Generally, for each danger region the avalanche bulletin not only indicates the danger level but also the terrain where the danger is particularly significant. Example: "The avalanche prone locations are found on west- to north- to southeast-facing slopes above approximately 2000 m in particular."
It has become customary in backcountry touring to assume the danger level to be one level lower on slopes not meeting these criteria. This rule of thumb has largely proven its worth but, like every rule, is subject to exceptions. It can be used when planning backcountry tours, but must not take the place of on-site assessment. The rule of thumb is disregarded when producing the avalanche bulletin.
Most fatal avalanche accidents occur on slopes whose gradient, aspect and altitude meet the criteria mentioned in the relevant avalanche bulletin. This analysis disregards the danger level prevailing on the day when the accident occurs.
A north-facing slope falls to the north. If you are standing on a mountain summit looking towards the north (with the sun behind you at midday), the north-facing slope lies immediately in front of and below you. In mid-winter, steep north-facing slopes are not exposed to any direct solar radiation. A south-facing slope falls to the south and also receives regular sunshine, even in mid-winter.
In most cases the slope aspects that are particularly affected are indicated in the avalanche bulletin and, if possible, also illustrated in graphical form.
Conditions change gradually rather than abruptly from one aspect to another. Therefore, the edges of the area coloured in black are not clearly defined boundaries, but show marginal areas that cannot be clearly assigned to either the favourable or the unfavourable area.
References to particularly affected altitudes are generally made in increments of 200 m. In the case of dry-snow avalanches, the altitude above which there is an increased number of avalanche prone locations is usually cited. For wet-snow avalanches, the cited altitude is the one below which they are most likely to occur. Formulations referring to a range of altitudes, such as "between 2500 m and 3000 m", are only used occasionally.
Definitions of altitudes
Low altitudes: below approximately 1000 m
Intermediate altitudes: between approximately 1000 m and 2000 m
High altitudes: between approximately 2000 m and 3000 m
High alpine regions: above approximately 3000 m
The tree line is also used as a reference. This denotes the transitional area between forest land, which is sheltered from the wind, and open Alpine terrain, which is exposed to the wind. The tree line is situated at the transition between intermediate and high altitudes. It lies at approximately 2200 m in Central Valais and Engadine, at around 2000 m in the other Regions and at about 1800 m in the Prealps.
Conditions change gradually rather than abruptly from one altitude zone to another. Therefore, the given altitude is not a clearly defined boundary but shows a marginal area that cannot be clearly assigned to either the favourable or the unfavourable area.
Most skier-triggered avalanches occur on slopes that are between 35° and 40° at their steepest point.
The slope gradient category given in the avalanche bulletin indicates that there is an increased number of slopes with at least that gradient that are affected. The indicated values (e.g. "steeper than 35°") must, of course, be assumed to be approximate. They are to be regarded as guidelines, and transitional areas must be treated with caution as well. If no gradient is given, it can generally be assumed that steep slopes, i.e. slopes with a gradient of around 30° or more, are meant.
Definitions of slope gradients
Very steep, extreme terrain
Particularly unfavourable, for example as regards gradient, terrain profile, proximity to ridge lines or characteristics of the ground. This is mostly used in the avalanche bulletin in connection with danger level 1 (low).
Steeper than 40°
43% of fatal skier-triggered avalanches
Steeper than 35°
Long-term average: 82% of fatal skier-triggered avalanches
Steeper than 30°
97% of fatal skier-triggered avalanches
Less than 30°
3% of fatal skier-triggered avalanches
Shaded or shady slopes are more prevalent in mid-winter (when the sun is low in the sky) than in spring (when the sun is higher). Depending on the extent of the shadow cast by the near horizon, slopes with any aspect, not only north-facing ones, may be shady. Usually settling and bonding take a long time on such slopes. Conversely, sun-exposed or sunny slopes are more common in spring than in mid-winter. The snow on sunny slopes usually settles and bonds more quickly.
Windward slopes face the wind. Snow falling on these slopes is usually blown away. Lee slopes face downwind (away from the wind). Snow blown off the windward slopes is deposited here, forming a 'snowdrift accumulation'. Lee slopes often have many times the average snow depth and are therefore sometimes called 'wind-loaded slopes'. Windward and leeward slopes are found not only near mountain summits but also on slopes a considerable distance away from the ridge line. In such situations, the wind can be diverted by the terrain and deviate significantly from the naturally prevailing direction.
Sometimes, especially in relation to snowdrift, a more detailed description of the particularly affected terrain profiles is provided:
- Gullies and bowls, so concave slope profiles
- Areas adjacent to the ridge line, so especially slopes near the ridge and the summit
- Slopes behind abrupt changes in the terrain: These are often also a considerable distance away from the ridge line.
- Bases of rock walls: These are often very deeply covered in snowdrift.
If the principally affected- terrain profiles are limited, the avalanche prone locations are usually somewhat spatially limited and relatively easy to locate – at least by experienced individuals when there is good visibility.
As regards the prevalence of avalanche prone locations, the following sequence generally applies:
- (Very) steep slopes: Avalanche prone locations can be expected on all (very) steep slopes of the indicated aspect and altitude. As well as areas adjacent to the ridge line and gullies and bowls, slopes considerable distances away from the ridge line are affected.
- Wind-loaded slopes: Avalanche prone locations can be expected, in particular, on steep slopes of the indicated aspect and altitude where there is (fresh) snowdrift. This includes gullies and bowls filled with snowdrift.
- Gullies and bowls/areas adjacent to the ridge line: Avalanche prone locations are found in increased numbers in gullies and bowls/adjacent to the ridge line with the indicated aspect and altitude.