AVALANCHE TYPES & SLOPE INCLINATION
If you are out in the mountains in winter, you should also be familiar with avalanches. To do this, you first need to know what types of avalanches exist and how they differ. Also the assessment of slope inclination is an important skill for winter sports athletes. For this reason, you will learn about the different types of avalanches and how to measure slope inclination.
Whether it is a snow lab-, sliding snow- or loose snow avalanche, all types are dangerous for winter sport athletes when we go ski touring, as well as freeriding, but how do they differ?
SNOW SLAB AVALANCHE
"The snow slab avalanche is the most dangerous type of avalanche.
It causes more than 90% of avalanche victims."
Wide, linear tear-off edge with flat lintel. The snow slab slides at high speed on a smooth layer of snow - little binding of the weak layer to the lower layer.
- Slope inclination at least 30°.
- Sliding surface or a weak layer in the snow cover
which is susceptible to failure (e.g. hardened old snow cover, snowed-in surface tyre)
- Triggered spontaneously, as well as by additional weight
The typical snow slab avalanche is 50m wide and up to 200m long. In dry and wet snow, drift snow can occur long after a snowfall! The wind is one of the most famous builders of snowboards.
As in a snow slab avalanche, the tear-off edge in a gliding snow avalanche is also wide and linear. However, the entire snowpack slides off here.
- Smooth ground: grass or rock
- The steeper the slope, the more likely the snow slides
- Damp snow layer on the ground
- Formation of a damp snow layer: warm and/or wet ground in mid-winter or melt water and rain seeps to the ground
Gliding snow avalanches cannot be triggered by additional loads, they go off spontaneously. A gliding snow avalanche often forms slowly - watch out for gliding snow cracks, so-called "fish mouths"!
© Henriette Morgenstern
WET SNOW AVALANCHE
In case of wet snow avalanches, a part of the snow slab in the area of the crack must be moist or wet. Due to the moisture penetration, there is a loss of stability in the snowpack.
- Soaking of the snow cover caused by rain or melt water
- High temperatures or rain
- Increased occurence in spring
Wet snow avalanches are mostly spontaneous and are only rarely triggered by winter sports athletes.
LOOSE SNOW AVALANCHE
The loose snow avalanche can be recognized by its punctiform crack and pear-shaped avalanche cone. The snow masses become wider and faster as they slide down.
- Slope inclination more than 35°
- Frequently after thaw or sunlight
- Unbound layer of new snow or even a moist snow cover as a warning sign
Avalanches of this type are triggered by skiers or other types of weight that cause the snow to move.
"1% of avalanche victims caused by loose snow avalanches"
© LWD Tirol
Dust avalanches often originate from snow slabs and can reach a speed of up to 300 km/h. They have a great potential for damage.
- Large or very large avalanche danger
- Cloud of dust due to high fall height and snow mixed with air
The danger for skiers and snowboarders: breathing in snow dust can lead to suffocation!
"97% of all avalanche accidents occur on slopes steeper than 30°"
The steepness of the slope has a great influence on the formation of avalanches. The steeper the slope, the easier it is for an avalanche to detach itself. With increasing steepness, the slope's downforce and the tension in the snowpack increase. In addition, the slope inclination and thus the angle of the solar radiation also influence the snow composition & the snowpack build-up.
TECHNIQUE FOR MEASURING SLOPE INCLINATION
WITH THE CARD
With the help of a topographic map, the slope inclination can be derived from the contour lines. To do this, the distances between the contour lines must be measured - the narrower they are, the steeper the terrain.
WITH THE HANDY
In the ABS A.WAY app you will find a built-in clinometer, altimeter and compass.
Bring both sticks to the same length. Put one stick with the handle in front down into the snow. Lift the stick by the handle, but do not pull it completely out of the snow - the tip will stick at the same point. Now the second stick comes into play: bring both sticks together with the handle. Let the second stick swing and let its tip touch the snow surface. If the tip of the stick reaches exactly the end of the imprint of stick 1, you can assume a slope inclination of about 30°. If the slope hits the snow below the mark, the slope is steeper than 30° - 10cm is about 3°!
HAIRPIN BEND & ROCK PENETRATION
Rule of thumb: If hairpin bends are necessary, one can assume that the slope is steeper than 30°. If the slope is rocky, the slope is usually steeper than 39°.
- If loose snow avalanches release themselves, the slope is steeper than 39°.
- With avalanche warning level 3 you should avoid 35° steep slopes
- With avalanche warning level 2 you should avoid 39° steep slopes
Anyone who travels in the mountains in winter should be familiar with avalanches. But not all avalanches are the same. Whether snow slab avalanche, sliding snow avalanche, wet snow avalanche, loose snow avalanche or dust avalanches - all these types of avalanches can be fatal for winter sports enthusiasts. But how do avalanches occur, what types are there and what are the differences?
EMERGENCE OF AVALANCHES
Basically, the term avalanche refers to the departure of a large amount of the snowpack. The structure of the snowpack has many influencing factors which, depending on their composition, can have a positive or negative effect on the avalanche probability. The snowpack consists of several layers which are formed under different meteorological phases. The weather plays a major role in the formation of the layers, which can have different properties depending on temperature, wind, solar radiation, snow or rain. The more these properties vary, the worse the layers bond. This leads to an unstable build-up of the snowpack and therefore to an increased avalanche danger. Especially the snow surface is exposed to weather conditions and can quickly change its character due to rising temperatures and sunlight. If the characteristics of the layers are too different, so-called weak layers are formed. There is only a small bond which, when overloaded, leads to cracks and triggers a snow avalanche.
