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AVALANCHE DANGER: THE CONTRIBUTING FACTORS

AVALANCHE FACTORS

Prerequisites for a slab avalanche

Avalanches result from the interplay of various factors. The critical factors are: weather and snowpack (conditions), terrain, people. 

The weather has a direct impact on the snowpack and, therefore, influences the current avalanche problem: if it is windy you must expect wind-drifted snow; if it snows you must expect a new snow problem, etc. Together, the weather and the snowpack make up the “conditions” factor. 

The terrain not only facilitates the formation of avalanches, it also determines the consequences of the avalanche. Rockfall beneath a steep slope can be a death trap, even in the event of a harmless little snowslide. 

Regardless  of the type of avalanche, they are most often triggered by people who go out onto a dangerous slope.

The set-up of a slab avalanche

The set-up of a slab avalanche

In order to correctly assess the avalanche danger, you must evaluate the three factors: “conditions” (weather and snowpack), “terrain” and “people” and determine how they relate to one another. This is done in three phases: at home when planning the tour, on-site in the mountains and on the individual slope. This is known as the 3x3 filter method. Conditions often seem to be the most difficult factor, because it requires you to consider both the weather and the current snowpack.

The set-up of a slab avalanche

AVALANCHE FACTOR CONDITIONS

The weather determines the snowpack. Together, they represent the “conditions” – a crucial parameter in avalanche danger.

AVALANCHE FACTOR CONDITIONS
AVALANCHE FACTOR CONDITIONS
AVALANCHE FACTOR CONDITIONS AVALANCHE FACTOR CONDITIONS AVALANCHE FACTOR CONDITIONS

AVALANCHE CONDITIONS

The snowpack  is determined by the weather, i.e. wind, temperature, sun exposure and precipitation. “Conditions” refer to the interplay between the factors “weather” and “snowpack”. 

Wind, known often in the past as “the architect of avalanches”, transports snow and forms “snow slabs”. The temperature influences the snowpack; it facilitates the formation of weak layers or leads to the snowpack heating up and, therefore, becoming unstable. Sun exposure also influences the snowpack. When it warms up the upper layers, they become wet and can later freeze. Precipitation – in the form of new snow or rain – often increases the avalanche danger level. A new layer of snow may enable a snow slab to form. Rain wets and weakens the snowpack.  Both rain and snow add additional stress

WEATHER

Precipitation creates and influences the snowpack over the course of the winter. A new snow layer often poorly bonds with the old snow layer. Therefore, avalanche danger typically increases during or shortly after snowfall. New snow is also an additional load for the old snowpack. Rain also constitutes an additional load, and warms and soaks the snowpack. During a period of precipitation, the amount of new snow, the temperature and the wind are the main factors for the development of avalanche danger. An unfavorable combination of these factors leads to what is called “critical new snow depth” – the avalanche danger level can be expected to increase.

CRITICAL AMOUNT OF NEW SNOW WIND TEMPERATURE RAIN

CRITICAL AMOUNT OF NEW SNOW

The “critical amount of new snow” is a rough rule of thumb for how much new snow indicates a significant increase in the avalanche danger. And both the wind and the temperature play a role. In unfavorable conditions during new snowfall, the wind causes drifting snow and, therefore, to massive, brittle snow slabs. Low temperatures slow down the process of the new snow sintering with the old snow surface.

WIND

The wind is said to be the “architect of avalanches”. However, this title is only partially applicable: While the wind is responsible for the formation of “snow slabs”, the slab can only slide away if it is on top of a weak layer. If this is the case, it is known as an “unfavorable layer”. Snow slabs such as these – also known as wind slabs – are predominantly found on untouched, lee aspects of steep slopes, behind terrain breaks, ridges and in bowls.

TEMPERATURE

The temperature influences the snowpack, the transformation processes within it and, in particular, the top 30–50cm of the snow layer. Cold, warm air, sun exposure and rain all influence the snowpack. Cold, for example, leads to “grainy”, non-cohesive layers that, when later covered in snow, can become dangerous weak layers. Heat initially leads to settling, but can also destabilize if the snow starts to melt and soak the snowpack.

RAIN

Rain is unfavorable for two reasons. Firstly, it adds a great deal of heat to the snowpack, destabilizing it. Secondly, it adds mass and, therefore, an additional load. This is because rain makes the top section of the snowpack heavier, which could overstress the weak layers below.

WIND-DRIFTED SNOW

Wind, the “architect of avalanches”, is responsible for the formation of wind slabs. New or old snow transported by the wind is especially brittle and perfect for forming a “slab”. This wind-drifted snow can optimally transfer stress and lead to the dreaded crack propagation. 

