Beyond the Surface

In the early days of skiing, if folks found snow conditions to be unfavorable they would—believe it or not—ski-pack a favorite run themselves, each after paying to ride the uphill transportation. Race-course preparation consisted of boot-packing, then sidestepping and finally sideslipping. As on-snow recreation gained popularity, various vehicles and towed contraptions appeared for the primary purpose of packing and loosening snow surfaces.

“Packing” became “grooming” in the late 1970s, when the power tiller came into use and snow surface preparation became a daily chore. At the same time, front blades—the single most important grooming implement—became a must-have, giving area operators the ability to break up, build, move, repair and maintain snow. Since the 1980s, grooming improvements have primarily been in increasing vehicle horsepower, efficiency and productivity as well as the quality of the surface.

Here’s an overview of available implements and techniques—some more unusual than others—as well as what they do, the effects on snow and advantages and disadvantages.

Snow Characteristics

Snow, whether natural or machine-made, begins to change once it lands on the ground. The metamorphic process is a natural occurrence, beginning with the transfer of water molecules by vapor movement from the extremities to the body of the snow crystal. This water molecule movement has the effect of removing sharp curves, causing snowflakes or grains to become rounded.

When two or more grains make contact, a highly concave region is formed between them. A tendency for water molecules to migrate to this region causes a bonding process called “sintering” between adjacent grains that forms links called “necks.” This rounding and bonding process also causes a reduction of air or “pore” space between grains. Bonding and the resulting pore space reduction can be augmented by downward pressure from gravity, skiers, snowboarders and grooming equipment, creating a high-density surface that skiers regard as hard.

Machine-made snow usually accumulates on the ground in somewhat a rounded form, although cold temperatures and high air pressures from snowmaking guns produce somewhat irregular, elongated grains. However, machine-made snow undergoes the same metamorphoses as natural snow.

Trackpacking

Early season run preparation often begins with the trackpacking of natural snow to create a firm base on which successive layers can be built. Grooming vehicles are driven over the slope, compressing snow crystals together and increasing density through pore-space reduction, leaving behind a rough, trackmarked surface.

The advantages of trackpacking are: compaction creates a firmer slope; more surface area is exposed, which can augment bonding deeper into the surface; thousands of miniature snowfences are created by this method.

The disadvantages of this method are: rough surfaces expose more transportable snow to high winds; warm winds on rough surfaces impact more surface area, which increases melting; solar radiation will have more surface area to affect.

Compactor Bars

The compactor bar is made of aluminum, steel or rubber and has a smooth, curved surface facing the snow. It is primarily used in new-fallen and low-density snow conditions to create a smoother, firmer surface, pulling snow off high spots and filling in low areas. As it is pulled across the snow, grains and grain clusters are dragged and pushed into the surface. A plastic comb is usually attached to the compactor and pulled over the surface to form a series of peaks and valleys known as a “corduroy” surface.

In lower density snow conditions, a vehicle-width compactor can provide more pounds per square inch (psi) pressure and more consistent density due to the added compaction of the vehicle’s tracks. Wider “winged” compactors cover more surface area, but provide lower and possibly inconsistent surface densities because parts of the area covered by the compactor are not first compacted by the vehicle tracks.

Advantages of compactor bars: minimal crystal impact for lower-density, softer surfaces; smooths surface with low compaction; low energy consumption; works well with low-density snow.

The disadvantages of these implements are: well-bonded, high-density old snow is not broken down by compactor; high-moisture new snow does not finish out well; cutting moguls and finishing with compactor is rarely a one-pass operation; pulls snow off convex surfaces.

Rollers

Though not as popular as they once were because of their awkwardness and non-corduroy finish, rollers still have a place in the implement yard, particularly in early season for compacting new snow in marginal depth conditions. Studies have shown that rollers create denser surfaces than compactors because they put a smaller surface area in contact with the snow and have a higher psi. Rollers are, however, infamous for entanglements with towing vehicles, often ending up like one of those tangled ring puzzles with only one tricky solution.

The advantages of rollers are: good compaction; doesn’t displace as much snow from convex surfaces as compactor; minimal snow crystal deformation; minimal vehicle effort to pull.

Disadvantages are: awkward in tight areas and descending steep slopes; certain snow types stick to roller surface; high density and deep snow conditions cause poor finish.

