Science of the Ski

How a Ski Glides across Snow

Extra Blue Conditions

Coined from the classic kick wax used in these conditions, Extra Blue weather is one of the most ideal places for a ski to glide because frictional forces are very low. The frictional forces that are present transfer some energy to the snow surface. This energy melts a very small portion of the snow pack and forms a thin film of water under the ski surface. The ski then glides on this film. The film is thin enough to insulate the ski base from the sharp snow crystals, but thin enough that capillary action does not create a large amount of drag.

Green Conditions

Green conditions are much different than blue conditions. They tend to be colder and or drier. In these conditions skis do not glide very fast, the frictional forces are much higher. In colder temperatures it takes much more energy to melt the thin water layer that occurs in blue conditions. In “green” snow the water layer under the ski is much thinner and so the snow crystals can rub against the ski base, creating dry friction. I was unable to find anything about why dry snow is slower, but I believe it is due to the low humidity. A water layer may be harder to form because as the snow melts the water is immediately sucked up by the dry snow around it.  

Yellow Conditions

Yellow condition snow is very warm and wet and has generally started to melt. In this scenario the water layer under the ski base is very easy to form, the snow doesn’t take much energy to melt and there may already be lots of melted water in the snow. In fact the layer is so easy to form that it becomes “too” thick. The thickness causes capillary forces to increase significantly, which creates a large drag often referred to as a suction feeling.