Why does snow crunch

You can hear your neighbors coming a mile off these days. That's why you hear squeaky snow only when it's cold outside. PH Your support matters. Support MPR News. Program Schedule Station Directory. Where they touch, they bond or weld together like a matrix.

These bonds are weak in fluffy snow and strongest in cold, dense snow. The crunchy sound occurs when the bonds between the ice grains in the snowpack break apart when you step on them.

A layer of snow is made up of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. 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.