GUIDELINES FOR SNOWMOBILE TRAIL
           GROOMER OPERATOR TRAINING


A Resource Guide for Trail Grooming Managers and Equipment Operators:


Chapter 1 - Introduction To Trail Grooming
      .
The Physics of Snow & Snow Surface Preparation:
 
It is useful for grooming managers and grooming equipment operators to have a basic understanding of the properties of snow in order to produce and maintain a durable trail.

Because snow (or ice) on the Earth’s surface exists so close to its melting temperature, it is unlike soils
or other construction materials used to build or
surface trails.

This section presents a general overview of how snow forms in the atmosphere, its response to environment, and external loads that are important to snowmobile trail grooming.

Formation of Snow The basic structure of snow, or ice, is a hexagonal (six-sided) crystal within Earth’s
atmospheric pressures and temperatures (see Photo 1.1).

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.


Figure 1.1 The basic structure of snow formed in the atmosphere is a hexagonal crystal.

A-axes growth produces a stellar crystal or “snowflake”. The Snowpack and How It Changes Once snow has been deposited on the ground, it begins to change, or metamorphose.

Gravity causes natural compaction and motion (or creep) to occur. Water vapor moves from areas of higher temperature or higher pressure areas to lower temperature or lower pressure areas.

Free water may be present in the snowpack and solar radiation can cause a change in the snow surface.
Three basic types of changes in the snowpack, i.e. snow metamorphism, are important for the groomer operator to understand.

These changes depend mostly on the snow temperatures, allowing water vapor to flow within the snowpack, or the migration of free water in the snowpack. It is important to note that the temperature of the snow, even at or near the snow surface, is not typically the same as the ambient air temperature.

Equi-temperature (ET) metamorphism occurs in regions where an “equal” or uniform temperature is present within the snowpack. This produces a high degree of sintering (neck growth and bonding) which yields a higher strength snow.

The snow crystals grow, become rounded, and bond at the expense of more faceted forms due to the transport of water vapor. Under equal temperature conditions, the transportation of water vapor is a pressure dominated process (see Figure 1.2).

Higher vapor pressures are present over convex surfaces. Lower vapor pressures exist within concave surfaces. The water vapor at high pressure moves to the low pressure regions, condenses, and forms necks and bonds.

This is the desired condition for producing a stronger, more durable snow surface.

    
                
Figure 1.2 Equi-temperature metamorphism.

Snow grains become rounded and bond to each other,
producing a higher strength snow. Temperature gradient (TG) metamorphism causes the formation of a poorly bonded, faceted TG crystal, commonly know as “depth hoar.”

It is typically seen at the base of the snowpack or underneath an ice crust layer. The formation of a TG layer typically occurs in a shallow snowpack during cold, clear nights.

The heat loss of the snow surface to the atmosphere through radiation creates a strong temperature gradient, or temperature difference, within the snowpack.

The ground temperature will be warmer than the snow surface temperature. A weak, hollow layer will be formed and will persist at the base of the snow. Under temperature gradient conditions, water vapor transport is dominated by temperature (see Figure 1.3 on the next page).

Water vapor at the higher ground temperature moves upward to the lower snow surface temperature, or more simply, hot moves to cold. When the net vapor transport is toward the snow surface, faceted cohesion less crystals rapidly form due to the excess vapor density.

It is important for the groomer operator to note a weather pattern of cold, clear nights with a shallow snowpack early in the season, particularly in mountainous regions, since the presence of a TG layer at the base of the snowpack can eventually produce an avalanche cycle.

        

Figure 1.3 Temperature gradient metamorphism – the flow of water vapor towards the colder snow surface causes the weak “depth hoar” to grow. Cold, clear nights following the passage of a front can also cause changes on the snow surface.

The development of surface hoar occurs when a temperature gradient, or difference, between the atmosphere and the snow surface develops. Again, hot moves to cold, so water vapor is driven from the atmosphere to the cooling snow surface, forming the cohesion less faceted surface hoar crystals (see Figure 1.4).

