Despite the mid-March thaw this week, this past winter will long be remembered for record snowfall across much of the U.S. Weeks of snowstorms, bitter cold, dangerous commutes and backbreaking shoveling sessions can can harden even the season’s most avid fans.
 
Still, for anyone standing in a gentle snowfall for the first time, watching the air turn into little bits of fluffy ice, the experience can be magical. And if the snow falls through particularly cold air, which holds less moisture than does warmer air, you will often hear tiny squeaks walking (or driving) through it. Much about snow formation is known science, but that last part—the squeak—is little studied and less understood.
 
Snow forms when supercooled water droplets collide with specks of dust suspended in the air and freeze. They can take the shape of everything from classic six-sided dendrites and needles to pellets and chaotic crystals. It is actually rare for a pristine dendrite to make the trip from the cloud to your tongue unaltered. More likely, flakes will hit other supercooled droplets to form heavier, hard-packed pellets called graupels. They make the best snowballs, but stepping on this denser snow usually creates a crumpling sound. The fluffiest snow is mostly aggregate—low-moisture dendrites that run into one another and freeze together. It is aggregate that offers up the pleasing squeak.
 
To get an idea why this might be, Scientific American spoke with W. Craig Carter, a materials scientist at Massachusetts Institute of Technology who has spent some time contemplating the squeaks of winter. Carter's lab works to produce predictive models for complex, micro- and nanoscale behavior of materials such as those in batteries. He has also collaborated on several art projects that incorporate aspects of materials science, natural design and mythology.
 
[An edited transcript of the interview follows.]
 
Not all dry snow squeaks. Why?
Freshly fallen dry snow goes "whoof" when you step on it. The snow has to sit awhile in cold conditions and collapse slightly under its own weight before it will squeak.
 
Fresh snow is just sitting there, with flakes loosely touching. Something has to happen to the snow, and I think that something is the collapse [when the snow melts minutely and refreezes]. Tiny bonds—they're like welds a couple-hundred nanometers in size—form between each flake. You know how a bucket of ice cubes in your freezer eventually becomes one big piece of ice? The cubes are bonding together in a process called sintering, and I think that's what is happening on a nanoscale with the snow. Sintering—the growth of these tiny necks between ice crystals—takes place over a matter of hours. The necks support the weight of the snow and stop it from collapsing any further under its own weight.
 
The necks squeak when they're broken?
I think so, yes. And they are breaking sequentially, from the top of the snow to the point where your foot stops sinking because you've compacted it. With wet snow, it's one of two things. The bonds might be more liquid or there also might be more water generally to lubricate the flakes. Stepping on that gives you the classic crumpling sound.
 
So the noise we hear is the sound of the tiny bonds cracking?
Yes, I think you have it: tiny bonds cracking. At larger scales, it would sound like compressing an old, stale Rice Krispies treat—the bonds being sugar bridges, perhaps. At larger scales, it might sound a bit like the crunch you hear as the garbage truck compresses the trash.
 
Does the squeaking happen only with classic six-sided flakes?
My hunch is that larger-pellet snow wouldn’t squeak so much as crunch. I believe the squeak depends on the fallen snow forming the welds to sinter together. These small particles can form from either [denser] flakes or aggregate flakes, either by partial melting and refreezing or as a morphological change that occurs just below the water’s melting point. If one were to look at snow that squeaks, it would look like a Rice Krispies treat, but at a very small scale.
 
What drew you to this specific question? Do you ski or maybe you've just suffered one too many Northeast winters?
It is just one of those things that I’ve observed, and it struck my curiosity. I did think about it on a recent weekend’s long walk with my dog. I like that squeaking sound very much—but I’ll be happy to see spring arrive soon.
 
How satisfied are you with your theory?
In my mind it's still just in idea. I can't poke any holes in it but I wouldn't say I have tremendous confidence in it. If I sat down and did some back-of-the-envelope numbers, I might be more confident.
 
Is there an envelope and pencil on your desk?
[laughing] It's one of those things. It's a question of time and a cost-benefit analysis. And no one is handing out Nobel's for solving the squeak in snow.
 
This is a fun topic, but I can't help but think there might be some practical applications for the information, maybe in materials science?
Yes, there are many similar phenomena in natural and technical materials. Most of the practical ones would have to do with the sintering process involved in making ceramics. The fundamental idea is that capillarity [the rise or depression of a liquid in small areas such as the space between fibers or small tubes] dominates at the smallest scales. Capillarity can cause things to stick together—and it can also cause an isolated particle to change its shape so as to reduce its surface energy.