A distinction is made between two scenarios in the development of avalanches. Avalanches can either occur spontaneously due to the composition of the snowpack in combination with weather conditions or they are triggered by external loads. Avalanches caused by additional loads are more frequent and are triggered by humans or animals. In periods when the avalanche danger is particularly high, extra blasting is carried out to deliberately release the accumulated snow from the dangerous places.
TYPES OF AVALANCHES
In order to recognize avalanches and then to behave correctly, one must first know the different types of avalanches. The types are as broad as the spectrum of triggering factors. Avalanches are divided into five avalanche types. Depending on the orientation of the slope, slope inclination and time, the different types of avalanches can occur more or less likely.
Snow slab avalanche
One speaks of a snow slab avalanche when an entire snow slab starts to slide. A slab of snow is usually triggered by external factors and is particularly dangerous because the break-off of the slab of snow is located above the point of failure. The fault in a slab avalanche is linear and parallel to the slope. These avalanches have a large diameter - the typical skier avalanche is 50m wide, 200m wide and 50m thick - and has a speed of about 50-100km/h. The snow cover in this type of avalanche is characterized by a poor binding of the snow crystals in the layers and therefore tends to form a weak layer. An unstable snowpack in combination with overloading leads to an avalanche, whereby winter sports enthusiasts usually have no chance of escaping from the avalanche and are dragged down the slope with it. The snow slab avalanche is the most dangerous type of avalanche and is usually responsible for an avalanche accident. Another requirement for an avalanche is a slope inclination of at least 30 degrees, only then can a slab of snow slide down the slope.
As in snow slab avalanches, the tear-off edge in sliding snow avalanches is linear. The difference between avalanches is that the entire snowpack slides off the ground and not just a single snow slab. This avalanche is not caused by a break in the weak layer, but by friction loss on smooth ground such as grass or rocks. Damp layers on the ground can also lead to friction loss. Sliding snow avalanches usually occur spontaneously, not through additional loading. Departures are possible at any time and are difficult to predict. For this reason one should pay attention to sliding snow cracks (fish mouths). These are cracks in the snow that reach down to the ground. Passages under fish mouths should be avoided as far as possible or crossed quickly and widely.
Wet snow avalanche
As the name suggests, in a wet snow avalanche, a part of the snow slab must be wet or damp in the crack. The loss of satiety is caused by water penetrating the snowpack due to rain or melt. Due to the high temperatures in spring, a release is possible especially in this season. Wet snow avalanches are mostly spontaneous and are rarely influenced by external factors. The wetter the snow, the flatter the terrain in which the avalanche starts.
Loose snow avalanche
Loose snow avalanches are always point-like and can be recognized by their pear-shaped cone. The loosened snow causes more and more snow to move and the avalanche therefore becomes bigger and bigger. Compared to a slab avalanche, it is slower and requires unsolidified snow on the surface as well as a weak bond to form. In this type of avalanche, a distinction is made between wet and dry loose snow avalanches. Dry avalanches are mostly spontaneous and occur after or during snowfall. However, skiers can also trigger the unbound new snow. The terrain usually has an inclination of 40 degrees or steeper. In contrast, wet avalanches are caused by strong solar radiation and high temperatures. When the snow cover is soaked through, the snow loses its firmness and slides off. They can already break off at an inclination of less than 40 degrees. Loose snow avalanches are often less dangerous for the trigger than for people further down the slope. The avalanche has even less power in the initial area, so the danger is higher with increasing snow volume.
Dust avalanches often occur after a large accumulation of new snow. They are caused by a snow slab and can reach a speed of up to 300km/h. Due to the large snow masses and speed, they have a high damage potential. The occurrence of dust avalanches is most often at a danger level from large to very large in the avalanche situation report. Because of the extreme drop height, the snow mixes with air. Dust avalanches are particularly dangerous for winter sports enthusiasts, as inhaling snow can lead to suffocation. Due to the high pressure, the snow is pressed into the airways and deposited in the lungs.
BEHAVIOUR IN CASE OF AVALANCHE DANGER
The preparation is extremely important before the adventure in the mountains. This includes not only choosing the route and packing the right equipment, but also research on the avalanche danger. The avalanche warning service provides information about the danger level, current avalanche problems and much more. Since avalanches can never be completely ruled out, you should assess the danger correctly and if it is too great, you should cancel the tour. However, if the danger of an avalanche is reasonable, the equipment should be packed up. Here you should have everything you need to be equipped in case of an emergency. Optimal is the emergency equipment (avalanche transceiver, shovel, probe) as well as an avalanche backpack to be prepared for snow avalanches. But the equipment alone without the knowledge of avalanche awareness does not help. The correct procedures in an avalanche should therefore be practiced continuously.
Just as important as the preparation is the behaviour on the mountain. Avalanche forecasts are always only guidelines that must be checked against the actual conditions. Even if the danger of avalanches was relatively low in the morning, this can change quickly during the day. Therefore, you should have a basic knowledge of avalanches if you are travelling in the mountains in winter. Especially arlam signs, hints and danger spots help to assess the avalanche danger on the spot. As soon as there is a danger of an avalanche, you should adapt your behaviour accordingly and not take any unnecessary risks. This can also mean changing the chosen route spontaneously.