Persistent Weak Layers – The Risk of Catastrophe Remains
- Henry
- Jan 15
- 5 min read
The peak of the danger has passed after a catastrophic week in the Alps – but the danger that remains still poses a critical risk.
This past weekend – 10th and 11th January 2026 – saw an insane amount of snowfall across the Alps, but concentrated in France. In places, around 1.5m of snow fell in around 24hrs, creating awesome conditions but drastically elevating the avalanche risk.
I don't know about you, but my Instagram has been full of videos of avalanches popping off all over the Alps, including through this week as the avalanche risk has supposedly decreases.
Catastrophe has reigned across the Alps and beyond. At least six people are confirmed to have been killed in avalanches, including a young ski patroller in Chamonix and a British man in La Plagne who was caught without an avalanche transceiver.
These catastrophic avalanches were made worse by what is known as a persistent weak layer, and this danger remains even as the avalanche forecast has reduced over this week.
This post will cover what a persistent weak layer is, how it forms, how it causes widespread carnage and why it remains a danger.
What is a Persistent Weak Layer?
At its most basic level, persistent weak layer (or PWL for short) is a thin layer of snow that splits the snowpack in two.
To dive in deeper, in an ideal world the snowpack – running from the surface down to the ground – is one consistent density; as snow falls over the course of the season, it morphs and sticks (sinters and binds) to the snow already there to create a single cohesive snowpack. Another option is the density of the snow gradually decreases from bottom to top.
A PWL is a thin layer of unique snow, measuring maximum 6mm in depth. It is unique in two senses; firstly, because it differs drastically from the snowpack around it, normally a very soft layer sitting on top of a rock hard layer of snow. Secondly, because the weak layer is made up of a type of snowflake called a “facet”, which are key to understanding why they form and why they are so dangerous.
Why do Facets Form?
Facets can form for a number of different reasons, but primarily due to a temperature difference in or on the snowpack that pulls moisture up or down through the snowpack.
Snow at the bottom of the snowpack always sits around freezing, 0°C. If there is large temperature gradient - such as a bitterly cold alpine night - to either the top of the snowpack, or a point in the middle of the snowpack, the snow will dry out.
As the snow dries out, individual snowflakes fail to morph (sinter) into the right shape that will allow it to stick (bind) to the snowflakes around it. These loose snowflakes – now a similar size and shape to salt flakes – stand proud either on top of the snowpack or deep within it, a layer of sugary, exceedingly low density snow.
You can see facets on the surface; an otherwise clean surface of snow will sparkle back at you like it is covered in diamonds. You can read more on this phenomenon in one of my first pieces here.
In this case, lots of snowfall in December were followed by long periods of mixed weather, creating large facets on the surface as early as the second week of December. Dry weather has then followed with very little, if any, snowfall until this past weekend. Then, suddenly, a metre-and-half has fallen on this weak layer that has been waiting to be covered up for nearly a month.
Why are PWLs so dangerous?
Facets on the surface are benign, until snow falls on top of them creating the PWL within the snowpack.
This clean layer of low density snow props up the layer on top of it, like a tray of champagne glasses supporting a tray sitting on top of it. When a skier puts energy into the snowpack on top, the necks of the champagne glasses shatter, collapsing the upper part of the snowpack down onto the lower part, with the broken sugary facets acting as a lubricant to allow the top layer of snow to slide over the other.
That’s step one. Step two is the consistency of the weak layer that allows this collapse to propagate far and wide throughout the snowpack. As a result, avalanches caused by PWLs are regularly far larger and more dangerous than those caused by other avalanche problems.
Why are PWLs so … Persistent?
The key part with PWLs is actually the first part – persistence.
That weak layer remains in the snowpack until a significant change in humidity or temperature of the overall snowpack allows the facets to sinter and bind as they should. More often than now, however, they are simply buried by more and more snow on top until they are no longer an issue again – the energy from a skier or snowboarder only impacts 1m down into the snowpack, meaning a greater snow depth than this will neutralise the weak layer.
This means that it is possible for weak layers to “reactivate” in the spring, when the snow begins to melt – as soon as the snow depth above the layer becomes 1m or less, suddenly it is a problem again.
Over the last two seasons, Avalanche Canada – the nationwide avalanche governing body for Canada – has issued blanket “no go” warnings for all or large parts of western Canada because of persistent weak layers refusing to go away.
On occasion, these facet layers can form deep inside the snowpack, rather than on the surface before being buried. This means that it is possible for a PWL to form without you even being aware of it in the first place.
PWLs, therefore, remain an issue even when the main force of the avalanche risk has passed. As the avalanche risk decreases – marginally, from Level 4 to Level 3 on the European Scale – the problem remains Persistent Weak Layers.
Vigilance, therefore, is key. Read your local avalanche forecast carefully, and plan backcountry travel in accordance. Carry all the right equipment – avalanche transceiver (turned on!), shovel, and probe – make sure you know how to use them, make sure you are making conservative terrain choices, and you manage your group accordingly.
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