Why Does Snow Squeak More When It's Colder?

Snow is a mass of tiny ice grains. According to the University of Wisconsin–Madison's weather guide (WxWise), when snow is warmer than about −10°C, the pressure of your foot slightly melts the surface of the grains. The snow then flows smoothly under your boot, so it makes almost no sound.

Grains with a thin film of water are slippery and don't catch on each other. The closer to 0°C, the more the sound turns slushy rather than squeaky.

WxWise explains that below about −10°C, pressure doesn't melt the snow. Instead the ice crystals under your boot are crushed, and that makes the squeak. The explanation from University of Wisconsin researchers reported by the British newspaper The Guardian is similar: as the temperature drops, grains stick to each other more, friction grows, and the sound gets louder.

W. Craig Carter, a materials professor at MIT in the United States, points to a process called sintering. Grains bond lightly, forming small ice connections, and when you step down, thousands of those connections snap at the same moment. Imagine the sound of running a finger along the teeth of a comb, split into tiny pieces that all happen at once.

The sound snow makes hasn't been studied thoroughly. Some explanations put the pressure-melting effect first, and others put the process of grains bonding and breaking first. The Guardian also reports that the sound varies with snow depth, the surface underneath, and how long the snow has been on the ground. So −10°C is not a sharp line.

What's certain is that the same snowy path sounds different depending on the temperature. Next time you walk on snow, listen for whether it's slushy or squeaky, and you can judge by ear how cold today is.