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ABS Avalanche Airbag US

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HAZARD PATTERNS

The avalanche hazard patterns are recurring situations or events. The containment of these hazard patterns is primarily based on a comparison of the respective snowpack structure in combination with the subsequent weather events. In this chapter we explain the 10 hazard patterns that can occur over the winter.

GM1 - THE FIRST SNOWFALL

  • Weak layer close to the ground from early winter, which is built between the first and second snowfall
  • Usually there is a time interval of at least one week or more between the first and second snowfall
  • Remaining snowfields from the first snowfall turn into loose crystals, which means that fresh snow can bond less easily with the existing layer
  • Leads mainly to snow slab avalanches, typical skier avalanches, which are responsible for at least 95% of fatal avalanche accidents
  • The decisive factor for a possible avalanche hazard is the coherent cover of old snow in combination with the subsequent overlaying of drift snow

GM2 - GLIDING SNOW

  • Movement of the snowpack parallel to the slope
  • Downwards gliding snow cover leads to the formation of cracks, sometimes several metres deep, known as gliding snow mouths
  • Triggering can also take place without additional load
  • Frequent occurrence of gliding snow avalanches as the snowpack becomes progressively soaked
  • When planning the tour, care should be taken to avoid the area underneath the gliding snowmouth if possible

GM3 - RAIN

  • Load on the snowpack due to additional weight and reduced firmness
  • destruction of the connection between the snow crystals; the rain also serves as a lubricant
  • Serves as a classic alarm signal and usually leads to avalanches
  • Formation of hard crusts immediately after rainfall, which contribute to the development of adjacent loose weak layers susceptible to failure
  • Thin weak layers in the area of crusts are frequent causes of avalanche accidents
  • Exception: With very cold snow cover the water is immediately bound by freezing
  • Loss of firmness only starts with a "warmed up" snow cover when it approaches the melting point (0°C)

GM4 - COLD TO WARM / WARM TO COLD

  • Strong temperature changes between two weather phases, with significant differences in temperature between the underlying snow cover and the fresh snow that is added
  • Unfavorable effect of large temperature differences during snowfall on the avalanche situation
  • The condition for this hazard pattern is a relatively cold or relatively warm snow surface
  • Snowfall in combination with a strong temperature jump connects the old and fresh snow well with each other for the time being, but a weak layer can form with a time delay

GM5 - SNOW AFTER A LONG COLD PERIOD

  • Fresh snow after a long cold period in combination with strong wind
  • Conversion of snow to loose crystals during longer cold periods
  • Poor bonding of loose drift snow and old snow
  • Deposit of fresh snow in slipstream slopes can be detached under additional load
  • Abrupt increase in danger of snowfall under the influence of wind
  • Rapid increase in avalanche danger and usually a slow decrease
  • The problem is the rapid change of conditions, which requires an equally rapid adaptation of behaviour

GM6 - LOOSE SNOW AND WIND

  • Influence of wind on falling and already deposited snow
  • Wind is one of the most important factors in the formation of avalanches
  • Increased disturbance of snow deposits at cold temperatures
  • Loose, dry snow in combination with wind leads to entanglement and thus to an increased avalanche danger
  • Increased brittleness in colder snow, making the snow more sensitive to stress
  • The difference to hazard pattern 5 is the short-term occurrence

GM7 - AREAS WITH LITTLE SNOW IN SNOWY WINTERS

  • Winters with heavy snowfall benefit from more stable snow cover in contrast to winters with little snow
  • Normally fewer avalanche accidents in winters with more snow due to the favourable snowpack build-up
  • Exceptions in winters with heavy snowfall are slopes exposed to wind (with little snow)
  • Formation of weak layers is more likely to occur in areas with little snow
  • Special hazard in transition areas with much too little snow
  • Decrease in snow depth near edges, protruding rocks or crests
  • Avalanche triggering depends on the type and size of the disturbance (additional load)

GM8 - SNOWED-IN SURFACE FROST

  • Surface frost due to deposition of water vapour on the snow surface
  • No danger as long as surface tire is visible on the snow surface
  • Danger only arises when the surface tyre is covered by further snow
  • Most critical weak layer in snow and avalanche knowledge
  • Frequent formation of surface frost during cool, good weather periods
  • The nigg effect represents a special form of surface frost-formation
  • overcoating of warm, humid air on a ridge or grat
  • Nigg effect is limited to the immediate ridge area
  • Difficult to assess, especially if the surface frost is already slightly snowed in again

GM9 - SNOWED-IN SLEET

  • Sleet is a spherical form of precipitation
  • Snow-covered sleet serves as ideal gliding layer for avalanches
  • Danger only arises when a thick layer of sleet is snowed in
  • Shooting snow accumulated above the sleet layer combines only poorly with the weak layer
  • Sleet acts in the snow cover like a ball bearing for avalanches

GM10 - SPRING SITUATION

  • Special conditions in the touring areas due to rising temperatures and increasing solar radiation
  • Complex interaction of air temperature, humidity, radiation and wind lead to an increase in danger in the shortest possible time
  • Decisive parameters are the increasing moisture penetration or soaking of the snowpack, which causes a loss of stability
  • Important prerequisites for safe conditions are dry air and clear nights
  • Unfavourable effects on the snow cover are humid air and an overcast night sky
  • Higher danger level in the afternoon due to intensive solar radiation and heating during the day
  • Timely termination of tours due to rapid increase in risk
  • Time discipline and flexibility in route planning play an important role

ATTENTION: DURING THE WINTER SEVERAL HAZARD PATTERNS MAY OVERLAP, BUT USUALLY ONE ALWAYS DOMINATES.

Gefahrenmuster Überlagerung

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EXAMPLE

  • The top layer of snow was influenced by the wind, underneath is a thin, cold and loose layer of fresh snow (GM6, dominant)
  • Underneath is a thin angular layer under a thin melt harsh cover, which has been created by a cold-hot change (GM4)
  • There is a very loose layer in the depth, which was formed during a long cold period (GM5)

You can find more information about hazard patterns in the book lawine. by Rudi Mair & Patrick Nairz