Freestyle Skiing Course Preparation

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[edit] Course Preparation and Freestyle Skiing

Changing temperature, radiation, and wind conditions drive changes in the properties of snow and firn. In turn, changing snow properties have a large impact on the processes that occur in snow. Snow metamorphism controls both the nature of the crystal surface and bonds, affecting thermal conductivity and chemical reactions, and also controls the nature of the interstitial air space, affecting the permeability of the snow and the nature of air or water flow through the snow.

Liquid water in snow causes crystal growth, which in turn increases the permeability if the water does not subsequently freeze. In polar and high-altitude regions, snow may survive for more than one year; snow more than one year old is called firn.

Snow and firn are dynamic and complicated media, and feedbacks in heat and mass transfer mechanisms impact both the nature of the snow microstructure itself and the very processes of change.

Snow microstructure plays a key role in snow and firn behavior on many scales. Often the large scale behavior or appearance of snow is due to the small-scale properties of snow; for example avalanches are launched because of weak hoar layers, and remotely sensed data from satellites orbiting the planet are sensitive to snow crystal type and size.

[edit] Snow Physics

The key to setting good tracks and fast smooth skating lanes lies in the snow grooming which is done before tracksetting. Grooming is the working or reworking of snow using special vehicles and attached equipment in order to provide consistent skiing conditions.

The objective in grooming is to create a smooth level trail bed that is firm enough to support the skiers and their poling and at the same time to have enough loose workable snow to mould tracks and shape skating surfaces.


[edit] Formation of Snow

The basic structure of snow formed in the atmosphere is a hexagonal crystal. A-axes growth produces a stellar crystal or “snowflake”
The basic structure of snow formed in the atmosphere is a hexagonal crystal. A-axes growth produces a stellar crystal or “snowflake

The basic structure of snow, or ice, is a hexagonal (six-sided) crystal within Earth’s atmospheric pressures and temperatures.

Three a-axes are perpendicular to the c-axis at 60o to each other. The direction of crystal growth along the c-axis or a-axes depends on temperature. This temperature dependence of crystal growth produces the wide variety in the geometric forms of snow, such as stellar crystals, plates, dendrites, needles, columns, etc. Prolonged rotation of a snow crystal in the atmosphere produces more irregularly shaped aggregations of crystals such as snow pellets or sleet.

[edit] Why Groom Your Ski Trails

Trail grooming” is the activity of producing a smooth surface of snow with a uniform high density through the use of mechanical equipment. Trails become rough and uniform primarily through the cumulative effects of skier traffic.

  • Grooming helps to make a skier’s visit or race fun from start to end
  • Grooming helps make a ski race fairer
  • Grooming demonstrates that cross country skiing is not only good exercise but also fun and exciting.
  • Grooming puts more glide in cross country skiing.

There is an ongoing debate as to whether grooming is an art or a science. The fact is that the basic medium, snow, comes in a bewildering variety of states, and groomers, in their attempts to define scientifically what is still a very inexact art, have come up with as many definitions of snow types as have the Inuit.

For this manual the basic snow terminology, snow conditions, grooming and tracksetting processes will be presented. For a more detailed discussion of snow physics, grooming and tracksetting please refer to the Cross Country Canada Trail Grooming and Tracksetting Manual.

[edit] Snow

A knowledge of how snow is altered due to changes in temperature will allow you to better understand what happens when you groom trails in different conditions. It will help you to:

  • pack the snow to achieve suitable density for a variety of different users
  • provide the same track conditions for all skiers in a competitive event
  • extend your skiing season by working the snow the right amount.


[edit] Heat Gain and Loss In The Snow Layer

Temperature Gradient

The temperature gradient is the difference in temperature between the snow surface and the ground expressed in terms of degrees Celsius per meter of depth. For example, consider one meter of snow lying on a ground surface the temperature of which is zero degrees Celsius. If the air temperature drops to minus 20 degrees Celsius there is a difference of 20 degree Celsius in one meter of snow depth, or 20 degree Celsius per meter. Because the temperature gradient influences the movement of water molecules within the snow pack, it has a significant effect on changes in snow structure within the snow pack. The physical processes which cause changes at and beneath the surface of the snow are driven by temperature gradient or the lack thereof and by transfer of heat to the snowpack.

