Preservation of snow
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
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.
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 This heat is withdrawn from the surrounding snow (see black arrows in figures A, B and C above), and causes the snow below and eventually above the salt solution to harden.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
- 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
- medium penetration depth
- faster reaction time
- slightly shorter duration
- slightly harder surface
- not so deep penetration
- pretty fast response time
- not as long duration
- pretty hard surface
- only very superficial effect
- immediate response time (a few seconds)
- short duration
- icy and hard, almost slick surface
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 contain too little snow
- 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.
- 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).
- 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:
- Salt is foremost a problem when and where “salty” water (from melting) reaches ground water reservoirs, which may cause drinking water degradation
- 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 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.