01Why the bulletin talks about 'problems', not just a number
Avalanche problem types are the recurring ways a snowpack fails, and every modern bulletin names the active one beside the danger level. Europe's EAWS uses five: new snow, wind slab, persistent weak layers, wet snow and gliding snow. The US avalanche centres split them into nine. Each has its own terrain, warning signs, behaviour and management.
At the top level, every problem is either a slab (a cohesive layer that fractures and releases all at once) or a loose-snow release (a single point that fans out as it runs). Open any Alpine bulletin — France, Switzerland, Austria, Italy — and you see a danger level from 1 (Low) to 5 (Very High) next to one or more avalanche problems (Fr. problème avalancheux, De. Lawinenprobleme, It. problemi valanghivi). The number gets the headlines. The problem is what keeps you alive.
Here is the trap. Two days both rated 3 — Considerable (the middle rung of the EAWS 1–5 scale, and the level at which most avalanche accidents happen — see how to read an avalanche bulletin) can demand opposite behaviour. On one, fresh storm snow will stabilise by tomorrow afternoon: you wait, then ski almost anything. On the other, a persistent weak layer buried two weeks ago can be triggered from 50 metres away on a slope that looks bomber, and you stay off whole categories of terrain for weeks. Same number, a different first question.
The danger level sizes the threat. The problem type tells you how to think. Always read both — and let the problem drive your terrain choices.
This is why Snow Trace shows the official avalanche bulletin on the map: so you read the active problem and the snowpack summary beside your route, not just the digit. (The rating comes from your regional avalanche service; we do not forecast it.)
Below, the five EAWS problems — new snow (storm slab), wind slab (plaque à vent, Triebschnee), persistent weak layer (couche fragile persistante), wet snow and gliding snow — then the extra problems North America names on its own. For each: where it forms, its field signs, how it behaves and how to manage it, read against the Aspect & Elevation rose and your Slope Angle habits.
How fast each problem fades
Illustrative reactivity curves — teaching values, not measurements or bulletin data. Every problem type heals on its own schedule; some barely heal at all. Educational only.
Park the time-probe at day 2 and flip between problems: storm slab has already fallen to ~24% and reads "Healing" — the mountain is mending it for you — while the persistent weak layer is still ~69% and reads "Still armed", essentially where it started. Two days after each was born, one is nearly a wait-and-ski problem and the other has not budged. Persistence is not a footnote; it is a shape you can feel by holding the clock still and switching problems.
021. New snow / storm slab — the overloaded interface
What it is. During and just after a snowfall, a slab of fresh, cohesive snow loads the surface faster than it can bond to whatever lies beneath — an old crust, a sun-baked surface, or a weak interface within the new snow itself. When the load wins, you get a storm slab: the most familiar avalanche problem, tied to one visible driver — it is snowing, and it is snowing a lot.
Where and when it forms. All aspects, all elevations. Intensity matters more than the total: 30 cm in three hours is far more dangerous than 30 cm over two days, because the snowpack has no time to adjust. What a given depth weighs depends on its snow-to-liquid ratio: a wet, heavy 30 cm loads the slope far harder than dry powder. Sensitivity peaks during the storm and for the first 24–48 hours after it ends, then fades as the new snow settles.
Field signs.
- Active, ongoing snowfall, especially heavy rates (>2–3 cm/hour).
- Recent natural avalanches on similar slopes — the single most important sign.
- Whumpfing and shooting cracks as you move.
- A soft, cohesive upper slab that you can pick up as a block.
- Poor bonding to an old surface in a hand pit or quick column test.
How it behaves. Trigger sensitivity is high during the active window. Spatial predictability is comparatively good: the problem is widespread but consistent, so a representative slope tells you a lot about its neighbours. Persistence is low — often gone within a day or two of clearing weather. Size scales with the depth of new snow.
How to manage it.
- Wait it out. Give a big storm 24–48 hours and re-assess.
