Why is melting snow slower than slush?

A snowHead

You may recall, as you've skied down the piste from cold snow into the dreaded 'spring conditions', that it can seem as if someone's suddenly operated the 'ski slowly' brakes and you almost fall over (if not prepared, or experienced enough to instantly recognise the symptoms), but then, further down, as you hit slushier snow, it's as if the brakes have been released again though the slush is still a bit slower than normal snow.

Apparently, the phenomenon is caused by the phase change of the surface snow from ice to water, which needs heat in order to make this change. Just as water will suck heat from your brow to evaporate (liquid to gas) giving you a cooling effect, so ice will suck heat from what ever is in contact with it to change into water (solid to liquid). However, ice can bizarrely extract this heat in the form of friction from whatever is sliding against it, thus the snow that is ready to melt (but hasn't quite yet) will appear to have a high degree of friction - upon such as a ski or snowboard. The thing is, the surface of the ski or board is at the same temperature as the snow, but gives up heat to it as friction (loss of kinetic energy).

The slush no longer has this effect because so much of it is already melted (water), and the friction effect of any ice to water phase change is insignificant compared to the lubrication of the water within the slush.

A snowHead

I am no expert, but I'm not massively convinced by that explanation, as the amount of water produced by friction with your skis is negligible compared to that already in the snow. It's far more likely to be down to the different compressibility of slush and melting snow, and the higher amount of pre-existing amount of water on the top of the base layer of slush (i.e. before your ski has melted it through friction). I'd be interested to read more about it, though.

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Surface tension.

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There is a certain type of snow, or whatever you want to call it that has a far higher level of friction than usual and it can catch you out skiing. I think it's more likely to be surface tension thing than the model implied.

Remember that ice is a unique substance in that is less dense than water which is quite a unique property, hence when you put pressure on it an compress it, it turns to the more natural denser state, water. This is why ice, as a solid appears to have very little friction, as opposed to other solid objects.

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I was told that slush strips the wax from your skis quite quickly

I suspect it's because the snow under the skis is at just that temperature/humidity level where there's a melt/freeze cycle going on and some of the melted snow is freezing back and sticking to the skis.

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Spring snow is a whole spectrum of things and it is only a particularly subset at a particular point of phase change which goes grabby/gluey - it's pretty much the same effect you get when fresh snow is rained on. Incidentally most of the stuff that tourists commonly call slush and unskiable is largely a matter of application of rule 5 and is easily addressed by a combination of skis, wax, attitude and pie-eating capacity and is usually actually delightful spring snow.

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Why is melting snow slower than slush?

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