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WHAT TYPES OF AVALANCHES ARE THERE?

Avalanches are differentiated into three types based on their mechanics: Loose snow, slab and gliding avalanches.

WHAT TYPES OF AVALANCHES ARE THERE?
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“Dry” loose snow avalanche
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“Dry” loose snow avalanche
“Wet” loose snow avalanche
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“Wet” loose snow avalanche
Powder avalanche
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Powder avalanche
Slab avalanche
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Slab avalanche
Gliding avalanche
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Gliding avalanche
“Dry” loose snow avalanche

“Dry” loose snow avalanche

Loose snow avalanches, also known as point release avalanches, can be either dry or wet. They characteristically start from a single point and have a pear-shaped fall path. These avalanches are most often relatively small and hardly have the potential to bury someone. Dry loose snow avalanches can frequently be observed after new snowfall and in extremely steep terrain (over 40°).

“Wet” loose snow avalanche

“Wet” loose snow avalanche

Wet loose snow avalanches can be observed when there is a strong rise in temperature, often on steeper rock terrain (from approx. 35°). Once triggered, these avalanches can also advance into flatter areas.

Powder avalanche

Powder avalanche

Powder avalanches are dry avalanches with a track that runs over larger escarpments. The masses of snow accelerate due to the freefall and mix with the air. Depending upon how much snow rushes over the escarpment, powder avalanches can be harmless (spindrift) or create an enormous air blast – with major destructive potential. These avalanches occur more frequently on very high mountains, at low temperatures and where there are large escarpments (e.g. the Himalayas).

Slab avalanche

Slab avalanche

Slab avalanches are the deadliest type for winter athletes – approx. 99 %  of all victims are killed in slab avalanches. They typically have a distinctive fracture edge ranging from 20 to several hundred meters wide. Slab avalanches require a slope steepness of at least 30° to be triggered, are mostly dry and slide over a weak layer. The prerequisite for the formation of a slab avalanche is a “bonded snow layer” (the slab) that lies on top of a weak layer (often a very soft, faceted snow layer). If the weak layer is stressed (e.g. by people or blasts), the crack propagates in seconds so that the slab starts to accelerate and slide down.

Gliding avalanche

Gliding avalanche

A gliding avalanche is a special type that is usually observed on steep meadow slopes (from 27°). In a gliding avalanche, a snow slab glides on a film of water. It is formed when, for example, moisture from the snow trickles down and accumulates on top of flattened, long-stem grass (warm gliding avalanche). Sometimes, the moisture comes from the ground (wet meadow) or accumulates when the first snow falls on very wet meadow slopes. Typically, cracks or “fish mouths” form above the slab before it starts to glide. Gliding avalanches cannot be triggered by people or blasts. It is difficult to predict whether or when they will trigger.

Avalanche Types Avalanche Types Avalanche Types

HOW DO THE TYPES OF AVALANCHES DIFFER?

Science differentiates between avalanches and defines them primarily according to their mechanics and their triggering mechanisms. The following three types of avalanches are significant for us winter athletes: gliding avalanches, loose snow avalanches and slab avalanches.

These three main avalanche types  can be subdivided and more precisely classified according to additional criteria, e.g. whether they are dry or wet, whether they trigger in a layer close to the ground or in a weak layer in the snowpack, and whether they mix with air or rather “flow” along their track. Depending on the applicable criteria, they could be classed as a wet snow avalanche, dry avalanche, ground avalanche or powder avalanche. However, each of these sub-types is classified as one of the three types mentioned above: slab, loose snow or gliding avalanches.

In order to identify danger in the mountains and adequately assess the risk, it is essential that you can differentiate between the different types of avalanches and understand their triggering mechanisms.

