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Information about the snowpack stability map

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During wintertime the snowpack stability map is normally updated daily, provided that new snow profiles are available. It contains snow profiles, mostly produced on slopes, and the results of rutschblock tests. The information is obtained by SLF observers and employees. Each individual profile is assessed by an avalanche forecaster and assigned to a stability class (red=weak, orange=moderate, green=good) and a profile type (see below). Clicking on the symbols displays the individual snow profiles.

Snow profiles provide a snapshot of the snowpack at a specific time and place.

  • A snow profile describes the snowpack at the profile site but, in view of the spatial variability of snowpack layering and, in particular, stability, gives only an approximate indication of conditions in its vicinity.
  • Depending on the snowpack and weather pattern, the information can remain valid for a long time or, in case of fresh snow, wind or moistening, for example, it can quickly become unreliable. Irrespective of such changes, the profiles are usually shown in the map for a certain period, usually for the first 7 days after they are produced.

corresponding map

 

Rutschblock testing

The rutschblock test remains the most reliable indicator of stability. For this reason the SLF observers usually perform the test at the slope profile sites; on rare occasions they conduct one or two extended column tests. Rutschblock testing furnishes information on how easily a fracture was produced (at the profile site) and how effectively the fracture propagated.

  • The rutschblock score describes how easily the fracture was initiated. Scores 1 to 3 indicate relatively unstable conditions, and scores 6 and 7 are symptoms of a comparatively stable snowpack. Rutschblock scores 4 and 5 lie in the intermediate range.
  • The size of the released portion shows how easily the fracture propagated. If the entire block failed, the fracture propagated easily. If only a portion of the block failed (directly underneath the skis, or only one corner), a fracture was relatively difficult to propagate.
 

Snow layering

The snowpack is the principal determining factor in the formation of avalanches. To facilitate reliable forecasting of the avalanche danger, when snow is starting to fall or to melt for example, the avalanche warning service must possess prior knowledge of the bonding of the old snowpack.

In order to assess the avalanche danger, it is important to know whether weak layers exist in the snowpack and, if they do exist, how extensive they are. Weak layers often consist of large faceted grains whose properties usually change only slowly. Information on the layering of the snowpack therefore generally remains valid longer than the results of rutschblock tests.

Ram resistance profile (general hardness pattern)

The ram resistance profile is an objective measure of layer hardness, but is unable to detect thin weak layers. It describes the general strength or extent of bonding within the snowpack and also helps analysts to determine whether avalanches triggered on the surface can release the old snow.Ram resistance profiles are assigned to one of the ten defined profile types. Each of profile types 1 to 5 has a weak base, and the other types have a strong base. Profile types 1, 4, 5, 7, 8 and 9 occur more frequently in less favourable snowpacks, while the other types usually indicate more favourable conditions.

„Lemons“

Certain attributes of a weak layer, or which exist at the interface between two layers, are likely to indicate the existence of a potential weak layer. The more of these attributes (‘lemons’) that are present in a layer, the greater the likelihood of weakness.

CriterionCritical attributeRemarks
Grain size in weak layer≥ 1.25 mm diameterThe larger the grains, the less bonding there is between the grains in the snow layer.
Grain size difference between two contiguous layers≥ 0.75 mmIn most cases there is only little bonding at distinct interfaces.
Hardness difference between two contiguous layers≥ 2 steps on the hand hardness index

Hardness differences give rise to stress concentrations.

Hand hardness scale: fist / 4 fingers / 1 finger / pencil / knife blade / ice

Hardness of weak layerHand hardness: fist or fist to 4 fingerFist: a fist can be pressed into the layer
Grain shape in weak layerFaceted crystals (FC)

There is little bonding between faceted grains of snow.

