While analyzing hundreds of mostly tragic avalanche accidents, Rudi and Patrick discovered that most accidents occur in typical, recurring hazard patterns. They therefore developed 10 typical, particularly accident-prone and frequent hazard patterns. Recognizing these avalanche hazard patterns should help winter sports enthusiasts to behave with appropriate caution - and thus avoid accidents.
Hazard pattern (gm) 6 - cold, loose fresh snow and wind "The wind is the master builder of avalanches": This classic saying by Wilhelm Paulcke from the 1930s still applies unchanged today. Wind influences both the falling and the already deposited snow and is one of the most important avalanche-forming factors. In the case of loose, dry snow, wind always leads to displacement and thus to an increase in the risk of avalanches!
The colder the snow, the more sensitive it is to stress, because the brittleness increases. This hazard pattern differs from gm.5 in that the cold, loose snow has not formed during a long period of cold weather, but in the short term. It either snowed shortly beforehand in cold temperatures without wind and then began to blow, or it began to snow without the influence of wind, with the wind increasing in strength during the snowfall. A pattern that can usually be recognized quite well.
An example: Avalanche HochgrabeAvalanche. A local mountain guide originally plans to go up one of his local mountains, the 2951 m high Hochgrabe, with a colleague in very adverse weather conditions. The friend cancels because of the weather. So the mountain guide decides to do the tour alone. He starts at around 7:30 am and heads towards the summit via the normal route. When he has not yet returned from the tour at 16:00 - as originally agreed - his wife alerts the Sillian mountain rescue service.
His car is found at around 17:30 and a small avalanche cone is discovered 30 minutes later. On this cone, they find the already lifeless ski tourer, partially sticking out of the snow.
Short analysis. At the end of March, the snow cover up to around 2800 m is usually completely soaked. High pressure prevails on 1. 4. The snow cover stabilizes due to intense nocturnal radiation in conjunction with very low humidity. A pronounced melted snow cover forms, which stabilizes the snow cover. Underneath it remains wet or moist at higher elevations. The weather changes on April 2. Exceptionally cold air enters Tyrol. It snows up to 40 cm in East Tyrol. In addition, the wind constantly increases in strength and blows from different directions. The temperature at 2000 m is a very cold -10°C. Fresh, cold and therefore brittle drift snow lies on a thin, loose layer of fresh snow, which in turn lies on a stable layer of old snow. The ski tourer is already on the descent when he triggers the avalanche while crossing the gully and is subsequently swept into the bed of the stream.
Bad luck: He is alone and cannot free himself from his predicament.
Where: Hochgrabe / Central East Tyrol / 2090 m / N slope / 35°
Who: 1 person involved / 1 person killed
When: 6. 4. 2003, approx. 11:00 a.m.
Avalanche: Slab avalanche (dry)/L 150 m/W 20 m/rupture 0.3-0.5 m/burial 0.4 m/approx. 7 hrs.
Regional hazard level: 2 (moderate)
Headline LLB: In high mountains, beware of drift snow accumulations
Hazard pattern (gm) 7 - areas with little snow in winters with lots of snow
Normally, significantly fewer avalanche accidents occur in winters with lots of snow than in winters with little snow because the snowpack structure is generally more favorable. Nevertheless, even in winters with plenty of snow, the phenomenon is regularly observed that slopes exposed to wind are relatively poor in snow due to prevailing weather conditions. The snowpack build-up there is correspondingly less favorable, making it all the more likely that winter sports enthusiasts will trigger avalanches there.
An example: Avalanche of small tasers
Avalanche. On April 22, 2006, a group of eight hikers choose a tour destination on the Kleiner Kaserer that is common for this time of year. They start at 7:00 am from Schmirntal and reach the summit without any problems. From the summit, they observe two South Tyrolean ski tourers descending the extremely steep northern flank. However, the group chooses the objectively safer route along the ascent track. From the ski depot, they ski at long intervals over an initially only moderately steep ridge to a small flattening of the slope. As four tour participants are already waiting at the meeting point, they hear a loud bang. As a result, a huge slab avalanche breaks loose and splits right in front of where they are standing. The other people are caught up in it. One of them manages to get out just in time, but the others are swept away over rocky terrain. One person dies at the scene due to serious injuries, the others sustain severe injuries.
