Preservation of snow

Introduction

After having produced or stored the snow, distributed and then groomed it properly for skiing or competitions, it is important to know how to best prolong and preserve the snow, especially in mild or wet weather conditions, or when old snow becomes very soft.  With proper knowledge and application, salting and watering can freeze, harden and preserve the snow. Salting is done in all snow disciplines, and mostly for competitions. Watering and water injections are mainly performed on alpine slopes and ski jumping hills.  Harvesting natural snow may also be a applicable technique for certain alpine resorts.

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Salting

Salting is foremost a way to make soft snow hard, such that the competition courses become consistently firm.   We can say that salt is a “snow hardener”.   In addition to understanding the freezing process that salt initiates, it is also important to be clever at other basic elements.

• understanding the local weather conditions 
• proper base preparation
• watering the snow (in some cases) 
• having sufficiently trained crewmembers and correct equipment for spreading the salt 

Salting should be done when the snow is very soft, «rotten» or very wet.  The goal of salting is primarily to save the competition and making it fair for all participating skiers, not to just create a faster and easier competition course. 

Salt and water can also be used to prolonge the snow in mild and wet conditions (see the chapter “Unique weather and snow treatment”).

Elements that will harden the snow 

• Pure salt – consists of natrium and chloride (NaCl)
• Naturally occurring salts (sea-salt, rock-salt, Himalayan-salt etc.) which all consist mainly of NaCl, but in addition contain small amounts of other minerals
• Road salt – consists mainly of calcium chloride (Ca Cl2)
• Nitrogen based fertilizers and other water soluble elements (even sugars) can be used as “snow-hardeners”  
  • Ammonium Nitrate (NH4 NO3) 
  • Ammonium Sulfate ((NH4)2SO4) 
  • Ammonium Chloride (NH4 Cl) 
  • Urea (CO(NH2)2) 

If there is moisture in the snow – 1% water or more (you should be able to make a snow ball) – all these elements can be used and will give approximately the same result. The weather and snow conditions may however create slight differences.

• Sea-salt often works best in very warm and wet (5% water) conditions
• In dryer and a bit colder condition – towards 1% water content and towards 0 degree Celsius air temperature – Ammonium Nitrate have resulted in the best “snow-hardening” in some studies

Chemistry 

When grains of salt or fertilizer are dissolved in the “free” water that is present in the snow, the energy equilibrium between freezing and melting of water and ice particles/snow grains in the snow is broken.

Ice- and snowgrains will continue to melt and create a water&salt solution, while the opposite reaction (from water to ice) will halt. This salt solution will cause additional snow crystals to melt as it sinks.

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This change of phase from solid (snow) to liquid (water) requires energy or heat.  This heat is withdrawn from the surrounding snow (see black arrows in figures A, B and C above), and causes the snow below and eventually above the salt solution to harden.

The size of the grains is important

Salt and fertilizers come in different sizes.

• small grains (table salt, road salt, some fertilizers) 
• large grains (some fertilizers, spesial order sea salt)  
• grains in different sizes (sea salt) 

A larger salt or fertilizer grain will sink deeper into the snow while reacting with the water.  A larger grain might also “pack” more energy and help harden the snow in a deeper and stronger way.  This process might however take some extra time compared to smaller grains. 

Spreading the salt 

Manual spreading: 

• Before spreading the salt on the competition courses, always test on a separate small area
• Throw/spread up in the air, not down onto the ground! 
• It is important that the salt is applied evenly across the course or hill

• Enough buckets (10 – 15 liters), bags of salt (15 – 25 kg), rubber gloves and a trained crew must be ready such that the salt can be spread as fast as possible

For alpine, the process starts with basic salting and is followed by a second application. It is important that the crew members walk or slide down the hill all using the same technique for throwing the salt.

Basic salting: Apply large grained salt and let the hill stay untouched for 10 -15 minutes.

Second application: After the race course is set, slide through before salting to create a smooth surface. Then apply salt from the top down.

After minimum 10 -15 minutes it is safe to slide through the course and check the quality of the salting application.
If the snow is not moist enough, too much salt may have an opposite effect. The more water in the snow, the more salt it can take. In some cases it may be required to add water to get the desired effect.
If there is a need to salt immediately prior to the competition, use small grained salt which reacts with and hardens the snow faster.

