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A symphony of snow
Kelly Foss
www.snowcrystals.com

If a snowflake falls in the ocean and no one is around to hear it, does it make a sound? Dr. Len Zedel has proof that it does… most of the time anyway. 

A professor in the Department of Physics and Physical Oceanography, Dr. Zedel has been collecting underwater sounds generated by snowflakes as they pitch on water. He has hypothesized that the resulting noise may provide data that can be used to estimate snowfall amounts.

Oceanographers have given a great deal of consideration to ambient ocean sound, especially as it relates to interference with acoustic ocean technology. Only over the last 20 years have oceanographers and meteorologists given special attention to the underwater sounds generated by precipitation.

It is well-known within the field that rainfall rates can be estimated by analyzing the underwater noise it generates. “Raindrops are really neat,” Dr. Zedel explains, noting that different rainfall processes generate different effects. “If the drops are sufficiently small, like mist, you don’t get any noise. Large raindrops generate sound caused by the direct impact of the rain on the water surface. Intermediate size raindrops introduce bubbles into the surface of the water on impact. That bubble resonates and that’s what you hear.”

Those sounds also vary depending on how the raindrop strikes the surface of the water; how hard the wind is blowing; and how large or frequent the wave pattern. But overall, rainfall is proportional to the sound level. However, up until now, there has not been a comprehensive analysis for the comparatively complex ambient “music” generated by snowfall. Dr. Zedel expects to change that with his research by determining whether or not the same information collected for rainfall could be gathered from various snowfall types and intensities.

“There was a preliminary paper published by some people down south and we thought ‘What do they know about snowflakes?’” he jokes. But what really stood out for his team in the earlier study was the researchers’ failure to distinguish between snowflake types. “Anyone living here in Newfoundland and Labrador knows there are a lot of different kinds of snowflakes.”

In fact, the international snow classification system for solid precipitation recognizes 10 types of snowflakes—plate, stellar, column, needles, spatial dendrite, capped column, irregular, graupel, ice pellet and hail. And there is a more comprehensive scheme that identifies as many as 80 different snow classifications. Enough to generate a symphony of sounds.

“The sound levels of snowfalls are very quiet, much quieter than rainfall rates,” explains Dr. Zedel. “If you listen to snowfall sounds it’s kind of like a hiss, a ‘shhh’ kind of sound. It’s not as striking as rain which sounds like the tinkling of a chandelier, a very beautiful sound.” During the study they also collected freezing rain data, and “it’s just overwhelmingly loud by comparison,” he says.

“In our study we wanted to make observations in a field setting, but the opportunity never arose,” explains Dr. Zedel. His graduate student, Tahani Alsarayreh, had the idea to set out a tank filled with fresh water and equipped with sensors to see what data they could capture. For one winter, whenever it was snowing, she would roll the tank out of the loading bay door in the Chemistry-Physics building into the parking lot and collect data. “We didn’t know what we were looking for, but we thought we would just make observations to see what we would get,” he admits.

While there were issues with background traffic and other noise, Dr. Zedel says that they discovered that the sound level generated by snowflakes was proportional to the snowfall rate. They observed that while the more solid forms of snowflakes made noise, the lighter structures did not. They also were able to determine that the snow types that made no noise were associated with periods of low precipitation. However Dr. Zedel believes it is possible that, if lighter structures were observed during heavier snowfall conditions, a recognizable signal might have been observed.

So exactly how difficult is it to measure snowfall rates at sea? Dr. Zedel says it’s actually quite difficult. “You can’t catch it in a tank and automated equipment eventually gets covered in snow and then what?” he asks. For their study they used an optical rain measurement device to reference snowfall rates. “But even they are suspect when it’s really windy because the snow and rain are going horizontally and not vertically, so you can’t actually measure the rate at which it’s coming down.”

Dr. Zedel says measuring sound is much easier, but quantifying it is a bit of an art. He’s hoping in the next couple of years to be able to put a hydrophone out in a bay to measure snowfall sounds, along with a weather station that measures snowfall and correlate the data. This should give them the information they need to determine how well this way of measuring snowfall works in the marine environment. If the results warrant it, he plans to try the hydrophone in a more exposed ocean situation, open to the dynamics of wave action, to see how it performs there.

According to the professor, there is an increasing need for this kind of observation and this method might be one way of meeting the demand. “Accurate information about snowfall amounts at sea can benefit ocean modeling and meteorology,” he says. “In terms of understanding ocean circulation, one of the factors is how much fresh water gets put into the ocean. That’s really hard to measure and the more accurate the ocean models become, the more accurate the weather prediction can be, because the weather depends on what the ocean is doing,” says Dr. Zedel. “This information helps generate insight into climatic processes at both regional and global scales.”

Jan 21st, 2011

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