Short analysis. There are two possible causes for the avalanche triggering: the triggering by the tour participants in the area of the moderately steep, relatively snow-poor ridge or the accidental breakage of the adjacent cornice, which initiates the avalanche. It is more likely that the winter sports enthusiasts triggered the avalanche in the area of the ridge with little snow, which is constantly influenced by the wind during the winter (during a very snowy winter). The snow profile we recorded shows a pronounced layer of floating snow there, although it is not poorly bonded everywhere. The crack is likely to have propagated from a super-weak zone after the load and the avalanche, including the cornice, may have subsequently run off.
Where: Kleiner Kaserer / Zillertal Alps / 2950 m / N slope / 30°-45°
Who: 8 people involved / 2 people injured / 1 person killed
When: 22. 4. 2006, 11:00 a.m.
Avalanche: Slab avalanche (dry)/L 800 m/W 150 m/rupture 0.3-1 m/burial 0.3 m/30 min.
Regional danger level: 1 (low) - 2 (moderate)
Headline LLB: Generally low danger in the morning - diurnal increase
Danger pattern (gm) 8 - snow-covered surface frost
Surface frost is one of the most beautiful types of snow and does not pose a potential danger on its own: It only becomes dangerous when it is covered by new, bound layers of snow and is therefore rightly regarded as one of the most critical weak layers in snow and avalanche science.
An example: Avalanche southern Löcherkogel
Avalanche. Four fit ski tourers set their sights on two long-awaited peaks, the Northern and Southern Löcherkogel, for the day. They quickly gain altitude and reach the Nördlicher Löcherkogel without any problems, keeping a safe distance. After a short rest, they descend first on powder and then on firn. In a small basin in the area of the Südlicher Löcherferner, they get ready again for the ascent and now set their sights on the Südlicher Löcherkogel. This ascent also takes place without any problems over very steep terrain. Once again, the ski tourers descend individually, but gather on the steep slope. The last of the group triggers a small avalanche below the summit, which spreads downwards. Two people are swept over a rockfall, one of them is seriously injured, the other suffers bruising. The other members were able to get out of the avalanche.
Short analysis. On March 26, 2007, the authors observed a pronounced, wonderfully glittering layer of surface frost on the slopes near the ridge of the 3038 m high Festkogel in the Southern Ötztal Alps as a result of the Nigg effect. On March 31 and April 1, it snows up to 30 cm with a rising southeast wind. Afterwards, another few centimetres fall under the influence of light winds. From then on, the situation is insidious and resembles the often quoted wolf in sheep's clothing. At that time, several avalanche accidents happen in this region in the shady steep terrain close to the ridge. The slope of the terrain ranges from steep to very steep to extremely steep in places. The avalanche is triggered in terrain that is just over 30° steep.
Where: Südlicher Löcherkogel / Southern Ötztal Alps / 2950 m / N slope / 40°
Who: 4 people involved / 2 people caught
When: 6. 4. 2007 / 13:45 h
Avalanche: Slab avalanche (dry) / length 400 m / width 25 m / depth 0.2 m
Regional danger level: 2 (moderate)
Headline LLB: Favorable touring conditions with a diurnal increase in danger
Danger pattern (gm) 9 - snow-covered sleet
Weak layers within the snowpack are often compared to ball bearings during avalanche courses. This image is only really appropriate for sleet: a spherical form of precipitation that tends to be deposited in spring during thundery showers. It is easy to imagine that drifting snow that accumulates on top is usually poorly connected to this weak layer, thus increasing the risk of avalanches. Sleet is often distributed over a small area and is usually difficult to recognize even by experts without looking at the snowpack. A thoroughly insidious affair, which fortunately only leads to problems in the short term.
