- Short and sweet: a description of the current situation.
- A detour: "More snow = more avalanches?"
Current situation
In far-reaching parts of Tirol, quite favorable conditions currently reign. Avalanche danger generally depends on altitude during the morning hours: above about 2400 m danger is low, below that altitude danger is moderate due to the gliding snow problem. During the course of the day, avalanche danger increases to moderate for the most part, due to solar radiation and daytime warmth.
We currently are faced with two major threats: a gliding snow problem on steep, grass-covered slopes (also on rooftops of houses) and loose-snow avalanches on extremely steep, sunny slopes starting in the afternoon hours.
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| Typical picture of current situation: magenta arrows: loose-snow avalanches in extremely steep terrain; red ellipses: glide cracks on steep, grassy slopes. Northern Massif (photo: 13.02.2019) |
Clear-and-simple, the major peril stems from glide-snow avalanches which, due to the huge snow masses, can grow to dangerously large size. With conscientious tour planning, you ought to be able to assess avalanche starting zones (not always visible) as to whether glide-snow avalanches could release there, and thus, circumvent those areas. However, the risks of being forced to take a fall into open glide cracks also needs to be taken into consideration, particularly on freeriding tours in regions where snowfall has been heaviest.
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| Glide-snow avalanche in the Tux Alps on 06.02.2019. In background, loose-snow avalanches. |
Today, 14.02.2019 at 15:55, headquarters in Tirol was informed about an avalanche accident at Bodenbachfall in Kaunertal. One person was severely injured, according to our information. We don’t yet have any details, but it is clear that even relatively small (loose-snow?) avalanches in a narrow stream bed can be very dangerous for ice climbers.
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| Avalanche accident at Bodenbachfall in Kaunertal. In depiction, right, slope gradient legend |
Relegated to back-seat status recently: the snowdrift accumulations in high-alpine, shady, very steep terrain adjacent to ridgelines which can, in quite isolated cases, be triggered.
Current conditions remind one of springtime: star-studded nocturnal skies cool the snowpack down. On sunny slopes, a melt-freeze crust forms during the night. This is currently still breakable. In a few days, however, according to current weather forecasts of the ZAMG Weather Service, a melt-freeze crust capable of bearing loads will form, at very least at low and intermediate altitudes on very steep to extremely steep slopes. During the course of the day, this crust will soften and forfeit its firmness. Whoever ventures into outlying terrain early enough, will be rewarded by superb corn snow.
In the central and southern parts of East Tirol (the regions where we are still facing an old-snow problem) the situation has improved noticeably, compared to the beginning of February; nevertheless, the old-snow problem remains relevant. In those regions, one needs to ski defensively, particularly in shady terrain. In sunny terrain, caution is required towards the heightened proneness to triggering of the weak layers near ground level due to the increasing wetness of the snowpack.
More snow = more avalanches?
In far-reaching parts of Tirol there are currently massive amounts of snow down to low lying areas. What effect does all this snow have on the course of the winter with regard to snowpack stability and avalanche danger?
A little detour: types of avalanches
We distinguish three major types of avalanches, depending on their trigger mechanism:
- Slab avalanches,
- Loose-snow avalanches &
- Glide-snow avalanches
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| Fracture of a slab avalanche: steep terrain, slab on a weak layer |
Loose-snow avalanches - as the name implies - are of loosely-bonded snow. As a rule it is dry powder or thoroughly wet snow. A snow crystal moving downwards on its path towards the valley in extremely steep terrain (>40°) sweeps along other snow crystals with it. The avalanche which is unleashed thereby increases in volume ongoingly. What is typical of loose-snow avalanches is the pear-shaped avalanche path.
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| Wet loose-snow avalanche in rocky terrain |
Glide-snow avalanches behave completely differently from slab avalanches and loose-snow avalanches. This is a pure-and-simple gliding process. The entire snow mass loosens at the borderline of smooth ground surface and snowpack, and slides towards the valley as a result of melting processes. The more moist the ground, the less friction there is, and the more likely a glide-snow avalanche becomes. What matters above all else is a regular, smooth ground surface, e.g. rock plates, meadows, house rooftops. Glide-snow avalanches, in contrast to loose-snow avalanches and slab avalanches, can also slide over slopes where the gradient is less than 30°.
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| Numerous glide-snow avalanches in Zillertal Alps (photo: 22.01.2019) |
A snowy winter
What about avalanches in winters when there has been lots of snowfall?
Dry loose-snow avalanches occur more frequently than in winters where snowfall is infrequent, due to increased intensive snowfall.
Slab avalanches are frequently observed during periods of intensive snowfall. Thereafter, as a rule the snowpack stabilizes relatively swiftly. What is favourable is that due to the huge masses of snow, temperature disparities inside the snowpack tend to be minimal and thereby, weak, loose layers cannot form as easily as they do in winters when snowfall is less frequent. In North Tirol the snowpack tends to build up continually in early winter so that the overall snow cover is usually quite stably structured.
In the central and southern parts of East Tirol, on the other hand, it took until the end of January until the big snowfall periods arrived. Until that juncture of the season, the shallow snowpack formed strikingly weak layers near the ground which then, at the beginning of January, were easily triggered. The latent old-snow problem still remains a threat in those regions.
Winters with lots of snowfall are “glide-snow avalanche winters”. Glide-snow avalanches frequently announce themselves a good while before their actual release in the form of glide cracks, so-called “wishbones” in the snowpack surface.
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| Glide cracks on the Saile. View of Inntal (photo: 20.01.2019) |
The glide cracks are formed from heavy gliding in those places where the lower part of the snowpack glides more quickly than the upper part. A glide crack which widens over the course of several days can suddenly accelerate and release as a glide-snow avalanche. The moment when a glide-snow avalanche occurs is unpredictable and avalanches are not inevitable. Glide-snow avalanches can also occur abruptly, without a glide crack ever appearing.
A deep snowpack reinforces increased occurrences of glide-snow avalanches in twofold ways:
- The greater amount of snow increases the weight pressing downwards;
- Snow insulates well. Thus, the snow near the ground is usually around 0°C, in other words, moist, a prerequisite for glide-snow avalanches.
Nonetheless, there is one species that loves glide-snow avalanches:
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| Chamoix are able to graze wherever glide-snow avalanches have released. (photo: 17.01.2019) |
Rooftop avalanches: Over the last few days, increasingly frequent rooftop avalanches have been reported in Tirol. These, in principle, are glide-snow avalanches: water at the interface between roof and snow decreases the friction, raises the danger of a rooftop avalanche.
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| Snow glides over smooth rooftops (photo: 10.02.2019) |
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| Visible here on a metal rooftop: an ice layer, indicator of flowing water between rooftop and snowpack. (photo: 10.02.2019) |
