Why snow squeaks below about -10C
Walk across fresh snow at -2C and it is nearly silent. Walk across the same snow at -15C and every step announces itself. The difference is not in your boots.
Snow grains at temperatures near melting carry a thin film of liquid water on their surfaces. That film lubricates. When a boot presses down, grains slide past one another and rearrange quietly. Take the temperature down and the film thins until it is effectively gone. Now the grains cannot slide. They resist, then fracture, and thousands of tiny fractures per second reach the ear as a squeak.
The transition is not sharp, which is part of what makes it a useful field observation rather than a thermometer. Humidity, grain age and load all shift it. Old, rounded grains that have been through a few melt cycles squeak at colder temperatures than fresh dendrites. But the rough rule holds: if the snow is loudly complaining underfoot, it is likely below -10C, and if you are hearing a low crunch rather than a high squeak, the grains are probably large and old.
I have started noting the sound alongside the temperature whenever I remember to. Over two winters the correlation is good enough to be interesting and bad enough to be honest.