- Avoid steep, unsupported and freshly wind-affected terrain during and right after the storm.
- Stick to lower-angle slopes (under ~30°) and mind run-outs and connected start zones above you.
- Re-test bonding to the old surface before committing to anything steep.
032. Wind slab — snow moved by the wind, not the sky
What it is. Wind moves snow far more efficiently than snowfall delivers it, packing the broken crystals into a dense, cohesive layer draped over lee and cross-loaded slopes. The result — plaque à vent, Triebschnee — is a wind slab on a weaker layer beneath, often with no snow falling at all. You can build a serious wind-slab problem under a blue sky.
Where and when it forms. On the lee side of ridges, in gullies, behind terrain features and on cross-loaded slopes where wind blows across rather than over. It is most pronounced at and above treeline, wherever the wind has recently been strong enough to move snow — which can be daily in the high Alps.
Field signs.
- Pillows, drifts and smooth, rounded loading on the lee side of features.
- A surface that feels dense, 'chalky' or hollow — a drum-like sound under your skis.
- Cornices, sastrugi and visible snow plumes off ridgelines.
- Stiff slab over softer snow; shooting cracks that radiate from your skis.
How it behaves. Trigger sensitivity is high while the slab is fresh. Wind slabs hide in specific pockets, so spatial predictability is moderate-to-poor — the loaded gully is deadly while the ridge two metres away is bare. Persistence is low-to-moderate: fresh wind slab can stay reactive for days, particularly on a weak interface. Size is usually small-to-medium but enough to bury you or carry you into worse terrain.
How to manage it.
- Read the wind. Know the recent wind direction, then picture which slopes were loaded and which were scoured.
- Avoid fresh, smooth, convex lee loading and the immediate area under cornices.
- Approach suspect slopes from windward or via supported ground; test the slab before committing.
- Because the problem is pocketed, one good test is not a green light for the next slope.
Wind slab is a problem of where, not whether. The skill is spotting the loaded pocket — and walking around it.
043. Persistent weak layer — the buried memory of bad weather
What it is. Sometimes a fragile layer forms at the snow surface — feathery surface hoar, sugary faceted crystals, or, near the ground, coarse depth hoar. Buried by later snow, it is preserved, sometimes for weeks, while the slab above refuses to bond to it. This is the persistent weak layer (couche fragile persistante), responsible for the majority of avalanche fatalities involving skiers. EAWS calls it persistent weak layers; German-language Alpine bulletins call it the old snow problem (Altschneeproblem).
Where and when it forms. After a cold, clear, settled spell that lets the surface turn into facets or grow surface hoar before the next storm buries it. It lasts longest on cold, shaded aspects (roughly N–E) and, for depth hoar, on early-season snow over higher, colder terrain, and can outlive several storm cycles.
Field signs. The field signs are unreliable and intermittent: a clean-looking slope can hide a primed weak layer.
- Whumpfing and remote triggering — slides released from a distance or from flat ground.
- Recent persistent-slab avalanches, often stepping down to a deep, smooth, faceted bed surface.
- Column tests that produce sudden, clean (planar) shears on an identifiable buried layer.
- A documented weak layer in the bulletin's recent history — read the snowpack summary, not just the number.
How it behaves. Trigger sensitivity is moderate but treacherous — the slab can feel solid until the one trigger point connects, often the thin spot at a rock or convexity. Spatial predictability is the worst of any problem (a single safe slope tells you almost nothing about the next), and persistence is the longest — weeks, not hours. Avalanches tend to be large, breaking wide and deep.
How to manage it.
- Respect time, not signs. You cannot reliably test your way onto a persistent-slab slope. The honest answer is usually terrain avoidance.
- Stay off and out from under steep slopes on the suspect aspect/elevation band until the bulletin confirms healing — which can take a long, frustrating while.
- Treat 'it skied fine yesterday' as meaningless: yesterday's safe slope is not today's.
- Build your slab-and-weak-layer mental model with Understanding Snowpack Stability.