Tillers

Tillers—both praised as saviors and cursed as snow killers—are the most versatile rear implements used by today’s groomers, with the ability to create a consistent, smooth surface on nearly all surface conditions. A rotating, tooth-covered shaft followed by a rubber or plastic mat with plastic finishers is towed by a lift frame behind a grooming vehicle. The shaft width usually matches the track width of the vehicle. Tiller shafts have infinitely variable rotating speeds from zero to 1,500 rpm.

In new snow conditions, tillers are used to reduce pore spaces, also known as “knocking the air out,” which speeds up the rounding bonding process by breaking down the delicate crystal structures and breaking up consolidated snowgrains. The friction caused by compaction generates heat, which causes minute amounts of melting and refreezing that add to the bonding of the snow. The trailing edge of the tiller and the plastic comb compresses the snow further, increasing density.

Tilling typically impacts only the top two to four inches, which usually are skied off or, if allowed to cure undisturbed, rebond and become hard again. Tillers can efficiently process only a given amount of snow per second, depending on the depth and hardness of the snow being worked and the speed of the shaft. Tilling hard snow too fast can cause poor penetration and granulation.

The advantages of tillers are: they are highly versatile in nearly all conditions; produce consistent, quality surfaces in a variety of snow types; promote a firm surface, quickly, from new-fallen snow; work well with high-density, well-bonded snow.

Disadvantages of tillers: require 40 to 80 HP to operate, increase fuel consumption; promote aging (rounding) and hardening of snow; overuse or misuse creates prematurely hard conditions or dead, non-bonding snow; low snow conditions can cause tiller damage and mix dirt or debris with snow.

Mechanical Cultivators

Mechanical cultivators, also known as West Mountain tillers or spider gang harrows, are farming implements towed behind groomers to loosen hard surfaces to approximately seven inches deep. The tines of the cultivator are set at an angle to the direction of travel, digging up the snow surface as they rotate. Lightly tilling snow that has been loosened by a cultivator has can create an excellent, resilient surface.

The advantages of cultivators are: fast way to loosen hard conditions; low energy consumption; year-round implement, if you happen to own a farm.

Disadvantages are: low snow conditions can mix dirt and debris into the snowpack; they are cumbersome to maneuver when towed; surface must be tilled or powdermade after cultivating for a finished product.

Expanded Screen Rollers

Expanded screen rollers, also known as Powder Makers, are used on hard snow to create a loose, granular surface. Usually pulled in a three- or five-gang configuration, they are expanded metal screens set at an angle to scuff the surface. Pulling them directly behind tillers creates a finely granulated, silky surface. However, they do not work in wet conditions or in areas where there are large temperature gradients between air and snow, such as one above freezing, one below, which can create interesting, unwanted cylindrical snow formations like giant globs following the grooming vehicle.

The advantages of expanded screen rollers are: a loose, non-compacted, granular surface is created; in optimal conditions, quality surfaces can be produced with low energy output.

Disadvantages are: they do not work in warm or wet conditions; are difficult to maneuver; implement wants to pass groomer descending steep slopes.

Powerharrows

Powerharrows are farm implements with long vertical steel knives that rotate and can penetrate the surface up to 18 inches deep. Carried on the rear lift frame of the groomer, they are effective at breaking up hard conditions. Though not in widespread use, the powerharrow is better than any other implement at breaking up deep ice.

Advantages of powerharrows are: hard, consolidated snow is loosened to considerable depth; year-round implement—again, if you have a farm.

Disadvantages are: slow, low productivity; high energy consumption; does not produce a finished surface.

Front renovators

Front renovators have been in use for a number of years in cross-country grooming, but only recently entered the alpine sector. Composed of hardened steel cutters resembling a serrated bread knife, the renovator is attached to the front of the vehicle instead of the blade. Pushed along at an operator-determined depth, it cuts a layer loose from the snow pack, which is then broken up by the vehicle tracks and further reduced by a mechanical cultivator, tiller or powdermaker.

Regular use of front renovators reduces densities and improves overall surface quality and consistency. When working hard conditions, mixing layers of different grain types and/or adding new snow crystals to the worked layer can increase pore space, providing more resilient surfaces.

The advantages of front renovators are: high productivity with fairly low energy consumption; works in nearly all snow conditions; minimal snow transportation is good on steeps, particularly downhill; rivals mechanical cultivators in loosening of hard conditions.

