The Science Of Snow And Skiing

Extratropical cyclone

A clockwise spinning extratropical cyclone shot by NASA

With the 2016 ski season in the Alps kicking in, we take a look at the science of snow and skiing that brings such a buzz for so many. With the spotlight on the Port Du Soleil, which is the biggest international ski area in the world with over 650 km of piste across France and Switzerland, we take a look at the variety of conditions.

Making snow

Snow is a granular material which is basically the falling of flakes of crystalline water from clouds. This kind of precipitation tends to form in regions of upward movement of air around a low pressure system known as a extratropical cyclone. Snow can fall poleward of these systems’ associated warm fronts and within their comma head precipitation patterns.

Lakes and snow

The good snow conditions around the region of Morzine and Avoriaz are influenced by Lake Leman (commonly called Lake Geneva) which lies 40 km to the north and can be seen from many of the upper slopes on clear days. Water evaporation from the lake is blown south towards the Port Due Soleil region where it connects with the cold cyclonic flow around the backside of extratropical cyclones to create heavy local snowfall.

Port Du Soleil

Lac Leman feeds the snowfall in the Port Du Soleil region

Snow flakes

Snow crystals form when tiny super-cooled cloud droplets (roughly 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C , because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. Then the droplet freezes around this “nucleus”.

Experiments show that this “homogeneous” nucleation of cloud droplets only occurs at temperatures lower than −35 °C . In warmer clouds an aerosol particle or “ice nucleus” must be present in (or in contact with) the droplet to act as a nucleus.

Cloud seeding

Ice nuclei are very rare compared to the cloud condensation nuclei on which liquid droplets form – clays, desert dust and biological particles are effective. Artificial nuclei include particles of silver iodide and dry ice, which are often used to stimulate precipitation through cloud seeding.

Snow types

When on the ground, snow is categorized as ‘powder’ when it is light and fluffy, ‘fresh’ when recent and heavier and ‘granular’ when it begins the cycle of melting and re-freezing. You will experience allot of the granular material during the Easter ski season in Europe on the back of the warm days and freezing nights in the mountain. When powdery snow moves with the wind from the location where it originally landed, forming deposits or snowdrifts that often have depths of several metres. After attaching itself to hillsides, this blown snow can evolve into a snow slab which can form a significant avalanche risk on steep slopes. When a snowpack like this stays on the ground throughout the year for several years it evolves into a glacier whereby the pack turns to ice.

Virgin fresh powder snow

Virgin fresh powder snow in Avoriaz

Snow trends

December is a critical time for ski resorts as it is a time when the snow base is typically built for the season. Looking at December 2014 records for the Alps , the results demonstrate the snowfall is connected with the conditions for production and not simply the coldest weather spot, as Morzine and Avoriaz actually benefit from the warmer precipitation.

snow records

Ski history

Skiing has been around for over 1000’s of years. After the last ice age, Stone Age hunters began strapping long pieces of wood to their feet to travel farther and faster over snow in pursuit of the game that flourished across Europe and Asia. Identified through early cave art, the earliest remaining skis come from Scandanavia where they were preserved in permafrost for 2500 years. Although todays modern skiing evolved from Scandinavia, it was practiced as early as 600 BC in what is now China.

History of Skis

Ski styles across the ages

Skiing mechanics

The reason that skis slide is that the temperature at which water freezes, changes with pressure. As pressure increases the temperature that water freezes at becomes lower. So quite simply the pressure a ski puts on the snow, makes the snow directly underneath it melt which creates a very thin layer of water between the skis and the snow. The water reduces the friction, making the ski slide very easily. The more pressure applied the more snow under the ski will melt. The diagram illustrates the thin layer of snow that melts, and how the force from the ski spreads out as it goes through the snow.

ski pressure

Skis slide when we are on a slope because the reaction force from the snow (R), is at a different angle to the force from our weight. This means there is a component of the force created by our weight in the direction of the slope (FM) as shown in the diagram below. The friction force (FF) is calculated by multiplying the coefficient of friction (μ) by the reaction force from the snow (R). Because the pressure of the ski has made a thin layer of snow melt, μ is very low, and in most conditions the slope angle (θ) will not need to be very big before FM is larger than FF and the skis start to slide.

ski side

If you want to check the conditions out for yourself, details for getting the best from the worlds biggest ski region can be found here:

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