a.proof 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 temporal occurrence
Danger period already in autumn, but typically in early winter.
Often on high (>2000m) and high alpine (>3000m) shady steep slopes.
Observation by reading wind signs in the terrain Keep an eye out for stored drifting snowpacks in the steep terrain PRACTICAL EXAMPLE
Schuchtkogel avalanche
Where - Ötztal Alps / 3350 m / NW slope / 40°
Avalanche most likely triggered by itself Snow cover build-up: glacier ice, floating snow, fresh snow influenced by the wind (second snowfall) © lawine.
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 temporal occurrence
Occurrence of gliding snow avalanches during the entire winter period
Occurs in all exposures, especially below 2500m.
With regard to the time of the triggering, gliding snow avalanches are among the most difficult to predict Avalanches are possible at any time of the day or night, even in stable snow conditions, as well as in warm and cold temperatures Recognition of the point of triggering through clearly visible sliding snow mouths Triggering of gliding snow avalanches are often observed in the same places as the previous winter PRACTICAL EXAMPLE
Avalanche Galzig
Where - Arlberg / 2000 m / S slope / 32°
Triggering independent of the additional load Detection of snowmelt snowflakes even before triggering Warm temperatures, high humidity, intensive sunlight Loss of strength due to soaking of the snowpack © lawine.
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) temporal occurrence
Rain can occur in any part of winter.
Risk patterns equally represented in all regions.
Rain is the easiest hazard pattern to detect PRACTICAL EXAMPLE
Avalanche Innsbruck Nordkette
Where - Western Northern Alps / 2000 m / S slope / 40°
Fresh snow with intense rain Several spontaneous avalanches © lawine.
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 temporal occurrence
Frequently in the south-facing (sun-exposed) area.
Avoidance of hazards through continuous monitoring of the weather pattern with regard to temperature jumps Hazard patterns are difficult to detect without looking into the snowpack Danger of avalanche triggering must be extracted from the avalanche situation report or verified by a separate snowpack examination PRACTICAL EXAMPLE
Avalanche Metzen
Where - Tuxer Alps / 2250 m / east slope / 45°
Days before the accident there were warm temperatures and rain After that came fresh snow with wind influence and cold temperatures © lawine.
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 temporal occurrence
Hazard patterns can often be observed in deep winter.
Occurs at all altitudes and exposures.
Fresh snow and/or wind after cold periods lead to an increased danger level Obvious signs of a snowpack that is susceptible to failure are spontaneous avalanches, wind vanes, snow cornices and wind angling PRACTICAL EXAMPLE
Avalanche Saupanzen
Where - Kitzbuehler Alps / 1950 m / east slope / 40°
Unfavourable snowpack build-up due to a cold January Spontaneous avalanches were obvious signs © lawine.
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 temporal occurrence
Hazard patterns can often be observed in deep winter.
Hazard patterns in all exposures.
The near-surface, wind-affected snow layer is affected. This means that this hazard pattern is quite clearly visible Snow plumes indicate large freight volumes A prerequisite is the correct interpretation of wind signals such as wind angels or weaves Forecast with the help of weather station graphics or a simple stick test, which can be used to detect tied snow packages under the loose snow Recognition of wind influence already possible during tour planning PRACTICAL EXAMPLE
Avalanche Kapall
Where - Arlberg / 2230 m / SW slope / 39°
Bitterly cold temperatures, fresh snow and wind New drift snow packages © lawine.
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) temporal occurrence
Hazard time period is often at the beginning of the year.
According to the weather conditions in very steep NW to W slopes.
Avoidance of hazards through continuous monitoring of the weather pattern with regard to temperature jumps PRACTICAL EXAMPLE
Avalanche Mutterberger Seespitze
Where - Stubaier Alps / 3200 m / N slope / 44°
Above average fresh snow volume Wind in the ridge area, resulting in snow transport and a snow cover that is susceptible to faults © lawine.
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 temporal occurrence
Special danger at the beginning of the year.
Surface frost is possible at all altitudes and exposures.
Difficult to detect and extremely dangerous hazard pattern It is important to observe the avalanche situation report Clues in the terrain are provided by spontaneous avalanches with rather low initial avalanche strength PRACTICAL EXAMPLE
Avalanche Fatlarspitze
Where - Arlberg / 2550 m / N slope / 35°
Days before surface frost during sunny, cold winter days Surface frost is covered by drifting snow and fresh snow Choice of tour destination not adapted to snow conditions © lawine.
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 temporal occurrence
Increased occurrence of sleet in spring with snow from cumulus clouds.
Mainly in alpine and high alpine terrain (>2500m).
Without digging into the snowpack, snow-covered sleet is one of the most difficult factors to detect that is relevant to avalanches Indication of imminent aggravation of danger through exact weather observation and basic meteorological knowledge PRACTICAL EXAMPLE
Avalanche Schalfkogel
Where - Ötztal Alps / 3180 m / ENE slope / 40-45°
Particularly pronounced layer of sleet together with further fresh snow Already spontaneous avalanches in moderately steep terrain choice of terrain too steep and poor visibility during the ascent © lawine.
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 temporal occurrence
Hazard time period is in late winter.
Earlier phase (February, March)
Late phase (April, May)
Signs of danger are strong moisture penetration and fresh wet snow avalanches Recognition of the hazard pattern through analysis of weather station graphics and weather and snow cover observation PRACTICAL EXAMPLE
Avalanche Malfrag
Where - Samnaun / 2250 m / SW slope / 40
Springlike temperatures and rising humidity Loss of firmness of the snow cover due to soaking due to rising temperatures during the day Only special situation report this winter season © lawine.
ATTENTION: DURING THE WINTER SEVERAL HAZARD PATTERNS MAY OVERLAP, BUT USUALLY ONE ALWAYS DOMINATES. © lawine.
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
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