The wind transports the snow from areas exposed to the wind to zones sheltered from the wind, e.g. behind terrain breaks and ribs. Dangerous wind-drifted snow gathers in bowls and depressions. This often occurs along valley slopes. 

Wind-drifted snow Zones

The wind transports the snow from areas exposed to the wind to zones sheltered from the wind, e.g. behind terrain breaks and ribs. Dangerous wind-drifted snow gathers in bowls and depressions. This often occurs along valley slopes.

Wind-drifted snow is deposited everywhere in zones that are sheltered from the wind. This is most often on the leeward side (sheltered) behind obstacles such as ridges and terraces. But snowdrifts can also be found on the windward side in bowls and in front of and behind escarpments.

HOW DANGEROUS IS IT?

Wind-drifted snow is dangerous when a weak layer is situated within or directly  underneath the wind-drifted snow slab. Furthermore, the additional load of a wind slab can cause a weak layer further down in the snowpack to become overloaded.

MASTER BUILDER OF AVALANCHES

MASTER BUILDER OF AVALANCHES

Wind transports the snow from the windward (exposed) side to the leeward (sheltered) side. The snow crystals are broken down by the wind.  This process is also known as the mechanical aspect of “rounding”. Large new snow crystals are broken into small pieces that can be packed together very densely. This is what makes the “slab” brittle and compact – features that are required for crack propagation.

SNOWPACK – DANGEROUS AREAS

The snowpack is not the same everywhere. There is more snow in some areas and less in others depending on the slope aspect, elevation and terrain features. In shaded areas the snowpack is often loose, whereas on sunny slopes it is settled and more compact or has formed a crust. These geographical differences mean that there are dangerous areas and areas that have almost no avalanche risk.

The avalanche bulletin describes dangerous areas using the compass rose, among other things. Avalanches can often only be triggered in certain sectors of the compass rose.

SNOWPACK – DANGEROUS AREAS

Elevation is also often used to narrow down dangerous areas, e.g. when the snow line or temperature are decisive. But the timberline, which serves as a windbreaker, can also be a criterion.

Terrain features are often the key factor determining whether wind-drifted snow is deposited or not. However, transitions from little snow to much snow (e.g. steep sections) can be responsible for slab avalanches being triggered in low-snow areas.

SNOWPACK

TRANSFORMATION PROCESSES

The snowpack is not a static structure – it is constantly changing. Snow crystals transform permanently. There are three possible transformation processes in the snowpack: faceting, rounding and melt-freeze metamorphism.

Rounding Faceting Melt-freeze metamorphism

Rounding

Rounding is a process that has three steps : First, hexagonal crystals turn into “decomposed” snow and then into small, round crystals. As part of this metamorphosis, large new snow crystals are either broken down by the wind, or the branched crystals accumulate external water molecules close to their cores over time. This process takes time and is accelerated by relative heat (-8 to - 1°C) or pressure (the snow layer above).

Rounding

Faceting

Faceting turns decomposed or small, rounded crystals into large, faceted crystals. The transport of water vapor in the snowpack plays a key role in this. Water vapor from the warm ground (0°) sublimates – changes from a liquid to a gas – and rises. When it reaches crystals higher up, the moisture resublimates – turns from a gas back into a liquid – and crystallizes as ice . This causes the bottom of the crystals to grow. This results in faceted crystals, or even larger cup-shaped crystals. These faceted crystals have only a few contact points with one another and, therefore, form a non-cohesive, often soft layer – the “classic” weak layer. Additional stress can damage and collapse this layer. If there is a large area of faceted crystals and a suitable slab is lying on top of them, it can cause crack propagation and a snow slab.

Faceting

Melt-freeze metamorphism

Sooner or later, every snow crystal will be subject to melt-freeze metamorphism. Regardless of whether small and round or faceted, heat (above 0°) melts snow. The crystals are rounded, water is released. If the snowpack cools again at night, the free water refreezes forming crusts, ice slabs and melt forms. 

Melt-freeze metamorphism

AVALANCHE CONDITION TERRAIN

Terrain is significant in risk assessment for two reasons. Firstly, there are terrain features that facilitate slab avalanches; secondly, the terrain often forms “terrain traps”.

AVALANCHE CONDITION TERRAIN
AVALANCHE CONDITION TERRAIN
AVALANCHE CONDITION TERRAIN AVALANCHE CONDITION TERRAIN AVALANCHE CONDITION TERRAIN

AVALANCHE TERRAIN

Slab avalanches can most often be observed on flat, level slopes or in slightly concave slope shapes such as bowls and extensive gullies. By contrast, slab avalanches are more seldom on ridgelines, in narrow gullies and couloirs and in undulating terrain. This is because crack propagation occurs more easily when the snowpack is as homogenous as possible, i.e. very little height difference within the snowpack. In addition, a snow slab requires a certain size (>20 x 20m) and have a steepness of at least 30°. Slab avalanches usually occur on slopes with a steepness of 38°.