An example: Avalanche Marchreisenspitze
The winter of 09/10 will be remembered more for its pronounced floating snow foundation than for its thick layers of sleet. Nevertheless, an unusually thick weak layer of sleet is impressive in this avalanche that went off happily. If you take a closer look, you can quickly find an explanation for the accident. The scenically impressive Lizumer Kar in the vicinity of the Axamer Lizum attracts mainly local ski tourers, including a loner who has set his sights on the Ampferstein. The route leads the last 400 vertical meters over very steep, shady terrain. The winter sports enthusiast ends his tour at a saddle just below the summit. During the subsequent descent, he enjoys the good snow conditions and reaches the vicinity of the rocky head that descends from the Marchreisenspitze. There he triggered a snow slab, which carried him about 150 m and buried him completely. After the avalanche stopped, his head was about 30 cm below the surface of the snow. He manages to move his head. In doing so, he secures his air supply from the outside. Several ski tourers on the Widdersberg opposite - including mountain rescuers - see the avalanche, raise the alarm and rush to the victim's aid. After 20 minutes, he can be freed from his predicament and then make his own descent into the valley.
Where: Marchreisenspitze / Northern Stubai Alps / 2300 m / N slope / 40°
Who: 1 person involved / uninjuredWhen: 4. 4. 2010, 10:15 a.m.
Avalanche: Slab avalanche (dry) / L 200 m / W 20 m / Start 0.4 m / Burial 0.3 m / 20 min.
Regional danger level: 3 (considerable)
Headline LLB: Fresh accumulations of drift snow, increasingly in shady steep terrain, are currently the main danger.
Danger pattern (gm) 10 - Spring situation
Spring poses a particular challenge for winter sports enthusiasts, but also for avalanche forecasters and avalanche commission members. Rarely are "safe" and "dangerous" so close together in terms of time, and rarely is the range of danger levels issued during one day so wide. On the one hand, the avalanche danger is hardly ever easier to assess than in stable firn conditions, but on the other hand, hardly ever are such large avalanches recorded during a winter as during critical spring situations, with the sometimes complex interplay of air temperature, air humidity, the influence of radiation and wind playing a decisive role alongside the snowpack structure. For winter sports enthusiasts, time discipline and flexibility in tour planning are more important than ever.
An example: Avalanche Pforzheimer Hütte
Avalanche. A family of three decide to head back down into the valley after a three-day stay at the Pforzheimer Hütte. They are equipped with snowshoes and head downhill along the ascent trail. Close to the hut - the so-called hut slope - they suddenly hear a loud bang. A huge avalanche breaks loose, sweeping all the people about 100 meters. Two of them are completely buried when the avalanche stops. They are able to push their heads or snowshoes through the snow cover. One of them is able to free himself, the other person is dug out with an ice axe. The third person is only partially buried, but suffers injuries. Almost every week from mid to late February 2009, the same picture is repeated: pronounced north-westerly weather conditions shovel moist air masses into the Alpine region. Then it clears up. The temperature gradually rises. The radiation becomes more intense. The snow cover structure is poor at this time. It is dominated by an alternation of thin, harder layers with very loose layers. Large amounts of fresh snow are repeatedly deposited on top, including on February 24. The snow cover is in a very unstable equilibrium at this time. In addition to the additional strain on winter sports enthusiasts, there was another decisive weather factor at the time of the accident. It was diffuse. This means that intensive counter-radiation leads to an additional weakening of the snow cover. Incidentally, the following day - February 28th - was the most avalanche-prone and dangerous day of the entire winter due to massive warming.
Where: Pforzheimer Hütte / Northern Stubai Alps / 2340 m / east-facing slope / 35°
Who: 3 people involved / 1 injured personWhen: 27. 2. 2009, 10:45 a.m.
Avalanche: Slab avalanche (dry) / L 350 m / W 180 m / avalanche start 0.5-1.5 m
Regional danger level: 3 (considerable)
Headline LLB: In areas affected by rain in the north-east of North Tyrol short-term high avalanche danger
