01Why the Snowpack Is a Layered History Book
Every backcountry slope you ski is a vertical archive. The snowpack is not a single homogeneous block — it is a stack of layers, each one the frozen record of a single weather event: a storm, a windy afternoon, a clear cold night, a thaw, a refreeze. By mid-winter an alpine snowpack on a north-facing Alpine slope may hold 8 to 15 distinct layers within a depth of 120–200 cm.
Avalanches happen when a cohesive slab (a bonded layer of snow) sits on top of a weak layer that fails, all of it resting above a smooth bed surface. Three ingredients must coexist: a slab, a weak layer beneath it, and a slope steep enough to slide — almost all slab avalanches release on slopes between 30° and 45°, with the statistical peak around 38°. Below ~30° dry slabs rarely propagate; above ~50° snow tends to sluff continuously rather than accumulate into dangerous slabs.
The entire discipline of snowpack stability assessment is really one question asked in many forms: is there a weak layer buried in this history book, how reactive is it, and how widely will a fracture in it spread? The rest of this guide breaks that question into data you can actually observe and read.
02Snow Metamorphism: The 10°C/m Threshold That Decides Everything
Once snow lands, the crystals immediately begin to change shape — a process called metamorphism. Which direction they change is governed almost entirely by the temperature gradient: how fast temperature changes with depth through the snowpack.
The single most important number in snow science is this: *a gradient steeper than ~10°C per meter (equivalently 1°C per 10 cm) drives kinetic growth — faceting — which weakens the snow. A gradient gentler than that drives equilibrium* (rounding) metamorphism, which strengthens it.
The physics: the ground beneath the snow stays near 0°C all winter, while the snow surface on a cold clear night can drop to −15°C or colder. The temperature difference drives water vapor upward from warm bonds to cold layers. Under a weak gradient, vapor deposits gently and grains round off, bond, and sinter — this is good, strong snow. Under a strong gradient, vapor moves so fast that grains grow new angular, stepped, cup-shaped faces with almost no bonds between them. These facets are the seed of nearly every persistent avalanche problem.
| Condition | Temperature gradient | Metamorphism type | Effect on stability |
|---|---|---|---|
| Deep, warm snowpack | < 5°C/m | Equilibrium (rounding) | Strengthens — grains bond |
| Transitional | 5–10°C/m | Mixed | Neutral / slow |
| Shallow, cold snowpack | 10–20°C/m | Kinetic (faceting) | Weakens — facets grow |
| Very shallow, very cold | > 20°C/m | Strong kinetic | Rapid weakening — depth hoar |
The cruel arithmetic of this is that a thin snowpack is a dangerous one. A 60 cm cold snowpack packs the whole 0°C-to-−15°C difference into 60 cm — a gradient of 25°C/m, deep in faceting territory. The same air over a 200 cm snowpack gives only 7.5°C/m — safely rounding. This is why early-season shallow snow, and shallow rocky zones within an otherwise deep pack, are so often the trigger points.
Same cold air, opposite outcomes
Illustrative snow-pit profile. Above ~1°C per 10 cm the pack facets and weakens; below it, grains round and bond. Educational only.
Same air temperature, opposite outcome: the 200 cm pack sits at 7.5°C/m and strengthens, while the 60 cm pack hits 25°C/m — well above the 10°C/m faceting threshold — and rots into depth hoar. Thin snow is weak snow.
03The Three Persistent Weak Layers You Must Know
Weak layers come in two families. Non-persistent problems (new storm snow, wet snow) stabilize within hours to a couple of days. Persistent weak layers can stay reactive for weeks or even the entire season, and they are responsible for the deadliest, least-predictable avalanches. There are three you must be able to name and recognize.
1. Surface hoar — the frozen equivalent of dew. On clear, cold, calm, humid nights, feathery crystals grow on top of the snow surface, sometimes 5–40 mm tall and glittering in the sun. Beautiful, and deadly: when buried by the next storm, surface hoar becomes a near-frictionless, sheet-like weak layer that fractures and propagates across entire bowls, even into low-angle terrain. It can persist 4–6 weeks.
2. Faceted snow (near-surface and mid-pack facets) — angular grains formed by the strong gradients described above, often around buried crusts where vapor pools. Sugar-like, non-cohesive.
