24: Avalanches and Snow

Perhaps unsurprisingly, snow is a really important feature of life in the Arctic and Antarctic, as well as in high mountain areas all over the planet. It affects how people and animals can move around, can turn a beautiful hillside into a death trap and can provide a safe haven for animals and plants who spend winter below it’s surface. I have just spent a week knee deep in snow digging a bunch of snow pits, so thought to write this plog all about why and how holes in snow are useful, not only for science but also reindeer herding and winter activities like mountaineering and skiing. To learn some snow skills, read on…

My trusty rucksack and gear in a Swedish forest, ready to measure some snow! Picture: Author

Onions have layers, Ogres have layers, and snow has layers

In places that get reasonably deep snow (deeper than just a few cm) and where it stays throughout most of winter, the snow pack starts to build up layers. On top is the soft fresh snow that has recently fallen, whilst on the bottom is more dense snow which has been compacted by the weight above it over time. Throughout this older snow you might get layers of ice and soft, hard or crunchy snow depending on the weather conditions when that snow fell, or any changes in temperatures when it was already lying on the ground. Looking at these layers, where they are compared to each other and how they are structured is the important bit about snow science.

In order to look at these layers then, you basically have to dig a big hole so you can look at the snow side-on, from the shallow fresh stuff on top to the oldest snow lying on the ground. This side-on view is called the snow profile, and looking at this profile can help to forecast where and when avalanches might occur, both on mountains near settlements as well as areas where people might do winter sports like skiing. If you’re an outdoorsy person yourself these are tests you can do too when on a mountain in winter.

Layers of different hardness and density in a snow profile. Usually they are not this clear, and you have to find them by feeling the snow. Picture: Centre for Snow and Avalanche Studies


An avalanche can happen for many reasons. Shaking caused by earthquakes and explosives might move the ground enough to make the snow slide down en masse. Perhaps more common are avalanches cause by people on skis and snowmobiles, animals walking or even just new snow or rain building up on the snow surface. In these situations the snow slides because a slab of snow is sitting on top of a softer weaker layer, and so when enough extra weight is added to it on top, it causes the weak layer to crumble. The slab slides and starts to build up momentum as it goes downhill, picking up and shaking off more snow with it as it goes.

Avalanches are more likely to happen in late winter (December-April) as the snow layers have had time to build up. They also tend to happen more often on slopes that are shaded from direct sunlight (often north facing slopes) as they stay cooler and so the snow layers are less likely to bond. However, later in spring sunnier slopes can have a greater risk because there might be more warming and melting, causing wet snow slides.

So, what can you and a shovel do about this? Well, you can’t really stop an avalanche but you can predict where they are more likely to happen and so choose where you go to avoid them. By digging a snow pit you can look at the profile and see if there are softer layers lower down in the snow pack, lying under a harder snow slab on top. You can then put this together with info like which way the slope is facing, how much direct sun does it get, and how steep is the slope (generally steeper slopes are riskier) to get a feel for the safety of the area. Forest rangers might also use this kind of info to purposefully trigger a controlled avalanche when no one is around so people can go there afterwards and use the mountain. Probably best to leave that up to the experts though…

On a little side not, if you ever do get into an avalanche (which I very much hope you don’t) try to get off the slab as fast as possible. Otherwise grab for a tree or if this isn’t possible try to ‘swim’ in the avalanche to prevent you being pulled to the bottom when the snow settles.

File:Tuck Postcards by Frank Feller-Courtesy of the Newberry Collection-In the Tyrol, an Avalanche-nby LL10454.jpg
Avalanches can be huge and deadly, when large volumes of snow plummets down a mountainside. Image: Frank Feller/Public Domain


If you remember back to an earlier plog where I spoke about the research I’m doing on reindeer ecology, it doesn’t involve mountains or avalanches, so why would I care about snow structure? Even when it’s not sliding around, snow can cause many problems or even solutions for wildlife.

Though it is cold, snow acts as a very good insulator. Many animals and plants benefit by being covered by snow over winter when they are hibernating or dormant, as being under it keeps them warmer than being in the open air. In terms of reindeer, snow and ice coming together can freeze lakes and rivers allowing the animals to walk across them and so saving them from having to make big detours when they are migrating. If it is hard enough, reindeer can walk on the snow surface, but if it is too soft and deep their weight might constantly break through, making movement really slow and tiring. These kinds of conditions might make the reindeer avoid an area even if there is good food around, as reaching that food would just take too much energy.

On the other hand if the snow is too hard, so for example if there is a thick ice layer somewhere in the snow profile, the reindeer might be physically unable to dig through it to reach the plants below to eat. This means that once again food is around but they can’t reach it, and so either have to move to a new area to get food or risk starvation. This isn’t a hypothetical scenario- ice crusts in snow recently caused as many as 61,000 reindeer to starve to death over winter in Siberia, so it’s a big problem. For this reason reindeer herders pay a lot of attention to snow conditions and the ice layers. This involved a lot of fine-tuned knowledge about the environment, as the herders need to be able to predict how snow conditions will change over winter so they can bring their reindeer to the right pastures at the right time to reach food. If conditions are really bad they may even have to buy special feed to help the reindeer through difficult weather which can be really expensive. It can also cause some issues when the reindeer’s stomach tries to adjust from their natural winter food (largely lichen) to the richer stuff found in commercial feed.

Reindeer digging through the snow to reach food. Picture: Timo Helle

Understanding snow is a key part of being in cold places, whether you are skiing, hiking, a lichen or a reindeer. Being aware of what is going on beneath your feet in these places means you can make the most of the snow whilst appreciating and respecting that it is a strong force of nature that should be prodded or jumped on with care.

For more info:

How to measure snowfall: https://weatherworksinc.com/news/how-to-measure-snow

Basic info on digging snow pits to test for avalanche risk: https://avalanche.org/avalanche-encyclopedia/snowpit/

Music: kongano.com

Title picture: Author

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