In alpine ski racing, salt has been used for many year (the product PTX was used already in the 1990’s). Lately, grains of sea-salt in special manufactured sizes have been used by FIS and Olympic Organizers.

• Very coarse
  • deepest penetration depth (> 50 cm)
  • slowest reaction time
  • longest duration (about 3 days)
  • not so hard surface
• Coarse
  • medium penetration depth
  • faster reaction time
  • slightly shorter duration
  • slightly harder surface
• Medium
  • not so deep penetration
  • pretty fast response time
  • not as long duration
  • pretty hard surface
• Fine
  • only very superficial effect
  • immediate response time (a few seconds)
  • short duration
  • icy and hard, almost slick surface

Mechanical application: 

If there is enough water present in the snow, salt will guaranteed work when applied either in front of or behind the grooming machine and tiller.   By experience, spreading the salt on the surface behind the tiller gives faster results. If the salt is applied ahead or in front of the tiller, the snow sometimes hardens a bit deeper.

Spreading salt – how much to use? 

The correct amount of salt depends on the width of the course or the size of the hill.   

• On an 8 meter wide Cross-Country competition course, maximum 100 kg salt per km should be used, and on a 4 meter wide course 50 kg per km is enough  
  • This equates to ca 10 – 12 gram per square meter snow
• On a normal slalom course maximum 400 kg should be used, and on a GS course maximum 800 kg 
• On a large- or normal size ski jump, 40 – 50 kg salt is enough 

The more water in the snow, the more salt it can take, but normally more salt than suggested above will not improve the results. Too much snow may work negatively and damage (dry out) the snow.

The applied salt should in most cases be a mix of large and small grains.

When does salt not work?

Salting will normally not work in certain snow and weather conditions

• When the air temperature is 0 degrees C or colder 
• When the snow or the snow surface contains too little water  
• With dry, new natural snow 
• When it snows (but rain is ok)   
• When the snow is “dead”; you can not make a snow ball, or there is no crystal structure in the snow/ice grains

Be careful when applying salt during rain and windy conditions – it will likely create very icy conditions. It is then important to use large salt grains, and be prepared to scrape the surface with an implement. If this happens on a Cross-Country course, the most practical is to use a snowmobile pulling a Ginzugroomer or equivalent drag. 

Fog can reduce the effect of salting (with 100% humidity in the air, the required evaporation from the snow may not take place)

The difference of salting natural versus man-made snow  

Moist natural snow will, after it is groomed, react faster with salt than artificial/man-made snow. Old artificial snow will react slower because the larger and older crystals contain less water (the water drains easier through old and rounded crystals).  The larger crystals will take longer to “freeze together” as the salt solution moves down the snow layer.

Practial experiences

• It is important that the snow is untouched while the first part of the freezing process takes place 
  • Close the course or the hill for 10 – 30 minutes after salting, depending on the amount of water in the snow or the type of snow crystals and snow – old artificial snow needs more time than moist natural new snow  
• Salt/fertilizer may be used every day for a long time if there is water present in or added to the snow.
  • During the Olympic Games in Sochi 2014, urea was used every day for 2 weeks on the Nordic Combined courses (moist snow was added some days, otherwise the warm sun and temperatures would increase the water content on the surface).  
• “Visible” water (it is possible to make a snow ball or even see water dripping when squeezing) is required for salt or fertilizers to start the freezing reaction. 
• Using too much salt will not improve the snow hardening, and will over time be detrimental to the snow quality.  
• Repeated salting may give negative results unless water is also added. The snow will then slowly dry out and eventually stop reacting with the added salt.  
• Rain or melting snow due to a warm sun can start the freezing process if the salt already is in the snow   
• Freezing and hardening can last up to 10 – 12 hours or more in optimal conditions and when using very large salt grains
• Salting on the snow surface gives faster results since the “free” water is usually in the top layer, especially during sunshine. This means that the salt should be applied behind the tiller if a fast hardening process is desireable.
• In alpine or ski jumping it is sometimes necessary to break up the top surface layer of ice before salt is applied (such that the salt has access to the more humid snow just underneath the ice layer).  

Conclusion

• Smart base preparation 
• Must be water present in the snow 
• Use sea-salt, but not too much
• Test first – always! 

