Avalanches and avalanche safety part 1

Table of contents

• Types of avalanches
• Risk factors
• Avalanche hazard scale

Types of avalanches

Before we discuss the different types of avalanches, we briefly explain how snow cover is formed. A snow cover consists of several layers. At the bottom is the first layer of snow. This layer fell during the first snowfall and is often very compressed by the pressure of the layers above it. Each time it snows, a new layer is created. This layer must be able to bind well with the layer of snow below. If a new snow layer does not bind well with the old one, the risk of avalanches grows enormously. However, there are several ways in which an avalanche can occur.

Slab Avalanche (Schneebrettlawine)

In slab avalanches, the top layer of snow slides down as one large slab (German: Brett). This can occur when there is a weak layer in the snow cover. It results in a large and powerful avalanche up to 3 metres thick with speeds of up to 140 km/hour. A slab avalanche is responsible for most avalanche casualties. Partly because the snow has a high density. Often, this type of avalanche is caused by the skier or snowboarder himself when entering the slope. A slab avalanche is clearly recognisable by the rip-off edge left behind.

Loose Snow Avalanches (Powder Avalanches)

A loose snow avalanche occurs when the top layer of snow on top of a more stable layer suddenly begins to slide. They are often associated with freshly fallen powder snow. These avalanches occur mainly on steep terrain and usually occur spontaneously. A loose snow avalanche starts small, but ends up larger (the so-called snowball effect). A loose snow avalanche has a low density and starts at about 100 km/h and can reach a top speed of up to 300 km/h.

Dust Avalanches

A dust avalanche occurs in areas with dry and loose powder snow. This snow has poor cohesion, making it difficult to maintain a stable layer. In this type of avalanche, the snow mixes with the air, creating a large dust cloud of snow. Dust avalanches are otherwise quite rare.

Wet Snow Avalanches

A wet snow avalanche occurs when the snow is saturated with water. Usually as a result of thaw or rain. The water in the snow deteriorates the joint and makes the snow heavier, resulting in an increasing risk of avalanches. At the same time, the heavier snow allows a wet snow avalanche to occur on a less steep slope. As early as about 20 degrees.

Wet snow avalanches often move slower than dry avalanches, around 10 to 40 kilometres per hour. These avalanches can also contain more debris, which, combined with the extra weight of water, makes them very dangerous. The extra weight makes the snow extremely compact, making it extra difficult to free someone/yourself. Wet snow avalanches are mainly seen at the end of the season.

Slush Avalanches (Ground Avalanches)

A gliding snow avalanche is a rare phenomenon. The entire snow cover slides away over the subsoil, leaving a huge hole in the snow cover, after which the subsoil becomes visible. These avalanches occur when the lower layer is weak and does not stay well on the surface. This can happen, for example, after a period of thaw, causing water to flow directly over the subsoil and weaken the lower layer. Usually, these avalanches occur spontaneously after the lower snow layer can no longer support the weight. Slush snow avalanches are very dangerous, as a lot of rocks and debris are carried away. Fortunately, it is rare that a skier or snowboarder ends up in one.

Risk factors

Weather conditions

Weather conditions obviously play a crucial role in the occurrence of avalanches. Rain makes snow heavier, making it less stable. At the same time, the snow saturates, significantly increasing the risk of avalanches.

Besides rain, new snowfall also has a major role in avalanche formation. New snow can lie unstably on top of the old layer, causing it to slide faster. Wind moves loose snow, which can create accumulations on the windward side of the mountain. These accumulations are unstable and can easily break loose, resulting in an avalanche.

Furthermore, temperature variations can change the structure of the snow. The combination of thawing during the day and freezing at night can create an icy layer, making it very difficult for the new layer of snow to bond with the old layer.

If all goes well, you can see a pattern now. It is very relevant to find out what the weather conditions have looked like over the past few days, as they say a lot about the possibility of avalanches.

Terrain

Another important factor in relation to avalanche risk is the slope of a descent. Flatter slopes are more stable. Steeper slopes obviously have a greater risk of avalanches. However, it is the slopes between 30 and 45 degrees that are generally considered most dangerous. This is because such slopes are flat enough so that snow can accumulate in large quantities and, at the same time, steep enough so that this accumulation can slide down at some point. At slopes below 25 degrees and above 55 degrees, the avalanche danger decreases again. Nevertheless, avalanches can occur on all slopes.

Plants and shrubs can make the snow cover more stable and therefore protect it. They can prevent large accumulations and retain snow. Avalanches are therefore more common in open areas without vegetation. So the chance of an avalanche is also affected by the surface of the snow cover. Something that you as a winter sports tourist cannot see, but that local organizations are knowledgeable about. Read more about such organizations below.

Avalanche hazard scale

That you can estimate avalanche risk based on the terrain and weather conditions is only right. But fortunately, you are not only on your own and estimates of avalanche risk are also made by professionals. The same official avalanche danger scale is used in every European ski resort. The level of avalanche danger is scaled to a certain level for each area by the local avalanche warning service. These levels/scales are as follows:

Avalanche hazard 1: low

At avalanche hazard 1, the probability of an avalanche is very low. The snow cover is quite firm and avalanches are possible only under very high loads on very steep slopes. If an avalanche occurs, it is likely to be quite small.

Avalanche hazard 2: moderate

At avalanche hazard 2, the chance of an avalanche is a lot higher. In most places the snow is still quite firm, but on some steep slopes the snow can be unstable. It often still takes a high load to cause an avalanche.

Avalanche hazard 3: significant

At avalanche hazard 3, the probability of an avalanche is really higher. On steep slopes the snow is not firm and even at low loads an avalanche can occur. The avalanches that can occur are medium to large. Many accidents happen at this level because people think it is not that dangerous and thus underestimate the danger.

Avalanche hazard 4: high

At avalanche hazard 4, it is easy to cause an avalanche. Even by a single person. From this level, the avalanche alarm will also flash. This is a light that flashes at every gondola and sign to warn people of the dangers. So always follow the local rules. At this level, it is unwise to go off-piste.

Avalanche hazard 5: very high

At avalanche danger 5, it is extremely unwise to go off-piste. This level rarely occurs, but in very heavy snowfall it can still take place. The snow is actually weak everywhere and on medium and steep slopes, large to very large avalanches can occur spontaneously. Unstable snow cover is guaranteed to cause problems.

So much for Part 1 of our 2-part series of blogs on avalanches and avalanche safety. In this blog, we looked at how avalanches can occur and how, to a certain extent, they can also be predicted. Fortunately, you do not have to rely solely on your own knowledge and estimations, but you can also use the advice/warnings from the local avalanche warning service. This way you can make an informed decision about whether you want to backcountry or not. In Part 2 we look at what you need to consider in case you actually go off-piste.