Disadvantages of front renovators are: not recommended for use in marginal depth situations; inability to transport snow (no blade-like ability); depending on hardness and bonding, surface may require an additional pass with a tiller for an acceptable finish.

Winch-assisted vehicles

Winches provide cable tension for grooming vehicles to descend normally inaccessible steep slopes or unconsolidated snow conditions. A single-cat system has a winch and boom (able to rotate 360 degrees) mounted mid-vehicle, with the hook end of the cable attached to a “dead man” at the top of a slope so the vehicle can raise and lower itself while grooming. A two-cat system uses a winch that is front- or rear-mounted on a vehicle parked at the top of the slope, paying out or reeling in cable attached to a vehicle working on the slope.

While winch systems cost more to operate than conventional grooming systems, the tradeoff of uphill snow movement, early season compaction and well-maintained steep slopes usually offsets the additional expense. It is important to have well-trained operators in order to gain the most efficiency from winch systems.

The advantages of winch-assisted vehicles are: early season compaction on steep slopes; enhanced ability to move snow back uphill efficiently; ability to groom steep slopes to a level of quality not possible by conventional means.

Disadvantages are: high price tag; possible higher cost of maintenance and operation; high energy consumption.

Tough Condition Strategies

There are as many different strategies for grooming snow as there are constantly changing variables. In addition to this variety is the fact that many of these strategies can only be applied at certain times or in specific conditions.

Rain-sodden snow can be one of the biggest grooming challenges, particularly rain-soaked new snow. Knowing what the short-term weather forecast is—whether it is going to continue to rain, warm up or drop below freezing—can greatly influence the action taken. Many operators stay off rain-sodden snow because grooming vehicles may slide on steep slopes. If a freeze is on the way, however, such snow can be trackpacked and left open to speed up the drying process; then tilled after the freeze.

Massive amounts of new snow can rearrange priorities fast. Areas can designate runs that take precedence in deep conditions or keep climbing routes open to the top of each lift and maintain at least one compacted run. Other options: wait until snow stops falling, then work outwards from the centers of the runs or compact the edges and let skiers cut up the centers. Another option is to strip groom, leaving five- to ten-foot strips between the down passes. Vehicles climbing these strips eliminate the berming or windrows that usually accompany deep-snow grooming.

Dead snow or sugar snow is old, large-grained snow that seems to plague resorts in mid-season after an extended period of low temperatures, no new snow and heavy traffic. Groomers go out at the end of the day to push and till moguls of loose, large-grained snow until the run is flat, but by 10 a.m. the next morning, the bumps are back.

Bonding this stuff back together takes time—remember, the colder the temperature, the slower the bonding process. Adding heat—from solar radiation, warm wind, water or chemicals—can cause partial melting, which provides water molecules to assist with rebonding. It also may help to groom deep or take snow from the edges of the run in order to mix smaller and/or newer, less rounded grains with the problem snow. Machine-made snow or natural snow can be mixed in with the dead snow to aid rebonding as well. Generally, this is a tough situation to correct—what helps the most is a major weather event.

Spring snow or melt/freeze occurs when the snow becomes wet due to rain or melting then reaches freezing temperatures, so large wet grains absorb smaller adjacent grains. As this melt/freeze cycle repeats, the grains become larger, forming what many call corn snow. Subtle weather changes can have big effects, prolonging or shortening either phase. For example, sun with high overcast can provide heat on the snow pack, prolonging the melt phase.

Timing is the single most critical element in dealing with melt/freeze conditions. In the melt phase, the surface can be groomed fairly well if it is not overly saturated, but in the freeze phase, it is difficult to produce a surface that is much more than textured, granulated ice crystals. Groomers usually have a narrow window of time just as the surface begins to refreeze in which to produce a good product, so workers’ shifts should be as flexible as possible to get pushing projects done prior to deep refreezing.

Compactors and tillers work well in melt/freeze conditions, particularly when free water is present between the grains. Tillers also can widen the window of opportunity for grooming because they can still be effective even as the snow is refreezing.

In the end, a ski area can have the latest vehicles and implements available and still do mediocre work if the operators are not well-trained or if they lack direction, challenges and imagination. Like farming, snowgrooming requires knowledge of the substance being worked with as well as a feel for both the surface consistency and the grooming vehicle as it interacts with the snow. As George Willet, the wise snow wizard of Showdown Ski Area, Mont., says: “Good snowgrooming is a form of art.”