Hansueli Rhyner

Schnee begegnet uns in den verschiedensten Formen. Die großen Unterschiede der Eigenschaften von Schnee werden beim Bearbeiten offensichtlich: Frischer Neuschnee lässt sich mit einem Gebläse wegschaffen. Eine Absprungkante auf einer Rennpiste hingegen kann nur mit der Motorsäge bearbeitet werden. Verantwortlich für diese sehr unterschiedlichen Materialeigenschaften ist die große Spanne der Dichte, die Nähe des Schnees zum Schmelzpunkt und der sehr unterschiedliche Grad der Sinterung, wie im Folgenden erklärt wird.

To maximize performance in elite winter sports, considerable effort has been made to find gliding surfaces, which decrease snow friction to a minimum. Shifting away the focus from elite sport to mass winter sport equipment, this study aimed to evaluate materials providing acceptable gliding performance and in the same time high structural strength. Therefore, we investigated wet and dry snow friction of known engineering polymers with different surface configurations. Static and kinetic friction coefficients (COF) on snow of twelve gliders made of different polymers were measured on a linear friction tester placed in a cold chamber. Of each polymer, two gliders were built and tested: one with a smooth surface, another one with a stone grinded surface. Roughness parameters and dynamic contact angles were measured to characterize the surfaces. Dry and wet snow samples were prepared by grinding ice into powder followed by sieving, compressing and natural snow sintering. The snow surface temperature was measured before each experiment with a pyrometer. Liquid water content was generated directly before each experiment by applying infrared light. COF on wet and dry snow were correlated with mean R_a-values and mean contact angles. Mean static COF ranged from 0.05 to 0.375 on dry, and from 0.133 to 0.674 on wet snow. On dry snow, only a moderate negative relationship between static COF and contact angle was found (r_pearson = -0.6), whereas on wet snow a strong negative relationship was found (r_pearson = -0.92). Static COF and Ra showed a moderate relationship only on dry snow (r_pearson = 0.62). Excluding HDPE and ABS, smooth samples showed distinctly lower static friction than structured samples. The study showed that snow friction is a complex interplay of surface topography, hydrophobicity and mechanical properties, especially on wet snow. The typical texture for HDPE ski bases did not sufficiently decrease snow friction of the tested engineering polymers. On wet snow, hydrophobicity was the most important factor for good gliding. On dry snow, surface roughness had a stronger influence than hydrophobicity. Snow had clearly the strongest influence on the polymer - snow friction because it is a highly variable material with quickly changing physical properties.

Snow sports remain very popular. As well as classic skiing on slopes in traditional winter sports destinations like the Alps, new and often fun alternatives are now available in snow parks. Meanwhile, populous countries in Asia are discovering the potential of skiing as a recreational sport. All these developments are imposing increasingly stringent requirements on snow preparation. At the same time, climate change is presenting additional challenges for managers of slopes and cross-country ski trails. [...]

The holistic assessment and decision framework «3 x 3» is well established for ski and snowboard tours. Whereas difficult decisions can be postponed in the first two filters, «planning» and «local evaluation», a final decision is necessary in the third filter «individual slope». At that point, we can only decide 'to go' or 'not to go'. So far, an approach asking the right questions and answering them systematically was missing. The latest edition of the popular leaflet «Caution Avalanches» now includes a risk-based scheme for decision making at the individual slope. Three elements of avalanche risk assessment are considered: the avalanche triggering probability, the consequences of being caught and the behaviour, i.e. measures to reduce the risk. Three colours (blue, pink and orange) corresponding to the three elements guide the user through the decision-making process. In a first step, important questions have to be asked related to each of these topics. The answers can then visually be combined with the help of a diagram to estimate the avalanche risk. The scheme suggests an explicit decision 'go' or 'no go' and helps not to forget anything essential. In addition to the established methods, such as the «graphical reduction method» or thinking in «avalanche problems», the presented tool is an aid to reflect important factors for making decisions at the individual slope. It especially applies to slopes steeper than 30° and to slopes that are either not obviously critical or unproblematic. We demonstrate how to apply this risk-based approach to make better decisions at the individual slope.