Decision matrix for using salt

The matrix below can be used for Cross-Country or Biathlon.

The day before competition or official training :

On the competition day (2-3 hours before start):

Salt and the environment

It is proven that large amounts of salt can damage the ground water and plants. This is seen in connection with using road salt during the winter season. It has also been studied if the same is true when using salt on alpine slopes. Result from this study shows that the salt used on alpine slopes comes out with the meltwater in the spring. and will not stay on the slopes for more than 1 year. The relatively small amounts of salt and resulting chloride ions in the meltwater were diluted such that the effect was proven insignificant. 

• The following text is taken from studies of road salt: 
   
  Water 
  • Salt is foremost a problem when and where “salty” water (from melting) reaches ground water reservoirs, which may cause drinking water degradation 
• Plants 
  • Leaves and needles become scorched when salt attach to the vegetation or is taken up by the root system. Chemical imbalance in the root system reduces the plants ability to absorb water and other nutrients. The photo syntheses is reduced. Trees become less resistant to attacks from insects.
  • Salt or salinity stress in plants is caused by high level of salt in the soil, from sea water spray, and from salting of pedestrian paths and roadways. Salt stress will influence both the natural balance of water and ions.
  • The salt will decrease the plants ability to absorb water. It will influence the metabolism, and the growth will decrease. It may also cause oxydative stress. The plants normally have a high concentration of Potassium (K⁺) and low concentration of Natrium (Na⁺), and the activities for over 50 enzymes depend on this ratio. Added Natriumn (through salt) will disturb this balance.
• Animals  
  • Animals who otherwise lack salt in their diet (for example moose and elk), are attracted to the salt on the side of the roads and can therefore cause danger for traffic.

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Watering and water injection

Watering and water injection of the snow is sometimes used to harden the surface and create safe competition conditions in alpine, snowboard and ski jumping. Added water, followed by colder temperatures and grooming will increase the density of the snow. This is sometimes necessary for slalom, giant slalom, half pipe as well as in the landing zone of ski jumps. The actual watering process takes a long time and is complicated, and should be done only by experienced organizers or with expert advisors present. Before the watering starts, the weather forecast must be checked since changing temperatures will affect the process. 

Watering 

Depending on the size of the hill or course that is being watered, the process may take several days and demand lot of equipment and manpower.   First the grooming machine creates a “potato field” to open up the snow surface such that water can easier sink in.   All equipment (hoses, bamboo poles, rope, helmets, etc) should be brought to the top of the hill/course where the watering starts.   The hoses are rolled down and connected to a hydrant such that the watering can take place on the hill/course both above and below the hydrant. The hoses are roped to bamboo poles that can be used as support if needed. 

The crew is divided into groups that hold the hoses (in its entire length), and with two persons by the nozzle. Those two persons make sure the whole snow surface is being watered.   Because the water pressure is high, it is important to coordinate with a person in the pumphouse via radio in case the pressure needs to be gradually turned up. The snow is watered until it changes colors, first to grey then to blue. 

The process is repeated down the hill/course. After the hoses have been moved and connected to new hydrants, it is important to check that they have no sharp bends before water is turned on. This will secure that the high water pressure does not cause any dangerous situations with a “wild” hose.  

Water injection  

Use of water injection equipment has become popular among World Cup and Olympic Games organizers.   The injection is done by connecting high pressure water – via hydrants and hoses – to an aluminum bar with small nozzles who spray water 20 – 100 cm down into the snow.   The process will create thousands of small holes where the snow around the holes are saturated with water. This snow will freeze due to heat evaporating from the holes or due to cold air sinking down into the holes. 

This process also takes long time, and the safety due to the high water pressure in the water injection bar and hoses must be prioritized. When the aluminum bar with nozzles is moved it is important that the bar is not turned upside down and cause the nozzles to be filled with snow. The nozzles should not touch the snow until the water flows through them.

Each injection takes only 2 – 5 seconds, depending on the snow density and depth. First the water digs a hole, then fills it almost to the snow surface (sounding similar to when filling a bottle). The bar is then dragged (not lifted) downhill 5 – 10 cm and the process repeated. The bar must always be used perpendicular to the direction of the race course. The area next to the safety fences and the jump tables are not water injected.

