology class. (P =Professor, J = Jennifer, D = Davis)
P: OK. It’s important to measure a hurricane’s intensity before it reaches land because it can help save lives and avoid the enormous costs of an unnecessary evacuation. But the factors leading to a hurricane’s intensity like wind speed is very tricky to measure cause they’re changing constantly. So can anyone tell me how we get information about the intensity of a hurricane that’s on the way? Jennifer?
J: Well, don’t we… like… fly planes into it?
P: Right. Planes that monitor hurricanes fly through the eye of a hurricane to measure the speed of the storm’s wind. But it’s very expensive and brings a lot of risks.
J: I understand the point about it being risky for the pilots with the high winds and lightning and everything. But urn… how can it be so expensive?
P: Well. You see, hurricane-monitoring planes are built to withstand strong winds. And they’re usually loaded with a lot of sophisticated equipment that will measure wind speed and other things. One plane costs about 100 million dollars and its single flight costs like… 50 thousand dollars. Plus, it can take as many as 10 flights to monitor a hurricane as it approaches land. From the readings of these direct measurements of hurricane force, we can determine whether to evacuate the area. But that’s an expensive approach. I mean, we can’t simply fly planes into every hurricane and cyclone. Now a couple of researchers think there may be a better way to measure a hurricane’s intensity… uh… a much more cost-effective way. It has almost no risks and requires much lower tech equipment so this method is very promising. It’s a microphone… an underwater microphone called a hydrophone. OK, so, how it works is… urn… it receives acoustic waves underwater. By measuring the noise of a hurricane underwater, we can predict the speed of hurricane winds with an amazing precision. Yes, Davis?
D: But how can we hear winds blowing over the water if the hydrophone is underwater?
P: Well, hurricanes don’t just blow over the water leaving it untouched, do they?
D: Oh yeah. The water gets churned up.
P: Hurricanes churn up the waves like crazy. The roiling action of the wind actually turns the water into a bubble-filled froth. And all this action creates a unique rumbling sound under the water whose volume is a good indicator of the intensity of the storm… uh… the speed of the hurricane winds. Uh… hydrophones can be deployed hundreds of meters below the surface ahead of the hurricane’s path while conditions are still safe. Also, the total cost for such a deployment would be a small fraction of the cost of even a single flight into the storm.
J: I wonder how they figured this out. I mean, how would it occur to anyone to put a microphone under water to measure the speed of the hurricane winds?
P: Well, this actually brings us back to something I’ve talked about in class before. It’s the… ah… value of combining scientific disciplines to deal with com-plex meteorological problems. In this case, the idea came to light when two researchers from different fields met a few years ago. One was Nicholas Makris, an expert of underwater acoustics and the other was Kerry Emmanuel, a hurricane expert. So…uh…well… the research was triggered by their conversation. Emmanuel asked Makris, "Is it possible that underwater noise could be analyzed to determine the intensity of a hurricane?" Makris said, "Yes, in theory at least." It was a commonly known fact that wind speed has something to do with underwater noise. But it was not until Makris met Emmanuel that the idea occurred to him to use the relationship to measure hurricane winds. So, Makris started looking for specific evidence to support the theory. He thought there may have been a situation where a hydrophone was deployed for some other purpose and unintentionally recorded hurricane noise. And he found it. In 1999, a hydrophone in the middle of the Atlantic, just under a kilometer below the surface, was listening for underwater earthquakes when a hurricane passed over it. The hydrophone picked up a low rumbling sound, like the thundering sound, from the water churned up. And on the same day, a plane had flown into the hurricane and made direct wind speed measurement. When Makris compared the data, he found that there was almost a perfect relationship between the power of the wind and the power of the wind-generated noise. And there was less than 5 percent of error, which is about the same as the errors you get from direct measurements from airplane measurements.