TYPICAL AVALANCHE TERRAIN

TYPICAL AVALANCHE TERRAIN

Typical terrain for slab avalanche is steep (over 30°), mostly untouched and homogenous. Large, undisturbed, homogenous areas or slight bowls are especially prone.

STEEPNESS AND DISTRIBUTION OF SLOPE ANGLES

On average, avalanches triggered by skiers occur on a slope angle of 38°. In isolated cases, slab avalanches can be triggered on slopes of 30° and above. Snow slabs that are triggered at lower angles stay in place due to friction. At lower slope angles, you will only hear a settling noise (whumpf sound) – a warning sign that the slab has been triggered and the crack has propagated. The only reason the snow slab does not glide down is the lack of steepness.

STEEPNESS SLOPE ANGLE AND DANGER LEVEL HOW TO ACT

STEEPNESS

Slab avalanches release from a slope steepness of 30°. The steeper the slope, the more likely that an avalanche will be triggered. Therefore: the steeper the slope, the more dangerous it is! On average, slab avalanches triggered by winter athletes release on slopes with a steepness of 38°.  At higher angles, the probability of triggering decreases – firstly, because extremely steep terrain is more rarely frequented and, secondly, because new or wind-drifted snow slides down as soon as it falls and more rarely forms unfavorable layers.

SLOPE ANGLE AND DANGER LEVEL

No matter the danger level, it is evident that slab avalanches release at an average steepness of 38°. At danger levels 2, 3 and 4, the distribution of avalanches is exactly the same regarding slope steepness. Therefore, it would be wrong to assume that a lower danger level means a greater slope steepness is required for an avalanche to be triggered. A higher danger level means only an increase in frequency or size of the dangerous area. Therefore, being buried is more probable at a higher danger level. (Source: S. Harvey)

HOW TO ACT

As a rule of thumb: At higher danger levels, you should avoid steeper terrain and act with appropriate caution. While slab avalanches do get triggered on slopes that are 30° or steeper (most often at 38°) regardless of the danger level, the size of the dangerous area and, therefore, the probability of stumbling across a dangerous slope increases with the danger level.

SLOPE ASPECT AND ELEVATION

There are often dangerous areas at particular slope aspects and elevations. There are around twice as many avalanche accidents on north-facing slopes as on south-facing slopes. However, it is not known how often the slope aspects are frequented in comparison with one another.

NORTH-FACING SLOPES EAST AND SOUTH-FACING SLOPES TEMPERATURE INCREASE AFTER NEW SNOW

NORTH-FACING SLOPES

The fact that most accidents occur on north-facing slopes could be because settling takes longer due to reduced exposure to sunlight, and because more weak layers form. Powder snow also lasts longer on north-facing slopes, which generally means that they are frequented more often.  

However, there are situations in which south-facing slopes are more dangerous than north-facing slopes.

EAST AND SOUTH-FACING SLOPES

When a temperature problem arises, sunny slopes – meaning south-facing slopes – are more dangerous. Cold temperatures and a strong north wind can also cause dry, wind-drifted snow slabs to form on south-facing slopes.

TEMPERATURE INCREASE AFTER NEW SNOW

If the temperature increases for the first time after fresh snowfall, the avalanche danger will often initially increase. This is because the settling of the snowpack causes the loose top layer of new snow to bond and thus develop the optimum characteristics for a slab. Only after a certain amount of time does the increase in temperature have a positive effect so that the slab bonds with the weak layer below. The danger level then decreases.

TERRAIN TRAPS

The terrain has a decisive influence on burial depth and mechanical injuries. This is why we talk about “terrain traps”. 

Ditches, bowls and obstacles in the runout zone increase burial depth and, therefore, decrease the chances of survival. Obstacles such as escarpments, rocks and trees in the path of an avalanche often lead to fatal injuries.

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Escarpments
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Rocks, trees
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Ditches, the course of a stream, bowls, crevasse in the runout zone
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Large slopes above (>60 vertical meters)

Escarpments

Risk of a fall and mechanical injuries. 

Rocks, trees

Mechanical injuries resulting from impact can be fatal. 

Ditches, the course of a stream, bowls, crevasse in the runout zone

All trap snow; there is a risk of deep burial, reducing the chances of survival. 

