July/August 2008

Listening to Storms

Hydrophones are as effective as hurricane flights, but cheaper

By Deborah Halber

An acoustic sensor vital to joint MIT-Mexico hurricane research is deployed off the coast of Isla Socorro by members of the crew of the Mexican research vessel Altair. Credit: Srinivasan Jagannathan

A half-mile below the ocean's surface, where giant squid and sperm whales swim, hurricane-tossed seas sound like a low roar punctuated by cannon blasts. Nicholas C. Makris, professor of mechanical and ocean engineering and director of MIT's Laboratory for Undersea Remote Sensing, has found a way to use the underwater audio signatures of hurricanes to predict the storms' severity. The technique is as accurate as conventional methods, at a fraction of the cost.

Examining data from 1999's Hurricane Gert, Makris found what he calls "almost a perfect correlation" between wind speeds determined using measurements from a hydrophone--the same kind of under­water microphone that helped the U.S. Navy detect enemy submarines during the Cold War--and measurements collected by U.S. Air Force aircraft. The hydrophone had been placed by the National Oceanic and Atmospheric Administration to monitor seismic activity along the mid-Atlantic ridge; Makris and his team found it serendipitously while looking for existing data that might help them determine whether hurricanes' sound and fury are aligned.

Makris envisions using boats or planes to drop arrays of these cylindrical devices in the paths of advancing hurricanes as an alternative to flying special planes directly into storms to gauge their destructive power. Hurricane flights are dangerous and pricey: the planes run about $75 million to build and $4,000 per hour to fly (monitoring a hurricane typically requires 11 hours of flight). But they're considered necessary because they offer crucial information about storm intensity that satellite imagery cannot, Makris says. Four hydrophones called sonobuoys, dropped in a 200-mile arc by an accommodating ­commercial-­ship crew on a routine crossing, would cost about $4,000.

Makris aims to confirm his method's accuracy with data collected from two permanent hydrophones placed last year near hurricane-prone Isla Socorro off ­Mexico's west coast. He is also exploring exactly what occurs between winds and waves at the surface to create the underwater booms and crashes that characterize hurricanes. All he knows for sure is that the low-frequency audio, barely within the range of human hearing, is so evocative of being on the deck of a storm-tossed ship that it makes him seasick just to listen to it.