It's not every day that scientists get all bubbly.

Lawrence A. Crum, a physics professor at the University of Mississippi, was trying to record the sound of snowflakes hitting the surface of water to determine why, counter to logic, these fluffy, air-filled assemblies of ice crystals seem to make a lot of noise underwater. Several years ago, Crum got his big chance during a visit to Yale University in Connecticut.

After a dinner of pizza and beer, Crum heard television forecasters predicting snow for Baltimore. So he and a colleague borrowed the Yale engineering dean's van and equipment and went south in pursuit, but not before he picked up the phone and called colleague Andrea Prosperetti, Hopkins professor of mechanical engineering.

"He threatened to wake me up at 6 a.m.," Prosperetti remembers. "Thank God it didn't snow here. I kept sleeping and he kept driving."

As Crum recalls: "We asked if we could use his office or lab to do the measurements. But the storm went south and we couldn't find it, so we were chasing."

What Crum and his Ole Miss colleague Ronald Roy found that trip after rigging up acoustic equipment in a motel pool in Roanoke, Virginia, led to a research paper published in October in the Journal of the Acoustical Society of America. Crum, now chair of the Acoustics and Electromagnetics Department at the University of Washington, is the study's lead author, in conjunction with snowchase colleague Roy, now at Boston University, and Prosperetti.

The journal article, also co-authored by Hugh Pumphrey of the University of Edinburgh in Scotland, reveals a bit of the scientist's joy of discovery: Early snowfall data was "so unique and contrary to our intuitions and expectations that it has inspired us to accumulate data from a number of storms," the authors wrote. After studying such data, researchers believe the high-pitched sound Crum and Roy recorded is not caused by the impact of the flakes but by vibrating bubbles created after the snow hits the water's surface.

They had already detected a similar phenomenon in rainfall back in the 1980s. Prosperetti, a wizard theorist on the relation between bubbles and sound fields, had teamed up with Crum to publish groundbreaking research on the role of rain-induced bubbles in underwater noise. Among other tests, they used a high-speed camera to capture the bubbles. This time around, Prosperetti analyzed the acoustic signature of the snowflake noise recorded in Virginia; he found a similar "footprint." In both cases, the signatures revealed the typical features of pulsating bubbles. "If it walks like a duck and quacks like a duck, it's a duck," says Prosperetti.

It's an odd duck at that. "We think it is a bubble, but how can a bubble be involved? A snowflake is mostly empty. It's 10 percent water and the rest is air," Prosperetti says of the mystery. "As a snowflake deposits itself, there is no impact essentially. Leisurely, bumm, bumm, bumm, it drops down." A serenely quiet scene by any standard.

"But what the layer of water engulfs is not solid ice, it is engulfing the air of which the snowflake is made," he adds. As water melts the ice, a bubble remains, researchers postulate. Water surface tension and pressure then would cause the bubble to pulsate. Those pulsations, or oscillations, create the sound. "It's like beating a drum," Crum says.

Prosperetti (pictured at right) adds, "It's a high-frequency sound. It would sound like a hissing noise if we could hear it, but we can't." The sound, ranging between 50 and 200 kilohertz, is too high for human ears (which can normally hear nothing higher than 20 kilohertz). Snowflake screeching, which was first recorded in the mid-1980s, adds 30 decibels to the underwater environment. "It's the difference between a private conversation and a rock band," Crum says. It's unclear just how much it disturbs underwater animals, though porpoises can hear sounds at high frequencies, he says.

However, the noise does wreak havoc with underwater sonar equipment.

Wildlife researchers using sonar devices to count salmon in the Pacific Northwest, and U.S. Naval submarine officers using sonar to detect enemy subs, dread storms because raindrops and snowflakes have the potential to create "background noise," which could interfere with a sub's torpedo detection. Concerns about sonar detection gaps led the Office of Naval Research to fund Prosperetti's and Crum's work on underwater rain noise. The researchers suggest one possible solution: change the frequency range of fish finders and similar sonar devices.

Their findings could lead to other applications. Scientists need to measure rainfall in the oceans, an important factor in studying worldwide climatology. But gathering such data is difficult; oceans are big. Yet researchers could analyze the signature of bubble noise picked up by remote sensors to determine rainfall (in short, louder sound means heavier rainfall).

The next research step in the snowflake study? Using high-speed cameras to visually record snow bubbles. "In principle, what one would like to see is a snowflake caught in the act," Prosperetti points out. Trouble is, there isn't much of a practical demand for that verification. "This stuff is nice, but it's not by accident that there's no money in it," the veteran researcher says.

In the end, the four-university study can't help but be a bit of science for science's sake. Says Crum (who among other things has climbed onto the roof of his lab on Christmas Eve to record snowfall sounds): "When scientists get around each other and talk about things, they don't talk about girls and cars. They talk about how to find the next data point."

--Joanne Cavanaugh Simpson