Persistent weak layers are the problem you cannot outsmart with cleverness — only with discipline. When one is active, the win is the run you chose not to ski.
One weak layer, reactive for weeks
Drag, hover or arrow-key across the season · Tap a marker to jump to that event
Illustrative season trace. The stability index blends snowpack tests and danger ratings; a buried persistent weak layer can keep the pack reactive long after the storm that loaded it. Educational only — always consult the local avalanche bulletin.
054. Wet snow — when meltwater takes the bonds apart
What it is. Add liquid water — from sun, warm air or rain — and the bonds between snow grains weaken. Wet-loose avalanches start from a point and fan out as the surface loses cohesion; wet slabs release a cohesive block when water reaches and weakens a buried layer. Both are driven by warming and water input, which makes them partly predictable by the clock.
Where and when it forms. Classic spring conditions: sunny aspects (S, SE, SW) in the afternoon, during heat waves, with the first rain-on-snow events, or whenever an overnight refreeze fails. The danger follows the sun around the compass through the day — the world of the melt-freeze cycle.
Field signs.
- Roller balls / pinwheels and the first natural wet-loose sluffs on sun-warmed slopes.
- Boot or ski punching through a failing crust.
- A weak or absent overnight refreeze; isothermal, soggy snow at depth.
- Water visibly running, dripping cliffs, slush underfoot by late morning.
How it behaves. Trigger sensitivity rises through the day with water content — moderate, but strongly time-dependent. Spatial predictability is moderate: it tracks sun and aspect, but a saturated wet slab can surprise you in scale. Persistence is short within a day (a good refreeze resets it), but the pattern recurs every afternoon of a warm spell. Wet-loose slides are usually small-to-moderate; wet slabs can be large and destructive.
How to manage it.
- Ski with the clock. Start early on a firm refreeze and be off consequential slopes by the time roller balls appear.
- Follow the sun: move to shaded or not-yet-warmed aspects as the day heats up.
- Treat a poor or missing overnight refreeze as a hard stop signal for sun-exposed terrain.
- Mind terrain traps and run-outs — even a small wet slide carries enormous mass.
065. Gliding snow — the whole pack on the move
What it is. On smooth ground — grassy slopes, rock slabs — the entire snowpack can slowly slide downhill as a unit, lubricated by a film of meltwater at the base. Tension cracks open across the slope: glide cracks, mouth-like fractures that gape over days. When one lets go, the whole pack releases in a glide avalanche, and there is no reliable way to predict the timing.
Where and when it forms. Over smooth, anchoring-free ground at any aspect, though more often on sunny slopes, wherever liquid water reaches the base — in spring melt and in mid-winter when a warm, deep snowpack stays wet at the ground. Glide follows the ground surface, not the sky.
Field signs.
- Open glide cracks — the only real warning, and an ambiguous one: glide avalanches also release without any visible crack.
- Snow visibly creeping, or bulging and folding below the crack.
- Known glide-prone terrain (smooth grass, slabby rock) carrying a deep, wet pack.
How it behaves. Trigger sensitivity to people is very low — it goes when it goes. But spatial predictability is as poor as it gets and timing is effectively random: a glide crack can sit open for weeks, then release at 3 a.m. or 3 p.m. Persistence is long — the crack is a standing threat for as long as it exists. Glide releases involve the full depth of the pack and can be very large.
How to manage it.
- Avoid, do not predict. This is the one problem where the official advice is to avoid exposure rather than try to time the release.
- Do not travel below, above or through an open glide crack — minimise time spent in its run-out.
- Route-find around glide-prone ground entirely when cracks are present.
07The North American problems: deep persistent slab, cornice fall & loose snow
US forecasts (the avalanche centres via Avalanche.org) list nine avalanche problems, not five; Avalanche Canada lists eight, without glide. They split a few of the families above into named problems, because each behaves differently enough to manage differently.
