Every ski tourer learns to read slope angle and aspect. Fewer stop to ask why the snow underfoot behaves so differently from one week to the next — or why a slope that felt solid on Monday can collapse under a skier on Thursday, with no fresh snowfall in between. The answer is almost always temperature.
We guide in Val d’Anniviers and Saas-Fee, two of the classic inner-alpine touring valleys of the Valais, and here more than almost anywhere else in the Alps, temperature — not snowfall totals — is what decides whether the snowpack holds together or fails. This guide explains why, and what it means for how we read conditions and plan a tour.
Table of Contents
- The Valais snowpack: a continental climate with a memory
- How temperature swings build weak layers
- Spring and the daily freeze-thaw cycle
- What this means for planning a tour
The Valais snowpack: a continental climate with a memory
Switzerland’s Alps don’t have one snowpack, they have several. Researchers at the WSL Institute for Snow and Avalanche Research (SLF) classify them into distinct snow and avalanche climates. Snow-rich, milder regions closer to the northern flank of the Alps have a maritime climate: frequent snowfall and frequent thaws mean the snowpack is dominated by rounded, melt-affected grains that bond well together.
Southern Valais valleys such as Anniviers and Saas fall largely into what SLF classifies as a continental climate, shared with parts of the Engadine. Continental snowpacks see less snowfall, colder average temperatures and far fewer melt cycles, so instead of rounding off, snow crystals are far more likely to facet: turning into loose, poorly bonded, sugary crystals that barely stick to their neighbours. Persistent weak layers — facets and depth hoar — dominate here in a way they rarely do further north.
That difference matters because these layers don’t disappear with the next storm. Once buried, a faceted layer can sit in the snowpack for weeks or months, waiting for enough load — new snow, a wind slab, or a skier — to bring it to failure. It’s a big reason why the “old snow” problem shows up so often, and so persistently, on the regional avalanche bulletin for our valleys.
How temperature swings build weak layers
Temperature-gradient metamorphism
Inside the snowpack, heat always moves from warm to cold. When the surface cools rapidly — most effectively on clear, still nights, when snow radiates heat straight out to a cold sky — while the base stays close to 0°C above the relatively warmer ground, a temperature gradient forms through the snow. Water vapour migrates along that gradient, from warmer layers toward colder ones, and that vapour transport is what drives the crystals to change shape.
Weak gradients let crystals round off and bond, the same process that strengthens a snowpack over time. Strong gradients do the opposite: they facet the crystals into angular, weakly bonded grains with little strength that also bond poorly to the layers above and below. The steeper the gradient, the faster and more completely this happens — and gradients are steepest exactly where the snowpack is shallow, because the same temperature difference between surface and ground is squeezed across less depth.
Why Val d’Anniviers and Saastal are prone to it
Put those two facts together and you have a near-perfect description of early-season conditions in our home valleys. High, dry, inner-alpine terrain with clear skies, strong overnight radiative cooling, and — especially before the base builds up in December — a thin snowpack sitting over rock and scree, is exactly the recipe for strong temperature gradients and rapid faceting. Shaded, wind-sheltered slopes, where snow sits undisturbed and cold, are particularly effective facet factories.
Once that faceted layer is buried under a storm or a wind slab later in the season, it becomes a structural flaw that can persist for months. It’s a big part of why touring here rewards paying attention to the whole season’s snow and temperature history, not just the last 24 hours before you go out.
Spring and the daily freeze-thaw cycle
When the freeze works for you
As the season turns and days lengthen, a different temperature-driven process takes over: the daily melt-freeze cycle. Sun and mild air soften the surface through the day; if temperatures drop enough overnight, that meltwater refreezes into a supportive crust and bonds between crystals strengthen. Each solid overnight refreeze is effectively a stability “reset” — it’s why the classic spring touring day starts early, crosses a firm, refrozen surface, and aims to be off the slope before the corn snow turns to slush.
When the freeze fails
The danger comes when that overnight recovery doesn’t happen — cloud cover trapping heat, a warm night, or air temperatures that simply don’t drop below freezing at your elevation. Without a refreeze, the snowpack starts the next day already weakened, and each additional warm day compounds the problem rather than resetting it. Conditions become markedly more dangerous once temperatures climb higher than the snowpack has experienced so far that season, because meltwater then penetrates deeper and can reach and weaken layers that had been stable all winter.
Rain does the same thing faster: it adds heat and water directly into the snowpack with no chance of refreeze, which is why a warm rain event — even a modest one — falling on a cold snowpack is treated as one of the more serious triggers for large, wet avalanches anywhere in the Alps, our valleys included.
What this means for planning a tour
None of this is meant to turn a ski tour into a physics exam — but a few habits translate the science directly into safer, better days out:
- Read the temperature curve, not just the forecast danger rating: overnight lows, the freezing level, cloud cover and wind all shape what the snowpack is doing before you even leave the car.
- Check which avalanche problem is flagged for your area on the SLF avalanche bulletin, not only the danger level — an “old snow” problem calls for very different terrain choices than a “wet snow” one.
- In spring, time your day around the sun: start early on a firm overnight refreeze, follow the aspects as they come into and go out of the sun, and get off steep, sun-affected slopes before they turn soft and wet.
- Treat persistent weak layers with respect even on a quiet-looking day — they give few visual clues, can propagate widely, and are exactly the kind of hazard where local, season-long knowledge (or a guide who has it) makes the real difference.
It’s this local knowledge — how the snowpack in Val d’Anniviers and Saas-Fee has actually behaved through the season, not just what fell last week — that we build every guided day around. If you’re planning a touring trip to the Valais, understanding why our snowpack behaves the way it does is as useful a piece of kit as your transceiver.