What is the friction between skis and snow?

There are several individual friction forces that contribute to the total friction between skis and snow; some of them are attributed to the skis, others to the snow.

What is low sliding friction?

The low sliding friction between skis and snow can be explained by a thin lubricating water film that is generated during skiing. This very thin water film is essential for sliding to occur.

The water film is generated by:

• The free water content in the snow (the ambient interstitial snow moisture)
• The percolation of snow moisture to the sliding interface due to snow compaction
• Meltwater due to high local pressures
• Meltwater due to frictional heat.

At low temperatures, the generated water film has a positive effect on the sliding friction since it separates the two surfaces, i.e., the snow and the skis. At temperatures close to 0°C and when the liquid water content in the snow is high, the water film may instead increase the sliding friction due to suction or viscous drag.

What about the snow?

When it comes to snow, there are different mechanisms that dominate during different weather and snow conditions. If it is very cold outside, the lubricating water film might be insufficient or there might even be an absence of a water film. In this case, dry friction might dominate which means that friction arises from elastic and plastic deformation of the surfaces and form very thin lubricating water films exhibiting high friction due to shearing.

Wet friction dominates the friction mechanism for wet snow if the thickness of the water film becomes too large. Capillary attachments will then increase the drag on the skis.

Another factor that may contribute to the total friction is dirt in the snow or on the skis, which increases the friction. In competitive cross-country skiing, an understanding of the dominating friction mechanisms is essential to choose the optimal skis, texture, and ski wax.