Deep persistent slab. The persistent weak layer at its worst: the weak layer sits near the ground (usually basal depth hoar or old facets), often under a metre or more of snow. It is less reactive to a light skier than a persistent slab but far more destructive, and it can be triggered remotely from low-angle terrain or a thin spot at a rock. Rooted in the buried snowpack structure, it can stay primed all season: stay off and out from under the suspect aspect/elevation band, for weeks.
Loose dry snow (sluff). A point release of dry, unbonded snow that fans out as it entrains more. Usually small, it can still knock you off your feet in couloirs, above cliffs and over terrain traps. Control your sluff on steep lines (ski off to the side and let it run).
Loose wet snow. The wet point release, driven by meltwater. EAWS folds it into wet snow, with the same clock-and-refreeze management.
Cornice fall. A cornice is the overhanging, wind-built snow curling over the lee edge of a ridge — built by the same wind as a wind slab. Cornices fail with warming, new loading or their own growth, often untriggered, and a falling block can set off the slope beneath. Give them a wide margin: never walk out onto a corniced ridge (the fracture line sits back from the visible edge) and do not linger beneath one as the day warms.
Europe folds sluff and loose-wet into new snow and wet snow, lists cornices only as an optional extra problem, and reads deep persistent slabs as the dangerous end of the persistent-slab problem. The management questions are the same.
08Comparing the avalanche problems: same number, five different days
The five problems differ on the behaviours that drive decisions, not on how scary they sound.
| Problem | Forms where / when | Key field sign | Trigger sensitivity | Spatial predictability | Persistence | Typical size |
|---|---|---|---|---|---|---|
| New snow / storm slab | All aspects, during & 24–48 h after snowfall | Recent natural slides, shooting cracks | High | Good | Low (hours–2 days) | Small–large w/ depth |
| Wind slab | Lee & cross-loaded, above treeline | Pillows, hollow 'chalky' slab, cornices | High | Poor (pocketed) | Low–moderate | Small–medium |
| Persistent weak layer | Cold N–E aspects, often higher | Whumpfing, remote triggers, clean shears | Moderate (treacherous) | Worst | Weeks | Often large |
| Wet snow | Sunny aspects, afternoon, spring | Roller balls, punchy snow, no refreeze | Moderate (time-driven) | Moderate (tracks sun) | Short per day, recurring | Small–large |
| Gliding snow | Smooth ground, any aspect | Open glide cracks | Very low (to people) | Very poor | Long | Often very large |
(Typical size is the destructive scale, D1–D5: roughly D1 is relatively harmless to people, D2 could bury, injure or kill a person, D3 can destroy a car or a wood-frame house, and D4–D5 can flatten a stretch of forest.)
Now re-read that 'Considerable' day. Storm slab: wait, then ski mellow. Wind slab: find and avoid the loaded pockets. Persistent weak layer: stay off whole aspect/elevation bands for weeks. Wet snow: check the watch and the refreeze. Glide: route around the cracks and stop trying to predict. Five first questions, one danger number.
This is why the response is non-linear: you get safer by identifying which problem is in play and switching to its playbook, not by skiing 'more carefully' as the rating rises. The Aspect & Elevation rose tells you which slopes each problem lives on, and disciplined Slope Angle management — staying under ~30° when in doubt, and out of the run-out of anything steeper — is the single most reliable defence across all five. Settle those limits at the kitchen table when you plan your line and turnaround time. When you cannot identify the problem, treat the day as if the worst plausible one is active.
Our own records show the terrain lean of the deadliest problem. Of the 6,659 geolocated avalanche records in Snow Trace's avalanche database, the N–NE–NW arc (three of the eight aspects) holds 54% of those with a known aspect, and 62% of the fatal ones: the cold, shaded slopes where persistent weak layers last. The records do not name the active problem, so read this as a lean, not a cause.
