Why Does Snow Squeak?

Here in the Northeast, it has been cold. Mighty cold. Earlier this week, the thermometer dropped to -23 degrees Fahrenheit (that's -730 Kelvin, for our readers outside the US). On top of which, some parts of the region are now covered in as much snow as usually falls all year. In my neighborhood, where the past few winters have been relatively mild, we've got snowbanks as high as my shoulder.

So I was not surprised when I stepped outside and heard this:

[youtube]http://www.youtube.com/watch?v=fRrxunfr8Aw[/youtube]

In my hardy youth, I spent three winters attending university in Montreal, Canada, where wind chills of -40 (Fahrenheit AND Celsius!) were not uncommon. Squeaking across the snow from my porch to my car this week took my back to those brisk, glorious days when the sky was crystal clear (because it was just too damn cold to snow, or even cloud up).

But now that I am a GeekMom who looks at everything as a science moment, the question occurred to me: why does snow squeak?

It seems obvious that the extreme cold would have something to do with it. According to the Cooperative Institute For Meteorological Satellite Studies at the University of Wisconsin-Madison:

If the temperature of the snow is warmer than approximately 14F (-10C), then the pressure exerted by your boot partially melts the snow allowing it to 'flow' under your boot and no sound is made. When the snow is colder than -10C, the pressure from your boot does not melt the snow, and instead the ice crystals beneath your boot are crushed making a squeaking, or creaking, sound.

Not every scientist buys that explanation, however. As physicist Stephanie Chasteen writes on her blog sciencegeekgirl:

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.

Stephanie found another explanation which more closely fit her own observation:

The mechanism behind [this is] 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 ...

So what's the real reason for squeaking snow? Apparently, even the power of Google is not enough to settle this question -- yet. But one thing scientists and weather geeks do seem to agree on: The magic temperature at which snow starts to squeak is -10 Celsius, or around 14 degrees ABOVE zero Fahrenheit.

Which means I could have been having this pleasant science moment in a lot less frigid conditions.

Now -- recalling another memorable phenomena from my days up North -- I wonder how cold it has to be for your nostrils to freeze shut...