Schnee und Eis nehmen rasant und weltweit ab. Es gibt einige Ausnahmen z. B. in Karakorum oder auf dem ostantarktischen Eisschild. Im Europäischen Bergland und besonders in den Alpen haben die Temperaturen bereits stärker zugenommen, und Schnee und Eis haben stärker abgenommen als im globalen Mittel. Diese Trends werden sich in den kommenden Jahren und Jahrzehnten fortsetzen. In unserem Beitrag fassen wir den Stand des Wissens über die natürliche Variabilität der Schneedecke im Bergland und über die zukünftige Entwicklung zusammen. Ausgehend von diesem Wissen werden spezifische Anpassungsmöglichkeiten für Skigebiete diskutiert. Aufgrund der hohen Variabilität (in Raum und Zeit) von Naturschnee, die mit der erwarteten Zunahme von persistenten Wetterlagen noch ausgeprägter wird, wird es immer wichtiger, dass sich Skigebiete mit technischen Mitteln wie Schneelagerung und Schneeproduktion absichern. Die höheren Temperaturen führen aber auch zu immer kürzeren Zeitfenstern für die Schneeproduktion. Deshalb sollten Anpassungsmassnahmen auch auf lokale Prognosen von Wetter und Schnee über Mittelfristzeiträume von
mehreren Wochen zurückgreifen.

Snow and ice are decreasing worldwide and rapidly with a few exceptions e.g. in some high mountains (Karakoram) and in some locations in East Antarctica. The European mountains and in particular the Alps have already experienced larger temperature increase and a stronger decrease of snow and ice than the global average and these trends will continue over the coming years and decades. In this contribution, we review the state of knowledge regarding the variability of the natural snow cover and its predicted changes. Based on this knowledge specific adaptation techniques for ski areas are discussed. Because of the high variability (in space and time) of natural snow, which includes an expected increase in persistent weather types, it will be increasingly important for ski areas to rely on technical adaptation measures such as snow farming and technical snow production. However, higher temperatures will also decrease the time that can be used for snow production. Therefore, potential adaptation should rely on improved local prediction of snow and weather especially if those predictions can be made for several weeks in advance.

«Learning by doing» is an obvious credo for every instructor. In avalanche education, however, trying out everything yourself can be dangerous. Furthermore, learning about avalanches requires some theoretical background, which has to be taught. Avalanche instructors tend to explain a lot without the participants having to deal seriously with the topic. Using cooperative ways of learning, the learning attention and the learning success can be increased distinctly. The idea behind this form of learning consists of the following: a) Give tasks and stimulate participants to think; b) Discuss in different forms of groups and exchange knowledge and ideas, and c) Present answers and results to understand the content. Based on the content of the leaflet «Caution Avalanches», we put together a training-kit for cooperative learning. The kit is especially designed for teaching in the field and includes topics such as interpreting the avalanche danger scale, typical avalanche situations, graphical reduction method, trip planning, decision making, human factors. Guides and instructors of avalanche courses as well as participants have had good experience of working with this kit and appreciate the diverse way of learning.

Data from 27 snow profiles taken in frozen rock walls at two sites in the Swiss Alps reveal that steep slopes have distinctive snowpack characteristics. Snow pits were dug in 50–65° slopes at elevations between 2900 and 3600 m asl on north- and south-facing slopes at Gemsstock and Jungfraujoch Sphinx in the winters 2012–2013 and 2013–2014. There were marked contrasts in snow characteristics between the two aspects, yet strong inter-site similarities. Under the influence of intense solar radiation, basal ice layers and multiple hard melt-freeze crusts formed on the south-facing slopes. Soft layers of facets and depth hoar developed between the crusts. On the shady north-facing slopes, thick basal melt-freeze crusts formed when snow persisted during stable weather periods in autumn. The dominant snow grain types in winter were facets and depth hoar. When solar elevation exceeded slope angle from mid-April onwards, gravity-driven percolation of melt water flowing parallel to the frozen rock surface from areas with warm protruding rocks led to the formation of thick basal ice layers in the north-facing slopes. Windward slopes were covered with rime and glaze during storms, regardless of aspect and season. Despite widespread snowpack instability, the formation of large slab avalanches was hindered by the pronounced roughness of the rock surfaces. The main drivers contributing to the distinctive character of snow covers in frozen rock walls are the negative rock surface temperatures, enhanced/minimized solar radiation and multidirectional fluxes of water, vapour and heat induced by the steepness of the rock slopes.