Snow Trace topos give each route's main aspect and high point, so one bulletin reads differently line by line. In Belledonne, Dôme de la Coche via the Pas de la Coche faces NW (2134 m), inside the persistent-weak-layer arc; Croix de Chamrousse via Lac Achard faces S (2250 m), wet-snow ground on a spring afternoon; Croix de Belledonne via the Doménon lakes faces W and reaches 2926 m, the alpine band where wind slabs build.
The expert move is not knowing more facts. It is asking the right first question — and the avalanche problem is what tells you which question that is.
Compare the five avalanche problems
What this means for travel
Trigger sensitivity: The defining trait of this problem — let it drive route choice and timing. How easily your weight sets it off. High means it goes with little provocation — a single skier can do it.
Typical terrain / timing: All aspects, during & after snowfall
Reactive and widespread during the storm window, but consistent and fast-healing — the problem that rewards patience.
How to manage it: Wait 24–48 h after heavy snow; avoid steep, unsupported terrain during loading; re-test bonding to the old surface before committing.
Tap a bar or arrow-key the radar to inspect a trait · arrow-key the tabs to switch problem
Fingerprint scores (0–5) are illustrative teaching values, not measurements. Read your local bulletin for the active problem. Educational only.
Notice how persistent weak layers and glide max out spatial unpredictability and persistence — the two traits you cannot test your way around — while storm slab, which feels scariest mid-storm, actually scores lowest on persistence. The bars that matter for decisions are unpredictability and persistence, not raw trigger sensitivity.
Each problem hides on different slopes
This problem is flagged on NE slopes at alpine. Treat this cell as terrain to avoid — favour aspects and elevations the bulletin leaves clear.
Manage it: Wind has loaded the lee NE–E–SE slopes above treeline — find the smooth pillows under cornices and walk around them; the windward ridge two metres away can be bare.
Illustrative terrain signatures (8 aspects × 3 elevation bands). Real avoid-zones come from your local bulletin’s aspect/elevation rose, not this teaching model. Educational only.
09Educational use only — and how to actually use this
This article is educational only. It is not avalanche forecasting, not a substitute for formal training, and not a basis for go/no-go decisions. The fingerprint scores in the interactive are illustrative teaching values, not measurements and not bulletin data.
- Read your local bulletin every day (Météo-France / Meteomont / SLF / lawinen.report or your national service): the problems and the snowpack summary, not only the number — see how to read an avalanche bulletin.
- Take a recognised avalanche course** and carry — and practise with — a transceiver, shovel and probe (see why the first 15 minutes decide everything).
- Travel with judgement, not articles. Nothing here replaces local knowledge, current observations and a trained partner.
Plan the day in one place, then trust your eyes on the snow. Snow Trace puts the official bulletin on the map next to your route, with snow-station data and community trip reports. It helps you read the mountain — it does not make the decision for you.
10Sources & further reading
Sources & further reading. This article reflects the consensus of the major avalanche-safety organisations and the standard references. Always defer to your local daily avalanche bulletin and hands-on training over any single article:
- **EAWS** — European Avalanche Warning Services: the standard danger scale and avalanche problems
- **Avalanche.org** — US National Avalanche Center: forecasts and free avalanche education
- **SLF** — Swiss Institute for Snow and Avalanche Research
- McClung & Schaerer, "The Avalanche Handbook" — the standard technical reference
Key takeaways
- The avalanche bulletin gives you a danger number AND an avalanche problem — the problem tells you how to manage the day; always read both.
- The same danger level (e.g. 'Considerable') can mean opposite behaviour: wait it out for storm slab, but avoid whole aspects for weeks with a persistent weak layer.
- The two behaviours you cannot test your way around are spatial unpredictability and persistence — which is why persistent weak layers and glide are the most dangerous to manage.
- Match each problem to terrain: the aspect/elevation rose tells you where it lives, and staying under ~30° (and out from under steeper slopes) is the single most reliable defence across all five problems.
- Wet snow runs on a timetable (the clock and the refreeze); glide has no timetable at all and must be avoided, not predicted.