Again, these crystals are very stable within the snowpack, and a layer of these weak crystals can persist over the entire winter season.

          

Figure 1.4 Surface hoar crystals form on the snow surface during cold, clear nights. Melt-freeze (MF) metamorphism occurs whenever free water is present within the snowpack.

Free water may be present due to a rain event or surface melting by solar radiation. Free water will percolate slowly through the snow and freeze within a colder region of the snowpack.

Near the snow surface, smaller grains will melt and the melt water will be retained by the surface tension of the larger grains. Refreezing forms larger, polygranular clusters.

The snow strength becomes increasingly dependent upon the degree of refreezing that occurs. Melt-freeze snow can become solid ice or completely de-bonded, depending on its temperature.

Grooming Snow, Physical Properties, and Metamorphism Regional and seasonal differences in snow quality (i.e. physical properties of snow such as particle size, wetness, density, temperature, etc.) will influence the ideal method for trail preparation. In general, the goal of trail grooming is to reduce the snow particle size and produce some different particle sizes in order to maximize the number of bonding, or sintering, sites within the snow.

Mixing a layer of snow should also temporarily produce an equal temperature layer, to some extent. In other words, the goal is to prepare a layer of the snow to maximize equi-temperature metamorphism within that layer, and allow sufficient time for bonds to form between the snow grains, i.e. “set-up.” Therefore, the overall quality, or physical properties, of the snow prior to and post-grooming are of some importance.

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.

For bonding to occur, the snow temperature must be below freezing, i.e., less than 32 o F or 0o C. Again, equi-temperature metamorphism is a water vapor pressure dominated process, so water vapor is probably more available for vapor transport in warmer snow.

This only implies that bonding may occur at a more rapid rate when the processed snow is only a few degrees below freezing. Well-bonded snow can be achieved at very cold temperatures (less than -40 o F or -40 o C). The critical factor is allowing sufficient time for the snow to sinter, or “set up.” It is highly recommended that grooming occur post-sunset, as the snow surface does absorb some solar radiation during the day which will increase the snow surface temperature.

An equi-temperature metamorphism condition, and therefore better conditions for trail set up, is more easily achieved after sunset. Relatively inexpensive rapid response digital thermometers are commercially available for snow temperature measurements.

Infrared temperature sensors are not recommended since solar radiation, the reflection of the snow surface, and the exhaust from the grooming vehicle can produce an inaccurate temperature measurement.

The free water content of the snow, or wetness of the snow, can influence the selection of the best method for processing the snow. This property is best determined by measuring the snow temperature. The groomer operator should examine the snow, by trying to make a snowball, for example. Very warm, wet, or saturated, snow will not be cohesive. However, if the temperature is dropping wet snow may refreeze overnight.

Also, freshly fallen, cold, dry snow will not readily stick together. However, grooming and compacting this type of snow will enhance its ability to form bonds or “setup.”

Snow hardness is the best indicator property for snow strength. There are many available methods for testing hardness, such as cone penetrometers, ram penetrometers, drop tests, etc.

For the groomer operator, simply walking or stomping on the snow with a simple “boot test” (see Photo 1.9) is probably sufficient to give an indication of the compressive strength of the snow.

When boots make a deep imprint, the snow is soft. A light imprint indicates a medium strength snow. When it is difficult to imprint the snow at all, the trail can be considered hard and grooming is working well.

Another simple means for the groomer operator to get an indication of strength from within the cab of the tractor is to watch the ski imprint of the last snowmobile traveling the trail. If the body of the ski is sinking into the surface, the trail is soft. If the skag is riding on the surface, it is hard.

Post-grooming, sufficient time must be allowed for the snow to sinter or “setup,” preferably overnight.

        
Photo 1.5 Example of a “boot test” that indicates a soft trail Additional References on Snow Physics and Metamorphism

 

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