SHATTERING SNOWFLAKES
Postmedia’s Tom Spears looks at snow and ice and wildlife to see what makes our coldest season tick. Today, we look at the science behind the unique sound made by stepping on cold, packed snow.
In years of living in Manitoba, David Shoesmith got used to the different kinds of ice where his son played hockey, and different types of snow where his wife went crosscountry skiing.
Outdoor rinks behave nothing like the ice in a heated arena, he noticed. The deep cold makes the skating conditions different. And when the snow became extra cold, his wife complained her skis wouldn’t glide as easily.
Shoesmith is a materials scientist, now teaching chemistry at Western University in London, Ont. These differences got him thinking, and they led him to some conclusions about how snow behaves when we walk on it, and where that squeaking sound comes from — especially when you walk on snow that’s very cold.
The answer lies not only in snow as we think of it, but also in liquid water.
“What’s probably not commonly known or thought about is that snow does have a thin water layer” on its surface, he said. That amount varies with the temperature.
In warm temperatures, a person stepping on snow melts a little of it. This allows skis to glide on it.
Below about -10C there’s less moisture, which is why there’s less glide for skis. The snow crystals you move over are not able to slide around without that slick water layer.
Instead, “you tend to compact the crystal with a fracturing sound,” and that’s the squeaking you hear under your boots.
Or at least, that’s one theory. Although humans have been crunching and squeaking on snow for thousands of years, Shoesmith notes there are still competing ideas about where the distinctive sound comes from.
So he also floats a second theory: It isn’t individual snowflakes that shatter, but rather the “networks” they form after lying on the ground for a while.
Snowflakes don’t keep that delicate shape of Christmas tree ornaments when they lie around for a few days — especially when someone packs the snow by stepping on it, or shovelling it.
“When that stuff comes down you could almost blow it around because it’s so loose, which means there’s a lot of air in it. The density of the snow is very low,” Shoesmith said. “When you come out the following morning, you try doing it again. You can’t.” The snow has settled.
And if the snowplow leaves a heap of snow at the end of a driveway, it is packed and dense.
“There is a theory that says the freshly fallen snow will melt a little bit, then of course when it freezes it will interconnect these crystals (snowflakes) so that you start to form a little network.”
In this theory of the squeak, “you get this effect at higher temperatures, and then as you go down in temperature and put your foot on it (i.e. snow), you fracture all these little interconnects that have formed because the snow melted slightly. You form this little network that will crunch when you put your foot on it.”
So there it is. The sound of boots on packed snow is the sound of shattering snowflakes, or maybe frozen networks of snowflakes.