The acoustics of snow

As I headed back up the walk this morning, after dropping my older son off at school, I listened to the crunch of the snow under my shoes. I don’t generally associate snow with crunchiness – except when it has developed a crust on the top. The other day, I dropped something in the snow and it made no sound. Then when I reached over to retrieve it, I fell in the snow also. I wasn’t really paying attention to the sound of the impact, as I was more concerned with how cold and wet it felt. But the snow seemed to swallow any sound that was made.

That’s usually how it is when there’s a blanket of snow on the ground. Not only do the snowflakes themselves fall silently, they seem to absorb whatever other sounds might be made nearby. I wondered if it was my imagination, or if it’s just that lots of the usual sources of noise stay inside when it’s snowy (other than children). But I found a website devoted to snow research that confirms what I thought.

The pores in the snow cover are responsible for the quiet conditions. When acoustic waves travel horizontally above the snow, the increased pressure of the wave momentarily pushes some air into the pores. This air returns to the atmosphere after the wave passes, but some energy has been lost from friction and thermal effects. Over a short distance, this mechanism can significantly reduce the sound energy in the acoustic wave.

So if snow tends to dampen ambient sound, why does it make so much noise when I walk on it? It doesn’t always do that, of course. I noticed that the noisy snow this morning was a pretty thin layer, and that there was ice underneath in at least some spots. Was it the snow being pressed against the ice that made the scrunchy sounds? I know I’ve heard snow crunch in some conditions, and not in others, but had really never paid close attention to what factors affected the sound. Today I decided it would be a good subject for a blog post.

I suppose I could try to set up some experiments myself, comparing the sound of walking on snow in different places, wearing different kinds of footwear, at different temperatures and different depths of snow. But I don’t own a decibel-meter (when my older son’s science fair experiment required one several years ago, I was able to borrow one from the company I worked for at the time). And while I like walking in the snow, I prefer to enjoy it as an aesthetic experience rather than having to control and record all the different variables.

So I prefer to sit in a heated office and see what I can find out online. This aspect of snow acoustics doesn’t seem to have attracted as much attention as the sound attenuation effects. I found one page which suggests that sound travels better in a snowy landscape, which doesn’t seem to match what I read previously.

Any winter hiker can tell you, the acoustics of snow amplify sound, like a whale’s song in the ocean. Noise seems to travel farther and sound crisper. As I walked, I heard with perfect clarity the squeaky crunch of snow beneath my footfalls.

I wonder if maybe the snow does dampen the sound, but other conditions more than make up for that effect. Hikers generally walk in wooded areas, which means that most of the year there are a lot of leaves on the trees. I suspect that the leaves absorb a good deal of sound. Plus they make their own sounds when the breeze blows through them, and they provide shelter for a great many woodland creatures that add their own sounds. One thing I enjoy about hiking is the concerto of bird calls, the rustle of leaves, and all those different sounds that insects make.

In the winter, most of that sound is absent, and a hiker is likely to hear little but the sound of his own footfalls. Perhaps also the atmospheric conditions are different in such a way that sound is transmitted better – but that’s another subject for another day. I’m still trying to find out what makes the snow crunch.

I found an acoustic engineer’s blog, and he also was wondering about what makes snow crunch. He didn’t find any research on the matter, but he does tell us that food researchers compare the crispness of food to the crunch of snow. Both have a lot of air trapped in a brittle framework. But why does snow crunch sometimes and not others?

A FAQ on snow provides some more information.

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. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures, closer to melting, this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It’s difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.

I kept looking. And I found someone (I link to a blog that quotes this person since the URL it links to no longer exists) who does identify the temperature at which snow starts to crunch. There are three factors involved: the thickness of the very thin layer of water surrounding the ice crystals (which varies with the temperature), pressure on the ice (but walking on snow doesn’t exert enough pressure to make a difference), and the shape (degree of pointiness) of the ice crystals. Apparently no detailed studies have been done to determine exactly how these factors work together to determine the degree of snow-crunchiness (otherwise perhaps there would be a more scientific term for it than crunchiness).

But there is a good rule of thumb about the temperature. “if it is colder than about -10°C, then snow crunches, if it is warmer, then it usually doesn’t.” (For those of us who haven’t learned to think in degrees Celsius, -10°C is 14°F.) So now I just need to look up the temperature history for this morning at wunderground.com. For some reason, however, there are is no hourly data available for my zip code from 4:55 PM Tuesday until 9:15 AM this morning. At 9:15 AM the temperature was 19.4°F, and a quick look at history from other days tells me that the temperature doesn’t usually change much between 7:30 AM and 9:15AM.

So maybe the snow on my walk was particularly pointy this morning. Maybe my part of town is a few degrees cooler than where the temperature readings are taken. (I do know that the temperature displayed on the sign at the credit union near our house is always a few degrees cooler than the one at the bank downtown, but I don’t know how much that has to do with differences in temperature and how much with the calibration of the two thermometers.)

Maybe I’ll have to try doing that snow-crunching research myself after all.

One Response to The acoustics of snow

  1. eagleon says:

    I am doubting that ice grains rubbing against each other produce as much noise as air being compressed and suddenly released by the breaking of the ice crystals that trap it.

    My guess is that there is some degree to which this effect is amplified by escaping air pushing against other loose pockets, an explosion as the foot comes down and forces a large-scale evacuation of the air in the matrix into the air above. The depth of the snow would dampen how much is allowed to escape to reach your ears. The shape of the ice crystals would affect the size and stability of the pockets of air. The temperature would affect the transition between liquid and solid under higher pressure – check it out http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Phase_diagram_of_water.svg/2000px-Phase_diagram_of_water.svg.png if the pressure is rising high enough at any point it could transition to water before -10 celsius, or 263 kelvin, whereas below that it becomes increasingly unlikely until you reach 251 K. And just a quick glance here http://en.wikipedia.org/wiki/Orders_of_magnitude_%28pressure%29 10 MPa for a stiletto heel, which is probably momentarily much greater within the confines of the brittle pressure tank of the snow.

    Examining the structure of crushed snow that has made a crunch should tell what fractures and terrible forces occurred as its fluffy universe collapsed 🙂 Though I could be talking out of my ass, I’m not any kind of scientist.

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