Firn mirror

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Firn mirror on the Rhone Glacier in June 2014

As Firnspiegel refers to a large-scale, very thin layer of ice on the snow surface with high reflectivity . It arises from the interaction of solar radiation, melting processes, wind and heat radiation under the right conditions. This creates a surface of an old snowpack that sometimes appears almost metallic in sunlight. This phenomenon occurs especially in late winter or spring when there is strong sunlight.

Emergence

When a layer of firn is formed, part of the water that melts due to solar radiation is held between the coarser grains of an old snowpack and freezes again at sunset. Thin ice disks form between the grains and grow together to form a kind of ice skin. The sun's rays can penetrate this translucent skin and melt the snow below. As a result, a cavity can form between the ice skin of the firn mirror and the snow below and the ice skin only rests on the snow below in a few places. The firn mirror itself cools down through radiation and evaporation and can be renewed from below through resublimation .

term

The term “firn mirror” was coined by Wilhelm Paulcke , who dealt with avalanche research at the beginning of the 20th century. The German term was also adopted in English-speaking countries, but the phenomenon is also referred to there as “sun crust”, “firn mirror” or “ice mirror”.

Alpinism

The appearance of a firn mirror indicates that the snow is twisted and has solidified. This means that the conditions tend to be good for ski mountaineers , since spontaneous loose snow avalanches are no longer to be expected. If fresh snow falls on a layer of firn, it is of decisive importance whether the surface is moist during the snowfall. If it is very cold during the snowfall and the surface is therefore dry, there is no strong bond between the new snow and the old snowpack. A weak layer can thus form in the snowpack structure at this point, on which the layer of snow slips off when loaded.

The Firnspiegel is not to be confused with melt-freeze or breakable crust . This occurs when a blanket of snow thaws through warming, is soaked through and then freezes again, especially if these processes are repeated several times.

literature

  • Alexander Stahr, Thomas Hartmann: Landscapes and landscape elements in the high mountains. Springer, Berlin 1999, ISBN 978-3-642-58466-4
  • Edward R. LaChapelle: Secrets of the Snow: Visual Clues to Avalanche and Ski Conditions. University of Washington Press, Seattle 2001, ISBN 978-0-295-98151-2
  • Wilhelm Paulcke: Practical knowledge of snow and avalanches. Springer, Berlin 1938

Individual evidence

  1. ↑ a b c Alexander Stahr, Thomas Hartmann: Landscapes and landscape elements in the high mountains. Springer, Berlin 1999, ISBN 978-3-642-58466-4 , pp. 151, 154.
  2. ^ Edward R. LaChapelle: Secrets of the Snow: Visual Clues to Avalanche and Ski Conditions. University of Washington Press, Seattle 2001, ISBN 978-0-295-98151-2 , pp. 80, 97.
  3. ↑ Thomas Schlatter: Weather Queries: Firn Spiegel: An “Ice Mirror”. In: Weatherwise. Volume 37, pp. 317-317, 1984, DOI: 10.1080 / 00431672.1984.9933268 .
  4. ↑ a b Wilhelm Paulcke: Practical snow and avalanche knowledge. Springer, Berlin 1938, p. 34ff. ( online )
  5. ↑ International Association of Cryospheric Sciences, Working Group on Snow Classification: The International classification for seasonal snow on the ground. In: Technical documents in hydrology. Volume 83, 2009 ( online )
  6. ↑ Eike Roth: Avalanches. Understand - Avoid - Practical Tips. Bergverlag Rother, Munich 2013, ISBN 3-7633-6085-9 , p. 48f. ( online )
  7. ↑ Tony Daffern: Backcountry Avalanche Safety: Skiers, Climbers, Boarders, Snowshoers. Rocky Mountain Books, Custer 2009, ISBN 978-1-897522-54-7 , pp. 57f.

Web links

Commons : Firnspiegel  - Collection of images, videos and audio files