GLIDING AVALANCHE SLAB AVALANCHE LOOSE SNOW AVALANCHE

GLIDING AVALANCHE

Although gliding avalanches often look like snow slabs, they very rarely claim any victims. This is because they cannot be triggered by winter athletes. A good rule of thumb is: don’t stay any longer than absolutely necessary under cracks in the snowpack or gliding avalanches that have already triggered. The risk of a gliding avalanches can easily be identified by the typical “fish mouth” cracks. 

  • Typical cracks above (fish mouths) 
  • Cannot be artificially triggered 
  • Steepness: >27°, often at low and mid elevations 
  • Glides on a film of water – most often on steep meadow slopes 
  • Very rarely causes fatalities 

SLAB AVALANCHE

Slab avalanche are most commonly triggered by winter athletes. 98%  of avalanche victims die in slab avalanches; 95% of those trigger the avalanche themselves. The fracture in the snowpack propagates rapidly with speeds between 80 and 200 km/h. This causes the weak layer to collapse and the snow slab to slide.   

  • Typical linear fracture edge 
  • Separate “slab” suddenly triggers 
  • Masses of snow slide as a cohesive plate (within seconds) 
  • Steepness: >30° 
  • Accounts for 98%  of all winter athletes falling victim to an avalanche  

LOOSE SNOW AVALANCHE

Loose snow avalanches account for around 2% of avalanche fatalities. They most often cause the athlete to tumble down rocky terrain, resulting in physical injuries rather than a fatal burial. These avalanches only usually occur on extremely steep terrain – dry loose snow avalanches after new snow; wet loose snow avalanches in the event of rain and a significant rise in temperature. 

  • Release from a single point and have a typical pear-shaped fall path 
  • Can be wet or dry, mostly occur on extremely steep rocky terrain 
  • Often catch winter athletes as they descend, dragging them down the mountain 
  • Steepness: around 40° (dry); from approx. 35° when the snow is wet 
  • Accounts for approx. 2% of avalanche fatalities 

QUIZ: TYPES OF AVALANCHES

Take our quiz to test your knowledge about the different types of avalanches. 

Select the correct answer and then click continue.

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Types of Avalanches
1 — What type of avalanche is shown in the picture?

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Types of Avalanches
2 — What type of avalanche is shown in the picture?

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Types of Avalanches
3 — What type of avalanche is shown in the picture?

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Congratulations!

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THE SLAB AVALANCHE
THE SLAB AVALANCHE

THE SLAB AVALANCHE

Slab avalanches are the dominant avalanche danger and the deadliest type for winter athletes. In 95% of cases, the victims trigger slab avalanches themselves. 

HOW DOES A SLAB AVALANCHE HAPPEN?

In a slab avalanche, the entire snow slab suddenly slides away. Once caught, it is almost impossible to escape. Every year, around 115 people die in avalanches in the Alps, most  of them in slab avalanches. Avalanche danger is not a constant. There are “avalanche periods” that are made very dangerous by the weather and snowpack situation – but this is often only the case a few days per year. Conditions can often be quite safe before and after these periods. Therefore, it is important to identify these specific dangerous periods. Avalanche danger frequently varies from place to place, and often only certain slope aspects, elevations or terrain features are affected. Therefore, it is generally quite possible to avoid dangerous areas.

THE “INGREDIENTS” OF A SLAB AVALANCHE

A snow slab is a rare occurrence. Very special conditions are required for a slab avalanche to occur. Only when all four of the following “ingredients” are present in the snowpack or terrain can a slab avalanche occur.

CONDITIONS FOR SLAB AVALANCHES

  • UNFAVORABLE LAYERING - Bound snow, often drift snow over soft weak layer, mostly with angular crystals 

  • INITIALIZATION - Human as a trigger of the system 

  • FRACTURE PROPAGATION - Homogeneous weak snow layer with good propagation tendency  

  • STEEPNESS (> 30°) – The snow slab will only slip if the slope steepness is over 30° 

HOW DOES A SLAB AVALANCHE HAPPEN?