Geological investigations and seismic refraction tomography reveal a series of 70° steep, parallel and continuous fractures at 2950 m asl within the Gemsstock rock ridge (Central Swiss Alps), at the lower fringe of alpine permafrost Temperature measurements in a 40 m horizontal borehole through the base of the ridge show that whilst conductive heat transfer dominates within the rock mass, brief negative and positive temperature anomalies are registered in summer. These have very small amplitudes and coincide with summer rainfall events lasting longer than 12 h. In contrast, a complete lack of anomalous thermal signals during spring snowmelt suggests that runoff does not penetrate the open joints, despite high snow water equivalents of around 400 mm. This is attributed to the development of an approximately 20 cm thick, continuous and impermeable basal ice layer which forms at the interface between the snow cover and the cold rock on the shady North facing rock wall during snowmelt. Spring snowmelt water therefore does not affect rock temperatures in the centre of the rock mass, despite the presence of deep open joints. The mechanical impact of snowmelt infiltration on rock wall stability at depth is thus assumed to be negligible at this site.

"Lawinen und Recht" ist ein besonderes Spannungsfeld. Die Freiheit der Berge trifft auf die Welt der Paragraphen. So ist es nicht erstaunlich, dass die Protagonisten der beiden "Lager" sich nicht immer verstehen. Selbstverständlich trägt da die allgemein zu beobachtende, gesellschaftliche Tendenz, nach jedem Unfall nach Möglichkeit einen Schuldigen zu benennen, nicht gerade zur Entspannung bei. Umso wichtiger ist es, dass die Vertreter des Wintersports, die Alpenvereine selbst, den Kontakt zur Welt des Rechts suchen und pflegen. Der Österreichische Alpenverein u.a. waren da mit ihrem Seminar für Richter und Staatsanwälte wegweisend. In der Schweiz hat das Schnee- und Lawinenforschungsinstitut SLF in Davos erstmals 1994 ein Forum zu diesem  Thema organisiert.

The ski manufacturing industry is characterized by short product life cycles and high innovation pressure in order to meet customer's expectations of progressive improvements of skis. To reduce development time a FE model was developed and validated to simulate the influence of changes of the ski construction on its bending and torsional stiffness. Moreover, the study aimed to evaluate the feasibility of a novel ski sandwich construction with a load dependent bending stiffness. Therefore, the FE model was used to define required strain-stiffening of a single construction layer material to reach a perceptible change of the overall ski's bending stiffness. Two existing skis with identical geometry but distinctive differences of their torsional and bending stiffness were modelled using ANSYS Workbench. The sandwich construction was modelled by 18 solid bodies with bonded contacts: core, sidewalls, edges, upper and lower face with 13 layers including 3 additional resin interlayers. Orthotropic linear elastic material properties were taken from data sheets of suppliers and from material tests of the manufacturer. Sweep meshing was used to create an all-hex mesh of solid elements. Static structural simulations of a 3-point bending test and a torsion test were run in accordance to existing laboratory tests to validate the model. For both skis, the FE model showed good agreement with the experimental data for ski A and B. The simulated overall bending stiffness (center spring constant) was, respectively 2.8% and 3.2% higher compared to the experiments. The elastic curves revealed the model as slightly too stiff at the afterbody and too soft at the forebody. The simulated torsional stiffness (torsional spring constant) was, respectively 2.1% and 4.2% lower than found in the experiments. In the development process, for example, the model was then used to quantify the influence of edge profile heights on the ski's overall bending stiffness (3% per 0.1 mm edge height). The application of strain-stiffening materials to realize a load depending ski stiffness turned as not feasible due to too small strains within the ski structure. A realistic representation of the different construction layers of an alpine ski is still challenging, especially due to the heterogeneity of the fibre compound layers and the resin distribution. Using bulk properties of the upper and lower face of the sandwich construction is not an alternative. To virtually test the influence of new materials and layups every single layer has to be represented. Using two skis for validation and additional resin interlayers for calibration appeared appropriate in order to achieve adequate model results.