- This is educational only — read your local bulletin daily, take a course, carry a transceiver/shovel/probe, and let trained judgement, not an article, make your decisions.
Frequently asked questions
What are the types of avalanche problems?+
The core problems are: storm slab (new snow), wind slab (plaque à vent / Triebschnee), persistent slab (couche fragile persistante), wet snow, and glide. The US avalanche centres split these into nine named problems, adding deep persistent slab, loose dry (sluff), loose wet and cornice fall. Europe (EAWS) groups the same behaviours into five families.
What is the difference between storm slab and wind slab?+
Both are cohesive slabs of new-ish snow, but the driver differs. A storm slab is loaded by falling snow, so it is widespread across all aspects during and just after a storm. A wind slab is drifted by wind onto lee and cross-loaded slopes — pocketed and localised, and it can build under a clear sky with no snow falling at all.
What is a persistent slab avalanche?+
A persistent slab is a cohesive slab sitting on a buried, long-lasting weak layer — surface hoar, faceted crystals or depth hoar — that refuses to bond. It can stay reactive for days or weeks, produces unreliable field signs, and can be triggered remotely. Its deepest, most destructive form near the ground is called a deep persistent slab.
What is the most dangerous avalanche problem?+
The persistent (and deep persistent) slab is responsible for the majority of skier-triggered avalanche fatalities. Its field signs are unreliable, it can be triggered remotely, it persists for weeks on cold shaded aspects, and it tends to break large and wide. You usually cannot test your way safely onto one — the honest defence is terrain avoidance.
What is a glide avalanche and can you predict it?+
A glide avalanche is when the entire snowpack slides on smooth ground (grass or rock slabs), lubricated by meltwater at the base, releasing the full depth of snow. The only warning is an open glide crack — and there is no reliable way to predict the timing. Official guidance is to avoid glide-prone terrain when cracks are present rather than try to time the release.
How do avalanche problems relate to aspect, elevation and slope angle?+
Each problem favours specific terrain: persistent weak layers live on cold N–E aspects often higher up, wind slabs on lee and cross-loaded slopes above treeline, wet snow on sunny aspects in the afternoon, and glide on smooth ground at any aspect. The aspect/elevation rose tells you where each problem lives, and keeping to slopes under about 30° — and out of the run-out of steeper ones — is the most reliable defence across all of them.
How many avalanche problems are there?+
It depends on the system. Europe (EAWS) uses five avalanche problems: new snow, wind slab, persistent weak layer (old snow), wet snow and gliding snow. The US avalanche centres name nine (Avalanche Canada lists eight, without glide), splitting a few EAWS families into their own problems: new snow becomes storm slab and loose dry, wet snow becomes wet slab and loose wet, and deep persistent slab and cornice fall get their own lines. The underlying behaviours are the same; only the naming differs.
What are the five EAWS (European) avalanche problems?+
The five harmonised EAWS problems used across the Alps are: new snow, wind slab (Triebschnee), persistent weak layer — also called the old snow problem (Altschneeproblem), wet snow, and gliding snow. Each has a standard pictogram on the bulletin, its own favoured aspects and elevations, and its own management playbook.
What is the old snow problem (Altschnee)?+
The old snow problem (German Altschneeproblem) is the German-language EAWS name for the persistent weak layers problem: a fragile buried layer of surface hoar, facets or depth hoar left over from an earlier cold, clear spell. A slab builds on top while the old snow beneath refuses to bond, so it can stay reactive for weeks, gives unreliable field signs and can be triggered remotely — which is why it is the deadliest problem for skiers.
What is the difference between a slab avalanche and a loose snow avalanche?+
A slab avalanche releases a whole cohesive layer at once along a fracture line — storm, wind, persistent and wet slabs all work this way and account for almost all fatal accidents. A loose snow avalanche (sluff, wet-loose) starts from a single point and fans out downhill, entraining more snow as it goes. Slabs are the deeper danger; loose slides mostly matter for the consequence of being knocked off your feet in steep terrain.