It’s 4 o’clock on Christmas Eve 2003. We are walking on the completely snow-covered narrow road along the edge of the Nordic pine trees. The sun has set, stars are already glittering in the sky, like only stars can glitter in the deep dark forests of Värmland, some hundred kilometers east of the Swedish-Norwegian border. It’s cold, -16 oC. Every step we take gives off a familiar dry crunching sound, the cozy sound of walking in the snow during a cold starlit winter night.
Silent steps
At the same hour the next day, Christmas Day, a low pressure from the Atlantic arrives. It’s not as cold any more, maybe –7 oC. Again, we take a walk. But now nothing is the same. Clouds obscure the stars, the previously crisp definition of snow-covered trees against the sky has become fuzzy and low in contrast. As if everything were wrapped in a thin layer of second-best cotton. More importantly, the cozy crunching sound of our footsteps is gone.
What has happened to the crunching sound? What is the physics of walking in the snow?
Three physical factors
There are two –- no, actually three –- physical factors affecting the crunching / noncrunching of trodden snow. The mechanism behind all three is the same –- 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 in all cases, coming from two sources, both of which are temperature-dependent:
(1) Ice crystals are always surrounded by a very thin layer of water (a phenomenon already observed by Michael Faraday). The thickness of this layer varies with temperature, ranging from a one molecule thick layer at about –10 oC, to hundreds of monomolecular layers at –1 oC.
(2) Pressure lowers the melting point of water. If you step on snow, then the crystals are pressed against each other. The ice at the contact points may melt and create a thin lubricating layer of water. Unfortunately, the pressure from the soles of your shoes is far to small to melt snow at any temperature, so this factor, interesting as it my seem in itself, is rather irrelevant in this connection.
(3) The third factor is the shape of the ice / snow crystals: crystals with a greater number of pointed edges crunches more readily. An extremely pointed structure of the snow crystals can sometimes offset the other factors, making snow crunch even when it is warmer than -10 oC.
It is difficult to say how these phenomena interact in order to lubricate (or not lubricate) the snow crystals, but in any case something seems to be happening at around –10
oC, enough to make a sharply noticeable
difference: if it is colder than about –10
oC, then snow crunches, if it is warmer, then it usually doesn't.
Ten-degree rule of thumb
These factors, taken together, determine the precise temperature at which snow starts crunching. But the –10 oC rule is a surprisingly good rule of thumb, if you want to predict whether or not you will experience the nice crunching sound of snow when you take a walk at Christmastime.
Submitted to The Ninjagirls Christmas Special.