Mid-winter Mystery: Why Does Some Snow Squeak?
Snow has its own vocabulary. For reasons not fully understood it even squeaks when you walk on it. Here's one theory.
There's a reason the U.S. Northeast invented Groundhog's Day, an unofficial holiday on February 2 every year fixated on snowy weather. During an average winter, that's when the maximum number of people in that region look at the graying mounds of stuff and think an eternity in hell might not be so bad.
These people need a distraction, and talking about a basketball-sized rodent and its shadow is all that their cabin fever addled minds could come up with.
In the interest of saving their minds and reminding people in snow belts everywhere just how miraculous tiny falling ice doilies are, here's something else to occupy one's mind at this time of year.
Clomping through fresh snow has a changing soundtrack. It crunches, it ploshes, it shhes. But every so often, the snow squeaks when you walk through it.
And it's a unique squeak. It sounds nothing like a mouse shout (which is sad and horrible, if you ask me), nor does it sound like a window being cleaned (a happy and goofy sound that should be played when the circus clowns run into the ring).
It is as fragile sounding as snowflakes themselves, which leads me to this question: What's with that, snow? Why do you squeak?
As it turns out, no one knows for sure, and exceedingly few researchers have even considered the mystery. As one physicist told me, "They aren't handing out Nobels for unraveling the sounds of snow."
Fair point, but shouldn't ski-resort grooming crews have made a connection by now between the squeak and some kind of environmental condition?
What about road crews? Do snow sounds offer some hint about driving conditions?
How about hunters? Hunters must have considered this phenomenon.
Answering in the order asked: No. No. No. I spoke to people in all three activities, and everybody acted like I was the crazy one.
Here's what we know about snow. Flakes aren't frozen rain (that's called freezing rain). A snowflake forms when water vapor (a gas) in cold clouds hits a speck of dust, and goes directly to a frozen, crystallized form, and then falls.
Flakes take the shape of well-known six-sided dendrites (which rarely make it to your tongue undamaged), chaotic forms, needles and pellets among others.
Heavy pellets, or graupels, make the best snowballs because they pack together so well. And six inches of graupels make the classic crump sound when you walk on it.
No one's exactly sure what causes the crumping, but one theory, put forward by Prof. Craig Carter, a materials scientist with the Massachusetts Institute of Technology, holds that that noise might be flakes sliding past each other on minuscule films of water as well as the sound of wet snow being compacted.
Carter works on the application of theoretical and computational materials science to microstructural evolution and the relationships between material properties and microstructure. So, not like talking to the guy ahead of you at Subway.
Fluffy snow — mostly drier dendrites that collide and freeze together on the way down — gives up the beautiful notes, he said, but not really fresh fluffy snow.
Carter said that fresh, fluffy snow in fairly cold air sounds like a quiet "huff" when it's stepped on probably because individual dry flakes are surrounded by a lot of air and only touch one another loosely. "Huff" is the sound of escaping air and snow crystals being squashed flat.
To squeak, he has hypothesized, the snow first has to lie undisturbed for a few hours. As light as the snow is, it begins to collapse a little under it's own weight, and the crystals crowd each other. All this results in energy released as heat, partially melting the flakes.
Carter said little weld-like connections, which resemble tiny necks, form between flakes. The necks stretch maybe 200 nanometers, or about the length fingernails grow in three minutes, and stop the snow from collapsing any more under its own weight.
"I think it's the necks that are squeaking," he said. They are breaking sequentially, top to bottom as you step on the snow.
The squeak, then, is the sound of millions of icy necks snapping, depending on the size of one's foot. Each neck might break with the tinging sound heard when breaking an icicle. Collectively, they squeak.
Assuming this is why some snow squeaks, I asked Carter if there was any practical application for the information. Could any sound made by snow maybe warn of an avalanche, for instance?
He again prefaced his answer by saying he was not an expert in this area.
"I don't think the sound (that snow makes as it is compressed) is likely to give a warning of an avalanche," he said. And besides, "any initial sound would likely be microseconds prior to the avalanche," making a warning impossible.
It would appear that squeaky snow has no practical use for humans. In that way, the snow is like rainbows. Pretty nice company for a weather mystery.