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Mountain Safety Series - Part 2 - Understanding The Avalanche

Posted by Jennifer Ashton on
Mountain Safety Series - Part 2 - Understanding The Avalanche

In our last blog we took you through some basic information in regards to mountain safety and had a look at the single most important piece of kit you should carry when venturing anywhere off piste, the transceiver.

Here we dive a little deeper into avalanches and look at two more equipment items that you should think about carrying for backcountry adventures.

So what exactly causes an avalanche? Put simply, an avalanche is created when a layer of snow collapses and dislodges from its current position and slides downhill. Typically you will find a snow bed, with a weaker layer on top of it that may collapse or move with a snow slab on top of it.

Avalanches are naturally occurring events. However, in most cases where someone is caught in an avalanche, it has been triggered by the skier/boarder or someone in their party. The weakened layer is put under stress by the weight of the rider, which initiates the slide. According to the US Forest Service National Avalanche Centre, almost 90% of avalanches are triggered by humans.

‘Slab’ avalanches account for almost all avalanches that end in human fatalities. Put simply, a ‘slab’ of snow slides as one over the snow bed underneath it. According to the American Avalanche Association, slab avalanches are typically between 30 and 80cm deep, reach speeds of 20km and hour in the first 2 seconds after fracturing and can accelerate up to 130km/h after 6 seconds. Weak layers below the slab of snow are sensitive, meaning a quick change in weight can initiate the snow fracture causing the slab to slide. In most cases, the quick change in weight is a skier or snowboarder. Generally, the fracture in the snow happens above the rider, so they’re caught out in the slide.

This January, 7 skiers were killed in 3 days in 5 different avalanches in Switzerland. Two of these deaths occurred in 2 different avalanches on the same day in the resort of Verbier. All were slab avalanches.

Photo thanks to Verbier 4 Vallees Patrouilleurs & Zermatt Matterhorn -Avalanche Courses 

Other types of avalanches:

  • Icefall and Cornice fall avalanches – both occur with an overhang of either snow or ice. Icefall, when a glacier over hangs a cliff. Cornices are created by wind blowing snow drift around on the mountain. Typically both of these avalanches don’t cause many deaths. Icefall avalanches typically kill or injure ice climbers caught beneath the falling ice. While Cornice fall avalanches are often triggered by a skier or snowboarder, however if treaded with caution, falling cornices can actually be a test of the snowpack of the slope below.
  • Loose snow avalanches- ever watched big mountain skiers or snowboarders riding down steep spines? The loose snow released from these riders trigger loose snow avalanches. They tend to be small snow falls and often occur below the skier or rider so not to be caught up in the snow fall. Of course, this isn’t always 100% of the time. Loose snow avalanches tend to be a sign the snowpack below is relatively stable if they fall without causing a slab avalanche.
  • Glide and Wet Slide Avalanches- both are typically slow travelling avalanches (especially in comparison to a slab avalanche). A glide avalanche occurs when the entire snow base slides as a unit away from the ground. They tend to shift slowly, often taken a few days to shift. Wet avalanches occur when the weather warms, or rain causes water to travel through the snow. A wet avalanche being heavier and full of water, moves different to a slab avalanche, the snowpack being heavier and denser, reaching speeds of 20-30km/h. The heavy snow though, with enough momentum behind it can fell trees, flattening fences and damaging roads and rail lines. 

To the second part of our back country series, we look at the essential equipment you need to keep you (and those you are skiing with) safe. Once you’ve got your transceiver sorted, your next purchase is a simple one, a trusty Shovel and Probe.

Technically 2 pieces of equipment, but they go hand in hand with each other. You want to have both!

Think of a tent pole- long and skinny, thrown across the snow to deploy it and this is your probe. A probe is used to pinpoint the location of the buried individual after you’ve found where the signal is being received from. It can save you valuable seconds in the exact location and depth of the individual (think if they’re 2 metres under the snow and you dig and miss them by centimetres- sounds small but makes a big difference when time is against you).

Probes are usually made from aluminium or carbon fibre. Aluminium being a little cheaper, but heavier and therefore easier to push through the snow and debris. Carbon fibre is lighter to cart around all day, but can sometimes require a bit of force to push through the snow. You should be looking to have a probe of a minimum of 2 metres length. Most probes are 2.4 metres- making them practical without being excessive to carry.

A shovel is exactly that. But when it comes to saving a life not all shovels are built the same! Ensure your shovel is made of metal. Plastic shovels can bend under the stress of carrying heavy/wet snow. Typically they’ll be made of aluminium. The blade should have a sharp cutting edge to it, straight or serrated. Some well-designed shovels will allow the shovel face to either be used in a traditional sense, or be flipped 90° and used as an axe.

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