For the purpose of grooming, the interfaces which most concern us are:

  • the ground and snow surface
  • the air just above the snow surface and the snow surface
  • the snow surface and the snow one to two centimeters below the snow surface

Factors which affect the above interfaces are:

  • Incoming Ultra-violet Radiation (sunlight)
  • This process will heat up the snow and cause melting within the top few centimeters of the snowpack. The amount of warming depends upon the albedo (reflectivity) of the snow, the amount of impurities (dirt) mixed with the snow, and the granular structure of the surface layers.
  • Machine groomed snow is not highly reflective and therefore a large percentage of the incoming solar radiation is absorbed which may create a significant temperature gradient within the top two to three centimeters or may cause melting of the surface layers.


[edit] Outgoing Infra-red Radiation

This cools the snow surface. In clear conditions, in midwinter, outgoing infra-red radiation may cool the snow surface at the same time as incoming radiation warms the snow beneath the surface, creating or enhancing a significant temperature gradient. During a clear cold night, a crystalline deposit of surface hoar may form on the snow surface.

Rain

Rain transfers heat directly to the snow. It may remain as liquid water in the snowpack.

Wind

A warm moist wind results in heat being transferred to the snowpack. A dry wind, while causing the snow to evaporate at a high rate, transfers little heat into the snowpack.

Metamorphism

Process of Rounding

Snow begins to change as soon as it reaches the ground (or at higher temperatures, in the air before it reaches the ground). The rate at which it changes depends upon the temperature. Close to zero degrees Celsius the change is rapid. Below about -20 degrees Celsius there is little discernable change from day to day.

When outside temperatures are moderate or when the snowpack is deep, the temperature gradients within the snowpack will be small. Snow will then change by a process known as “rounding”. The natural process of minimizing surface area breaks down the intricate crystalline snow structure of the ice crystals into smaller, more rounded ice grains.

At the same time, because of the reduction in volume of the snow particles, the snowpack consolidates and settles. When snow is first deposited it is light and fluffy, the crystal branches interlocking to form a cohesive mass. After a period of time, water molecules are transferred by vapour movement from the extremities to the body of the crystal. Eventually, the ice grains lose all sign of their previous crystalline structure and become more and more rounded. The larger ice grains grow at the expense of the smaller particles resulting in a uniformity of size within each snow layer.


[edit] How Snow Gains Strength and Density

You have probably noticed that soft new snow, when packed, will harden overnight. The process where snow gains strength by the joining together of ice grains, is called Sintering.

In the case of the ski trail, the snow crystals or ice grains are forced close together by the mechanical compaction of grooming equipment, at which time energy is introduced into the snow by the mechanical action, resulting in a partial melt, and thus increasing the density of the snow. As a result a transfer of water molecules, necks of ice form between adjacent grains, strengthening the snowpack. On re-warming, the necks between the ice grains will be reduced thereby weakening or destroying the bond between grains.


[edit] Importance of Density

If your trails are to withstand their intended use, you should pay attention to the density of the groomed snowpack.

The following table gives typical snow densities and indicates suitable densities for various levels of use. Kilograms/cubic metre (Kg/m3) is the usual measure of density.

  • New Snow 150 - 200 Kg/m3
  • Wind packed snow 250 - 300 Kg/m3
  • Packed with snowmobile alone 300 - 350 Kg/m3
  • Support required for racing basket >350 Kg/m3
  • Recreation trails (moderate use) 450 Kg/m3
  • Racing trails 500 Kg/m3 or greater
  • World Cup and higher events 540 - 560 Kg/m3

Details on equipment and the process required to measure snow densities is available through your Cross Country division office or from Cross Country Canada.

[edit] The Melt-Freeze Process

When the sun is sufficiently strong to melt the top layers of the snowpack during the day, and when night-time temperatures fall below zero degrees Celsius, cycles of freezing and thawing will occur. In this process smaller grains will melt before larger ones. During the course of a number of melt-freeze cycles, larger grains will grow at the expense of smaller ones. The meltwater wetting the surface of these larger grains eventually re-freezes and firmly cements the grains together. Melt-freeze grains have a tendency to freeze together in clusters, leaving large pore spaces unless packing is done.

For grooming, the most important indicator properties of the snow are particle size, temperature, wetness, and the final snow hardness, or strength. Snow density, or the mass per unit volume of the snow, is not necessarily a good indicator property of snow strength since very wet, unbonded, melt-freeze snow can be of very high density but have very low strength.