3. Depth hoar / basal facets — the largest faceted grains, cup-shaped and striated, up to 5–10 mm, formed at the very base of shallow, cold snowpacks under gradients exceeding 10°C/m sustained for weeks. Depth hoar collapses with an audible whumpf and produces full-depth, often unsurvivable slides. It is the classic continental-snowpack killer but appears in the Alps in cold, snow-starved early seasons and in shallow, shaded, rocky terrain.
The defining property of all three: they do not heal quickly. A storm slab forgives you in 48 hours. A buried surface-hoar or depth-hoar layer can punish a decision made a month later, which is why bulletins flag a Persistent Slab or Deep Persistent Slab problem long after the last snowfall.
Read the column, find the failure
Buried surface hoar
weak layerA persistent weak layer of feathery surface-hoar crystals that grew on a cold, clear night and then got buried. It stands the crystals up like a house of cards — low strength, poor structure, and reactive for weeks. This is the layer that collapses with a whumpf.
Illustrative snow-pit log, surface to ground. A cohesive slab resting on a persistent weak layer above a smooth bed surface is the classic slab-avalanche setup: load it and the weak layer collapses, fracturing under the slab. Educational only — dig and test your own pit.
04Slabs: Wind, Storm, and Persistent Problems
A weak layer is only dangerous if there is a cohesive slab sitting on it. Three slab problems dominate decision-making, and each has a different signature.
Storm slab — fresh, bonded new snow overloading a buried interface. Highest danger during and 24–48 hours after a storm. Sensitive to snowfall rate: loading faster than ~2–3 cm/hour, or a total of >30 cm of new snow, sharply raises reactivity. Generally stabilizes fast.
Wind slab — wind transports snow up to 5–10× faster than it falls, stripping windward slopes and depositing dense, chalky, hollow-sounding slabs on leeward aspects and in cross-loaded gullies. Wind slabs form even on bluebird days with no new snow, are often localized, and are the most common cause of skier-triggered avalanches in the Alps. Look for smooth, pillowy, matte-textured snow and cornices pointing to the lee aspect.
Persistent slab — a slab over one of the persistent weak layers above. The dangerous trait is low spatial predictability and remote triggering: you can trigger it from flat terrain below, or from a thin spot on the slope, and the fracture can run hundreds of meters. These demand the widest safety margin and the most conservative terrain choices.
| Problem | Forms in | Lifespan | Trigger sensitivity | Spatial predictability |
|---|---|---|---|---|
| Storm slab | During/after snowfall | Hours–2 days | High then fading | Moderate |
| Wind slab | Wind events, lee slopes | 1–4 days | High, localized | Moderate (read terrain) |
| Persistent slab | Over buried facets/SH | Weeks | Stubborn but high-consequence | Low — remote triggers |
| Deep persistent slab | Over basal depth hoar | Weeks–months | Low probability / extreme consequence | Very low |
05Reading the Avalanche Bulletin: The EAWS 1–5 Scale
In the Alps, your most important data source is the daily regional avalanche bulletin, issued on the European Avalanche Warning Services (EAWS) five-level scale. Crucially, the scale is not linear — danger and the number of avalanche-prone slopes roughly double with each step up.
| Level | Name | What it means | Skier reality |
|---|---|---|---|
| 1 | Low | Generally stable; isolated, hard-to-trigger features | Mostly favorable — still verify steep, extreme terrain |
| 2 | Moderate | Triggering possible on a few steep slopes | Most accidents happen at 2–3. Careful route choice |
| 3 | Considerable | Triggering likely on many steep slopes; some natural release | Demanding. Experts only on steep terrain; reduce slope angle |
| 4 | High | Triggering likely even on moderate terrain; large natural avalanches | Backcountry travel strongly limited |
| 5 | Very High | Numerous large natural avalanches, even on low-angle terrain | Avoid avalanche terrain entirely |
The counter-intuitive but vital statistic: the majority of avalanche fatalities occur at levels 2 (Moderate) and 3 (Considerable) — not at 4 or 5. At high levels people simply stay home; at moderate levels the hazard is patchy, tempting, and easy to underestimate. Treat the level as a starting point, never a green light.