Large slopes above (>60 vertical meters)

The larger the slope above, the more snow can slide, causing deeper burial. 

TOPOGRAPHY AS A POSSIBLE CAUSE FOR DANGER

Terrain influences the risk of an avalanche in two ways. Firstly, slab avalanches are prominently seen on homogenous, extensive, slightly concave slopes. Wind and terrain also determine the lee areas where dangerous snow slabs can be found. Secondly, escarpments, rocks and trees often create life-threatening terrain traps. Ditches, bowls and pile-up zones in the runout zone of avalanche slopes can lead to deeper burials, causing increased fatalities.  

 The slope shape also influences a valanche formation. Avalanche accidents are particularly frequent at large, homogeneous and slightly concave slope shapes as well as wide gul lies and hollows. Accidents in convex or rugged terrain are less common. 

AVALANCHE FACTOR PEOPLE

A slope at risk of an avalanche is not a problem in itself – it is the people who create a problem by approaching it. 

AVALANCHE FACTORS – PEOPLE

In addition to the snowpack and terrain, people are a crucial parameter for avalanche accidents and (too) often forgotten. Yet snow slabs are almost always triggered by people. Beside the “hard facts” such as behavior, group size and ability, it is often soft skills – group dynamic, motivation, lack of communication and objectivity – that contribute toward avalanche accidents.

PEOPLE AS TRIGGERS

PEOPLE AS TRIGGERS 

Slab avalanches are predominantly triggered by skiers  or snowboarders . 95% of winter athletes trigger the snow slab themselves. In 90% of cases, it is the first person to step foot on the slope who triggers the avalanche. 

HARD FACTS – SOFT SKILLS

It is always helpful to be aware of classic group phenomena  when it comes to making avalanche decisions. In a group, people often feel safer than when they are alone, which tends to make them bolder and more willing to take risks. And when it comes to big groups, it’s often the loudest person who gets their own way. An open group dynamic, transparent communication and an ear for the “quiet voices” in the group can help make good decisions at critical moments.

Backing down can also be a mark of success – particularly in mountain sports!

Hard Facts Soft Skills

GROUP SIZE 

Large groups are often a problem. Firstly, they often move more slowly. Secondly, safety intervals pull the group far apart, which presents another challenge regarding timing and leadership; for this reason, the necessary distances are often not kept.

ABILITY 

Good skiers  are able to keep to the specified routes in the mountains and avoid falls in precarious sections that would lead to a heavy load on the snowpack. Good fitness will enable you to quickly pass critical sections one by one while keeping the necessary distance from other skiers.

GROUP HOMOGENEITY 

Similar motivation and comparable goals, equal ability and a speed matched to the group all help avoid tension and conflict.

GROUP PHENOMENA 

We often feel safer in a group than we do alone. This makes us more confident and willing to take risks – known as group polarization.

Large groups often have no clear decision-making structure: The person who argues loudest is often the one to get their own way.

Motivated people often argue to their own ends – with little room for the skepticism of others.

DECISION TRAPS 

Exhaustion reduces our attentiveness; other people can then easily influence our decisions.

The desire to experience something extraordinary and record it for Instagram and other social media sites can influence  our actions.

PRESSURE AND PROJECTION 

People who want to be accepted by a group and not stand out are often tempted to avoid asking awkward questions.

Your own personal motivation or even other people’s desires make objective decision-making more difficult.

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LAB SNOW QUIZ: AVALANCHE RISK

Avalanches result from the interplay of various factors. The critical factors are: weather and snowpack (conditions), terrain, people. In addition to danger levels, slope steepness, aspect, wind and warning signs, there are other important factors to consider when assessing avalanche risk.

Match the following conditions to the categories “increase avalanche risk” or “decrease avalanche risk”.

LAB SNOW QUIZ: AVALANCHE RISK

Match the following conditions to the categories “increase avalanche risk” or “decrease avalanche risk”.

Avalanche risk reducing
Avalanche risk increasing
GOING ON STEEP SLOPES INDIVIDUALLY
LARGE SLOPE ABOVE THE GROUP
EXTREME WATER INFILTRATION IN THE SNOWPACK
HIGH LOAD DUE TO A FALL
PRECIPITOUS TERRAIN
FRESH AVALANCHE DEPOSIT ZONE
TRACKS IN THE SNOW
SMALL, DISCIPLINED GROUPS
GOOD SKIING ABILITY
SMALL SLOPES WITH SMOOTH RUNOUTS
DISPARATE MOTIVATIONS
FRESH WIND SIGNS – SASTRUGI
Avalanche risk reducing
Avalanche risk reducing
Avalanche risk increasing
Avalanche risk increasing
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