The particle size and sorting can be determined by simply examining the snow prior to grooming. A particle size range from 1/32 in. to 3/16 in. (0.5 mm to 4.5 mm) is ideal. Large particles or clumps that have developed perhaps due to melt-freeze changes (MF metamorphism), may require a more aggressive grooming technique, such as tilling the snow. In many regions, the snowfall consists of relatively low density, small particulate snow and the snowpack remains dry. In such areas, a multi-blade drag can provide sufficient remixing of the snow surface. It is important for the groomer operator to become familiar with the variations in snow particle sizes for his/her specific region and snow conditions in order to determine the appropriate grooming technique.

[edit] Types Of Snow

  • Falling or Newly Fallen Snow
    • In cold conditions (-One degree Celsius and lower): The snow will be low density, highly crystalline, possibly interlocking crystals, matting and building up on trees. The snow will be hard to pack if the new snow is allowed to get too deep. It packs well in 10-15 centimeters (four - six inches), layers at temperatures close to zero degrees Celsius, and becomes more difficult to pack as temperature decreases.

In warmer conditions (zero degrees Celsius and above): The snow will be wet and heavy with little remaining crystalline form. If accompanied by wind, the snow is plastered on trees, signs, buildings, etc. It packs easily into a hard, dense layer at temperatures close to freezing and may turn to slush at higher temperatures.

  • Partially settled Snow (Fresh Powder)
    • This type of snow has begun the rounding process and, if left alone, will settle and strengthen naturally over a period of time as pore spaces are reduced and sintering occurs Mechanical disturbances such as blading, tilling or packing will reduce the air spaces by pushing the ice grains closer together allowing better sintering. The snow, now “Machine Groomed Powder” becomes both denser and stronger.
  • Settled Snow
    • When snow is settled, the grain size becomes smaller and more uniform. Pore space decreases, sintering increases, and density increases. The ability of the snow to re-crystallize due to large temperature gradients, and hence loosen-up, is reduced due to small pore spaces. In continental snow climates such as Alberta and the Rockies, Saskatchewan, Manitoba, Yukon, and North West Territories, a well packed trail system will not present re-crystallization problems. The snow should be packed early and well.

Dry Granular Snow This type of snow may present itself in three ways:

  • faceted surface grains. This is unlikely to occur at low elevations of most Cross Country ski areas.
  • faceted grains in bottom of snowpack. This is common in continental snow climates, but will not be a problem if trails are well packed.
  • re-frozen melt-freeze grains. These are enlarged grains produced by several cycles of melting and freezing, often a function of over-grooming the same snow pack. They may present as a loose surface layer, but more likely will be frozen clusters which will break up as temperature rises and skier traffic increases. When partial melting has occurred (free water content less than eight percent) it is known as “Corn Snow". Excessive grooming will tend to loosen and enlarge these grains.

Surface layers of refrozen melt-freeze grains (or sugar snow) can be reconstituted (strengthened) in two ways:

  • It can be mixed with fresh snow, either new snowfall or old dry snow from layers beneath the granular layer.
  • A power tiller or, for snowmobile groomers, a compaction drag may be used to mill the snow. This reduces

the size of the particles, allowing partial melting and sintering to take place. If melt-freeze cycles continue, the snow will eventually become sloppier, and renovation will be needed. There is little you can do at this stage to alter the physical characteristics of the snow grains. Tilling will help to dry out the snow by exposing more surface area to evaporation. Wind will speed up this process.

  • Wet Snow
    • There is an old field “squeeze” test for the definition of snow moisture levels. A handful of snow is scooped up and

squeezed in a gloved hand. Dry snow will crumble when released. Moist snow will compact to form a snowball, and water can be squeezed out of a handful of wet snow. Wet snow creates special problems for groomers. It should never be packed or groomed if there is any chance that temperatures will drop later in the day or night. Wet snow which is compacted and later frozen can become a “bullet proof’" groomer’s nightmare. Wet snow can sometimes be broken up into coarse chunks with a front renovator or rotary hoe or tiller and left loose to aerate for several hours before further grooming. This will allow water to percolate downward and encourage evaporation through the increased surface area. Later, when temperatures have started to drop below freezing point, further renovation and tilling can refine the surface layers.

In wet conditions where there is no chance of freezing prior to a race, snow can again be coarsely renovated or tilled to promote drainage and aeration, but final grooming and tracksetting should be left until just before the race. It is normally difficult to till wet snow as it “balls up” in the tiller.


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