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I've been quite stunned during the last Winter Olympics in Sochi by the following fact.

Temperatures were quite high, sometimes 10° Celsius, and the organizers had to do something to prevent the snow from melting. I heard on the TV the commentators saying that they were discharging salt on the tracks.

However where I live (France), we have occasionnally snow in the winter, and to keep the roads clear for the vehicles or the sidewalks clear for the pedestrians, city services also discharge salt !

How is it possible to have these two different effects with the (seemingly) same component ?

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    $\begingroup$ Salt kinda covers ice. In Olympics, salt was used to cover and shield ice from high temperature. In your area, France, they use salt to shield it from further low temperature resulting in formation of more ice... Btw, IMO, you should ask this on Physics.SE (I might be wrong at this point so I am not flagging as off-topic...) $\endgroup$ – user3459110 May 2 '14 at 14:41
  • $\begingroup$ @Awal I do believe it is the right place to ask this question - I do not have an answer, but I would also like to know. $\endgroup$ – Martin - マーチン May 2 '14 at 14:56
  • $\begingroup$ There are different kind of salts and interactions with water. Salt is a really generic term. When using salt on highway the objective is to lower the melting point of water in order to keep it liquid instead of solid. In Belgium where we use CaCl2 and it only work above -4°C. Bellow, salty water freeze. Do you know what kind of salt has been used in Sochi? $\endgroup$ – jlandercy May 4 '14 at 19:07
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    $\begingroup$ @bela83 Wouldn't it also be possible for the snow to melt then refreeze over night forming a layer of ice which is then able to be used by the skiers? NY Times article, nytimes.com/2014/02/17/sports/olympics/… $\endgroup$ – Jun-Goo Kwak May 5 '14 at 0:23
  • $\begingroup$ @jlandercy Totally agree that salt is a generic term. I don't know though what kind of salt has been used in Sochi. Sports commentators are seldom keen on sciences ! The point on CaCl2 is interesting. I would really enjoy an answer explaining this phenomenon. $\endgroup$ – bela83 May 5 '14 at 13:35
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This article and this article (posted by @Jun-Goo Kwak in the comments above) say that the salt was used to lower the melting point of the snow on the surface, which would cause it melt. It would then re-freeze (I am assuming either at night or due to being in contact with snow beneath it), forming an icy layer on the surface. I don't know anything about Alpine skiing, but it seems that an icy surface is better than a slushy surface.

The ice would also form an insulating layer on the surface, which might help keep more snow from melting. Ice is a better conductor of heat than snow is (snow contains a lot of air, which is a terrible thermal conductor), so it is unlikely that it would act as an effective conductive insulator. However, it would shield the sub-surface snow from convective heat transfer caused by the movement of air. This is how igloos and snow caves work.

There could also be an effect due to the phase transition of melting - when ice melts, it is an endothermic transition, which means it absorbs heat. Some of that heat comes from the air, but some of it will also come from the snow directly below the surface. The result could be that the snow melts more slowly than it would if it were not covered by a layer of ice. The reverse of this effect is the reason that spraying an orange crop with water right before a hard freeze can save the crop (pg. 3) - as the water freezes, it releases heat (freezing is exothermic) and some of that heat goes into the orange trees.

It is hard to guess which effect is most important (insulation vs. endothermic heat transfer), but I would say that both should be considered.

So, to answer your question, in both cases the idea is to melt snow and/or ice, but in the case of roads, we want the ice to melt immediately and flow away, so we use lots of salt. In the case of Alpine ski events, we want a little snow to melt and then re-freeze, so we use a little salt.

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