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Layers
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Initiation
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Propagation
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Steepness

Layers

The first basic prerequisite for a snow slab is “unfavorable layering”, meaning that a bonded layer of snow is lying on top of a weak layer. The term “bonded” snow is not synonymous with wet or sticky snow. Even the loose yet lightly interlocking crystals of new snow are “bonded” enough to pass on stresses that could trigger an avalanche. Typical weak layers – layers with low cohesion between the snow crystals – include snow-covered surface hoar, depth hoar (cup-shaped crystals) and faceted crystals.  The propagation mechanism of a snow slab must always be considered as the interaction between the “slab” and the “weak layer” underneath.

Initiation

Secondly, this weak layer must be initiated, or triggered. The avalanche is usually triggered by a winter athlete  who impacts and stresses the weak layer. Whether and how easily a weak layer can be stressed depends upon the thickness and hardness of the snow slab above. One way to measure this is by observing your sinking depth and the depth of the weak layer.

Propagation

Thirdly, it depends whether the initiated fracture can propagate. If the slab is sufficiently bonded and the weak layer underneath correspondingly unstable, the fracture will propagate through the snowpack within fractions of a second. If a weak layer has low density (consisting mostly of large, faceted, non-cohesive crystals), the slab can be very soft and seem “unbonded” yet still slide away. Conversely, a resistant weak layer requires the slab to be harder or better bonded for the crack to propagate.

Steepness

The fourth prerequisite is a slope steep enough for the slab to slide down. A slab avalanche requires a slope angle greater than 30°. The slab also needs to be a certain size (at least 20x20 meters) in order to detach at the sides and slide down. The median steepness taken from the observed “skier avalanches” is 38° at the steepest part of the triggered snow slab.

What conditions are required for a slab avalanche to form?

SKI TOURING QUIZ:
THE SLAB AVALANCHE 

Slab avalanches are the dominant avalanche danger and the deadliest type for winter athletes. In 95% of cases, the victims trigger slab avalanches themselves. Drag the four ingredients for a snow slab to the area on the left.

SLAB AVALANCHE
THE SLAB AVALANCHE back

SKI TOURING QUIZ: THE SLAB AVALANCHE

What conditions are required for a slab avalanche to form? Drag the four ingredients for a snow slab to the area on the left.

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SLAB AVALANCHE

Drag the four ingredients for a snow slab to the area on the left.

SLOPE STEEPER THAN 30°

STRONG WIND

NORTH-FACING SLOPE

A WEAK LAYER ABOVE WELL-BONDED SNOW

ADDITIONAL LOAD

NEW SNOW

LOOSE SNOW

ABILITY OF THE WEAK LAYER TO PROPAGATE

SOFT SLIDING SURFACE

LOW TEMPERATURES DOWN TO -10°C

WELL-BONDED SNOW ABOVE A WEAK LAYER

NEW SNOW

Drag the four ingredients for a snow slab to the area on the left.

CON­GRAT­U­LA­TIONS!

Everything correctly assigned.

Quiz
SLAB AVALANCHE
THE SLAB AVALANCHE back

SKI TOURING QUIZ: THE SLAB AVALANCHE

What conditions are required for a slab avalanche to form? Drag the four ingredients for a snow slab to the area on the left.

1
2
3
4
SLAB AVALANCHE

Drag the four ingredients for a snow slab to the area on the left.

SLOPE STEEPER THAN 30°

STRONG WIND

NORTH-FACING SLOPE

A WEAK LAYER ABOVE WELL-BONDED SNOW

ADDITIONAL LOAD

NEW SNOW

LOOSE SNOW

ABILITY OF THE WEAK LAYER TO PROPAGATE

SOFT SLIDING SURFACE

LOW TEMPERATURES DOWN TO -10°C

WELL-BONDED SNOW ABOVE A WEAK LAYER

NEW SNOW

Drag the four ingredients for a snow slab to the area on the left.

CON­GRAT­U­LA­TIONS!

Everything correctly assigned.