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SnowFlurry 17 2016/17 | The altitude delimitations in the situation report.

How are the altitude delimitations for the prevailing avalanche problems in the avalanche situation report arrived at?

by Lukas Ruetz 02/24/2017
The indication of altitude in conjunction with avalanche problems and danger levels is the backbone of the avalanche warning. In most cases, the delineations are surprisingly accurate.

Drift snow problem & Fresh snow problem

The prevalence of a drift snow problem depends on the one hand on the wind force, the amount of transportable snow on the ground and the amount of fresh snow. On the other hand, it depends on the temperature: the colder it is, the easier it is to disturb drift snow packs (especially when they are fresh). It can happen that fresh drift snow packs are relatively difficult to disturb due to high temperatures. As a result, the height separation increases even though the wind speed is higher than the transport strength even at lower altitudes. The delineation is primarily based on station data in combination with knowledge of the current snow surface conditions. The situation is quite similar for the fresh snow problem. With both of these avalanche problems, the avalanche danger usually increases continuously with increasing altitude, so it becomes more dangerous as you go up.

Wet snow problem

The vertical extent of the wet snow problem can be predicted on the basis of the air temperature and the snowpack structure: Where is the snowpack already isothermal (has the same temperature from the ground to the surface, in this case 0°C - here the temperature measurement is just as important as the layer profile) or where does it still have temperature reserves, i.e. are there still layers with a snow temperature of a few degrees below zero? If it is isothermal, how wet is the snowpack at what altitude? This again requires weather stations, temperature forecasts and field surveys on the condition of the old snow cover. The risk of avalanches normally decreases with increasing altitude due to the colder temperatures. In the classic spring situation, the danger situation first arises at low altitudes before it calms down here due to thawing or several melting/freezing cycles. The danger zone then rises continuously with periods of fine weather and warm temperatures and increasingly affects shaded slopes. The hazard potential also shows a flowing transition in the terrain.

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Old snow problem

In the old snow problem, the height delimitation works most accurately on average: weak layers often form in the vicinity of melt crusts (explained in detail in SnowFlurry 4 2016/17) and melt crusts usually form in early and mid-winter - i.e. when the sun is low - due to rain. Rain in winter can be narrowed down very precisely in terms of height. Depending on the region, it is not uncommon to be able to determine the distribution of weak layers in at least the lower or upper, perhaps even both altitude limits to a maximum accuracy of +/- 50 altitude meters for an area.

If surface rime forms the weak layer of the old snow problem, the altitude delimitation is primarily based on observer reports during the formation phase of the surface rime.

In the case of a cold to warm / warm to cold problem, field investigations are again required regarding the condition of the old snow cover, especially the old snow surface: Where was the surface moist? At what altitude, slope steepness and exposure? Together with the surface temperature measurement of the stations, the possible formation can be estimated and the actual formation can be determined and assigned to certain altitude limits.

If an old snow problem improves - as a rule, the lower limit of the problem area slowly rises due to warmer temperatures at lower altitudes - the crystals that have been transformed in a build-up process will therefore bond better again due to the onset of degradative transformation, the change in the altitude limits can be recorded by stability tests in the limit areas using the size of the load and tendency to fracture propagation.

Old snow problems can occur with significantly sharper boundaries in relation to other avalanche problems.

Summary

Avalanche problems are narrowed down using all methods - not least because they often occur in combination. In the case of drifting snow and fresh snow, the focus remains on station data and weather forecasts. In the case of wet snow problems, snowpack investigations play a major role alongside these, whereby the focus here is more on the snow temperature, less on the layer profile and even less on stability tests. In the case of old snow problems, snowpack tests are the be-all and end-all when it comes to both formation and decay: here, the focus is on the layer profile, i.e. layer hardness, grain shapes and sizes, and primarily on the load test. In this case, the temperature measurement is quite useful for estimating the progressive build-up transformation, but is of little help with regard to the current status.

The common factor here is the integration into the overall situation with reflexive analysis and process thinking. This creates an overall picture that can classify the problems according to severity, region, altitude, exposure and slope steepness.

Note: The significance of the altitude data for the distribution of avalanche problems is relatively high. The problem areas can be narrowed down very precisely in many - but not all - cases, particularly with regard to the distribution of weak layers that have been transformed by building up.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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