Don't read only the number. Every bulletin specifies which avalanche problem is active, the critical aspects and elevations (shown on a rose/clock diagram), and a trend. A 'Considerable, persistent slab, north through east above 2200 m' tells you exactly which slopes to avoid — far more actionable than the headline digit alone.
06Field Tests: Compression Test and Rutschblock
The bulletin is regional; your slope is local. Snowpack tests let you sample the layering yourself. They are most useful for finding and characterizing weak layers — a clean, sudden failure is meaningful evidence of instability — but a 'stable' result on one test pit does not prove a slope is safe. Treat them as one data point among many, never as a clearance.
Compression Test (CT) — Isolate a 30 × 30 cm column. Tap from the wrist (10 taps), then elbow (10), then shoulder (10), recording when and how a layer fails: - CT 1–10 (Easy): very weak, alarming - CT 11–20 (Moderate): suspect - CT 21–30 (Hard): stronger, not safe by itself
The fracture character matters more than the number: a sudden, clean 'pop' (SP/SC) that slides as a block indicates a propagating weak layer and is a strong red flag. A resistant, rough, non-planar break is less concerning.
Rutschblock Test (RB) — A larger 2 m × 1.5 m block isolated on three sides, loaded by a skier in progressive steps (RB1 = fails while isolating, up to RB7 = won't fail even when jumped). It samples a more realistic, skier-sized area: - RB 1–3: poor stability — fractures under low load - RB 4–5: fair — fails under heavier load - RB 6–7: good — hard to trigger
Extended Column Test (ECT) specifically tests propagation: an 'ECTP' (propagation across the full 90 cm column) is one of the clearest warnings that a fracture will spread. The golden rule for all tests: they can confirm a slope is dangerous, but they can never confirm it is safe.
Will the fracture run? Tap to load the column
Illustrative ECT. A propagating result (ECTP) is one of the clearest warnings that a fracture will spread; a non-propagating (ECTN) or absent (ECTX) result never proves a slope is safe. Educational only — no substitute for training or the local bulletin.
07The Daily Go / No-Go Decision Framework
Stability assessment becomes useful only when it converts into a decision. Combine four data streams in order, treating any single strong red flag as a veto.
Step 1 — The Bulletin (the night before & morning). Note the danger level, the active avalanche problem(s), and the critical aspect/elevation rose. This sets your terrain budget.
Step 2 — Recent weather (the loading history). The biggest stability drivers are: - New snow: > 30 cm in 24 h, or > 20 cm on a buried weak layer, = elevated storm-slab danger. - Wind: sustained > 30–50 km/h moves snow; expect fresh wind slabs on lee aspects. - Rapid warming / first sun: spring warming or a +5°C swing can trigger wet-loose and wet-slab cycles within hours. - Rain on snow: an immediate, severe red flag.
Step 3 — The 'Big Five' observable red flags in the field. Any one means scale back: 1. Recent avalanches on similar aspects/elevations. 2. Whumpfing — collapsing sounds (a weak layer failing under you). 3. Shooting cracks radiating from your skis. 4. Heavy recent loading (snow or wind). 5. Rapid warming / wet snow (rollerballs, pinwheels, ski-pole punch-through).
Step 4 — Terrain choice (the only variable you fully control). You cannot change the snowpack; you can change the slope you stand on. Apply the Reduction Method heuristic — at higher danger levels, cap your maximum slope angle:
| Danger level | Default max slope angle (whole run, incl. above/below) |
|---|---|
| 1 — Low | Steep terrain acceptable with caution |
| 2 — Moderate | Avoid the steepest slopes (> ~40°) on flagged aspects |
| 3 — Considerable | Keep slopes < 35°, avoid flagged aspect/elevation |
| 4–5 — High/Very High | Stay on < 30° terrain, not connected to steeper slopes above |
Measure slope angle with an inclinometer or your phone — eyeballing routinely under-reads steepness by 5–10°. Remember: a gentle slope is only safe if nothing steep looms above it (connected terrain and runout zones count).
If the bulletin, the weather, and the field all agree the conditions are favorable for your chosen terrain — go, with spacing, one-at-a-time on suspect slopes, and rescue gear on. If they disagree, the conservative read wins. The mountain will be there next week.