I’m cross-posting this from a fun little discussion we just had over at Morning Coffee Physics. (Perhaps ironically, all my posts took place in the wee hours of the night, sans coffee). Jasper wrote a really neat little post about why snow sparkles and I asked him if he knows why snow crunches underfoot when it’s cold.  It’s been really cold (really cold, whimpers this recent transplant from San Francisco) in Boulder lately and my tires and feet have been making cacophonous sounds in the snow.  I always had this sense that maybe it was because of snow crystals rubbing together.

Jasper wrote:

I did a bit of googling. From the few (sketchy) sources I saw, it looks like the crunch sound comes from the sudden release of air from the air pockets in a pile of snow. That I can believe, however, one explanation includes the following:

“When you walk on snow, your boots apply pressure. If the snow is warmer than about 14 degrees F (-10 degrees C), the pressure partly melts the snow, which “flows” under your boot instead of breaking. If the snow is colder, it does not melt, and your boot crushes those innocent ice crystals, accounting for that plaintive scrunching sound.”

As elegant as that explanation sounds, I suspect it won’t really add up… (literally even). It sounds a lot like the physicist’s myth of ice skating being explained by a similar process (Pressure from skate -> melting ice -> sliding). In one of my classes we did this calculation and it turned out that the freezing point of ice under a skate would only change by about 1 degree maximum. I suspect something similar for the preceding claim about crunching snow.

Going on physics intuition alone… I’d probably say the temperature dependence of the squeakyness of snow has more to do with the temperature dependence of the structure of the snowflakes. Maybe the shapes that snow crystals take on at low temperatures are better at making noisy air pockets… * shrug *

I shared his skepticism of the online explanation that he found.   It seems implausible that crushing would create that sound, but maybe my experience misleads me. It just seems like most of the sound is coming from the sides of my shoes in the snow, creating friction, rather than from my shoe coming down.  If I step straight down, rather than grinding my foot sideways into the snow, it is quieter.  But I also don’t think that a “different shaped snow crystal” explanation works for me, since the snow has already fallen to the ground and thus its crystal shape is already determined.  Once it’s on the ground, it crunches when you walk on it if it’s really cold, and doesn’t if it’s not.

After I wrote all that, I found a good link that seems to support what I just wrote (don’t you love it when that happens), and also incorporates the idea of different shaped snow crystals, but not in a temperature dependent way.

There are two — no, actually three — physical factors affecting the crunching / noncrunching of trodden snow. The mechanism behind all three is the same — lubrication, good or bad. When snow does NOT crunch, then the grains / crystals in the snow are well lubricated. When snow DOES crunch, then lubrication is poor. The lubricant is of course water in all cases, coming from two sources, both of which are temperature-dependent:

(1) Ice crystals are always surrounded by a very thin layer of water (a phenomenon already observed by Michael Faraday). The thickness of this layer varies with temperature, ranging from a one molecule thick layer at about -10 oC, to hundreds of monomolecular layers at -1 oC.

(2) Pressure lowers the melting point of water. If you step on snow, then the crystals are pressed against each other. The ice at the contact points may melt and create a thin lubricating layer of water. Unfortunately, the pressure from the soles of your shoes is far to small to melt snow at any temperature, so this factor, interesting as it my seem in itself, is rather irrelevant in this connection.

(3) The third factor is the shape of the ice / snow crystals: crystals with a greater number of pointed edges crunches more readily. An extremely pointed structure of the snow crystals can sometimes offset the other factors, making snow crunch even when it is warmer than -10 oC.
It is difficult to say how these phenomena interact in order to lubricate (or not lubricate) the snow crystals, but in any case something seems to be happening at around -10 oC, enough to make a sharply noticeable difference: if it is colder than about -10 oC, then snow crunches, if it is warmer, then it usually doesn’t.

Ten-degree rule of thumb

These factors, taken together, determine the precise temperature at which snow starts crunching. But the -10 oC rule is a surprisingly good rule of thumb, if you want to predict whether or not you will experience the nice crunching sound of snow when you take a walk at Christmastime.

But if anyone knows something more, please let us know!