The number and size of teams with equipment for watering and water injection depend on the length of the course and number of crew members available.

• 15-30 meter with snowmaking hoses
• Tools for opening and closing hydrants
• Nozzles or water injection bar (ett sett har som regel 3 rør)
• Crampons
• Bamboo poles to assist holding the hoses (during watering)
• Rope to assist holding the hoses (for watering)
• Drill with extra batteries and auger for drilling holes for the poles
• Propane or heatgun for melting nozzles or water injection bar if icing
• Whistle (for organizing and timing the activities)
• Walkie-talkie
• 1 person leading and organizing
• 1 person controlling the hydrant
• 5-8 persons holding the hoses or the water injection bar (depending on the length)

After the water injection it is important the the course is closed until the snow has been transformed (it is especially important that the holes from the injection are not covered).

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Unique weather and snow treatment

Mild and rainy

Many ski clubs and venues have experienced uniquely mild and rainy periods leading up to or during the winter that destroyed the snow cover, even man-made snow. This is especially frustrating when big events are planned.

Without making any guarantee, the following “snow treatment” methods have successfully preserved snow on Cross-Country competition courses during “impossibly mild” weather conditions, especially if all are used together (methods designed and implemented by Geir Olsen): 

• Method 1: Cover the snow with geotextile cloth; the cloth will protect against further melting

• Method 2: If geotextile fabric is not used, spray (a lot of) water on the snow and then salt. Water and salt with “freeze” the snow and preserve it before more mild weather approaches. Nozzles can be removed from lances (if available) and used for watering.

• Method 3: Use efficient and modern snow guns, and install a Snomax pump such that snow can be made on -1.7 degrees C wetbulb temperature.

Sugar snow

Over time, with no precipitation and with low humidity, heavy use (many skiers) and frequent grooming may cause very soft and “loose” snow, often called “sugar snow”. This is seen more often in venues with mostly man-made snow. The ice/snow crystals are rounded, and it is imposible to even create a snowball. It is very difficult to fix this problem, but the following three solutions may work: 

1. Add (lots of) water, groom, then salt the snow
2. Scrape away the sugar snow (manually with showels or mechanically with the front blade) such that a deeper and more humid layer of snow is exposed. This will harden better after grooming.  NOTE: Before attempting this, check with a drill and auger if the deeper snow layer has more humid snow.
3. Add new snow with a higher water content and/or better crystal structure (ice scraped off indoor ice rinks may be used). Then mix, groom and salt the snow.

If sugar snow is a frequent and annual problem, the venue operator should consider reducing the frequency of grooming with large machines, or only groom in the evenings such that the snow can always harden for many hours during the night. For Cross-Country and Biathlon venues, a snowmobile pulling the appropriate attachment should be used instead of the larger machines some days. This will reduce the destruction of the snow crystals.

Indoor ski arenas

Indoor ski arenas have a stable and controlled “weather”, but will often see challenges with the snow quality (perhaps due to the high humidity) where the snow after a while becomes “tired” and similar to sugar snow.

The solution for indoor arenas is to renew the snow in some way, similar to the methods suggested for outdoor venues.

• by adding and mixing in new snow with higher water content and sharper crystals
• by watering and salting the snow (although this is usually a short.term solution)
• by installing special made snowguns (for example in the roof structure) that the create snow daily without increasing the local humidity. This requires an air temprature of at least -2 degree Celsius.

Snow farming

At certain alpine resorts, snow fence is used to catch and harvest natural snow, which is later groomed. 

These fences effectively protect snow from alpine gusts that would otherwise blow it off the peak — at least until snow cats can pack the powder down and pave the trail. The fences are then transplanted to another area, and the process repeats itself. 

If the alpine terrain lies above the tree line, this technique may be applicable on a large scale. When used, the process begins in early September, long before any snow dusts the peaks. Pieces of 1.8-meter steel are pounded into the ground, which later freeze into place so that fencing can be tied around it securely. 

Sunshine Village on the Continental divide in Canada is probably the ski resort in the World that does most snow farming through this snow fence technique. Sunshine Village’s low-energy “snow farming” technique uses several thousand pieces of steel and roughly 20 kilometres of plastic fencing to trap snowfall on its upper peaks, reducing the need for traditional snow-making.

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