Acoustic Properties of Snow

When people think of snow, chances are they think about how it feels (cold and wet) or how it looks (the white expanse of a picturesque Christmas card), but they probably don’t give much thought to its acoustic properties. How does snow sound, and how does it affect other sounds? Meteorological researchers have made many interesting discoveries:

* Walking on snow sometimes produces a distinct crunching or occasionally squeaking sound. But not all the time. Why does snow sometimes make that crunching sound, while other times it’s as silent as if one were walking across a field of cotton balls?

A layer of snow is mostly empty space with tiny ice grains far apart in that emptiness. When something compresses the snow, such as your boot landing on it as you take a step, ice grains are pressed against each other. If the grains are melting and wet, they make little or no sound. If they are hard and frozen, the friction as they strike each other causes the familiar crunching or squeaking. Thus the colder the snow is, the more likely it is to be noisy.

* When scientists have measured the (obviously very slight) sound that is transmitted through water when a snowflake lands on the surface of a body of water, they have determined that it is distinctly different from the sound made by any other form of precipitation. On the other hand, though there are many different types of snowflake, the sounds they make when they land on the water vary hardly at all.

* When everything outside is covered by a blanket of snow, there’s often a kind of hush over the area, a peaceful quiet.

This is accounted for in large part simply by the fact that fewer people are out and about generating noise after a heavy snow. There are fewer cars, voices, footsteps, cell phones going off, etc.

But that doesn’t account for all of it. Even if those factors are held constant, things will be a bit quieter in the snowy scenario. Why?

It evidently doesn’t have to do with falling snow in the air, because the difference remains when it has stopped snowing and all the snow is on the ground. Nor is it a matter of sound not traveling as well in the cold winter air; in fact, there is often a temperature inversion on a cold day, where a layer of cold air is directly beneath a layer of warm air, and those conditions actually slightly facilitate the movement of sound waves through the air.

The explanation turns out to be twofold. One, less sound is generated by something that has snow on it. Two, more importantly, a layer of snow on the ground can absorb a substantial amount of existing sound. Only a fraction of the sound that would otherwise be reflected by the ground or the street will be reflected when it’s all snow covered. This is especially true when the snow is soft. The harder and more frozen it is, the more sound it will reflect.

In these and other areas, scientists continue to learn more about snow’s acoustic properties.