Typical faceted forms are: large faceted snow crystals (depth hoar), cup-shaped crystals, surface hoar

Snow-covered weak layer 1 m In the case of even deeper weak layers, triggering by winter sport participants is less probable

Temperature

The greater the temperature gradient, the more quickly faceting takes place. Water can penetrate the snowpack to a significant depth only if the snow is (at least approximately) 0 °C ‘warm’ (zero degrees isothermal).Moisture/water contentA mid-winter snowpack is typically colder than 0 °C and therefore dry. The snow is often 0 °C ‘warm’ only at the interface with the ground, even in mid-winter. If the snowpack becomes wet at this boundary, gliding avalanches can be released on a smooth substratum.Moist or wet snow is 0 °C ‘warm’. Distinctions are made according to the following scale:

Moisture/water content

A mid-winter snowpack is typically colder than 0 °C and therefore dry. The snow is often 0 °C ‘warm’ only at the interface with the ground, even in mid-winter. If the snowpack becomes wet at this boundary, gliding avalanches can be released on a smooth substratum.

Moist or wet snow is 0 °C ‘warm’. Distinctions are made according to the following scale:

1 drySnow colder than 0 °C, difficult to form into a ball
2 slightly moistSnow is sticky
3 moistWater is present but does not run off when light pressure is applied
4 wetWater runs off when light pressure is applied
5 very wetsaturated with water ('slush')

From moisture code 3, the snowpack may contain free circulating water (funicular regime). The initial moistening of weak layers consisting of large grains is a particularly critical process in the formation of wet-snow avalanches. If water seeps into the snowpack, it accumulates at the interface between fine and coarse-grained layers. This raises the water content at the layer boundary and significantly reduces the strength of the snowpack.

 

Estimating stability

The stability of the snowpack at the profile site can be estimated by reference to the rutschblock results and the number of ‘lemons’. The following rule of thumb applies:

CriterionUnfavourable
Rutschblock score≤ 3
Rutschblock: released portionEntire block
'Lemons'≥ 5

The greater the extent to which the criteria are satisfied, the more likely it is that snowpack stability is unfavourable.

  • If two or all three of the criteria are in the unfavourable range, snowpack stability is said to be poor.
  • If only one criterion is in the unfavourable range, stability is assumed to be more or less moderate.
  • If none of the criteria is in the unfavourable range, this indicates that snowpack stability at the profile site is fairly good.

Nonetheless, a favourable snow profile should never be the only criterion applied in avalanche-related technical considerations.

 

Example

(Large image PDF)

Hardness

Snowpack hardness is indicated by the ram resistance (blue) and hand hardness (grey) profiles. The deepest 60 cm consist of layers of medium hardness, while the next 30 cm are hard (hand hardness index 4-5). Immediately on top are a good 30 cm of soft snow (‘fist’), which is covered by an approximately 7 cm-thick harder layer. The very top layer consists of soft snow.

  • Weak layers are typically characterised by a hardness of ‘fist’ (hand hardness index 1) or ‘four fingers’ (hand hardness index 2). In the illustrated profile, this criterion is satisfied by a thin layer at 55 cm and several soft layers between 85 cm and 115 cm.
  • Hardness differences between contiguous layers of at least two steps on the hand hardness index indicate areas that are especially prone to fracture propagation. This criterion is most clearly satisfied at the layer boundaries existing at 58 cm (from index value 2 to 4-5) and 115 cm (from index value 1 to 5).

Grain shape and size

Typical weak layers consist of large faceted grains, which generally do not bond together well. There is less bonding between faceted grains than between large, rounded grains, and less bonding between large grains than between small grains – measured by volume. In the illustrated profile, these poor bonding conditions exist in most of the soft layers described above. Layers with highly disparate grain diameters are often poorly bonded as well. The layer released in the rutschblock test falls just short of the critical value for this criterion (≥ 0.75 mm).

Rutschblock

The rutschblock was released at the 115 cm interface with a score of 4 (skier jumps once). This corresponds to a failure triggered by a ‘medium’ loading. The failure of the entire block is a symptom of good fracture propagation.

Temperature

The snow is colder than 0 °C all the way through and therefore dry. These conditions apply at the base of the snowpack as well, which indicates the absence of a gliding snow problem at the profile site.