THE AVALANCHE PROBLEM TYPES

The Technique

1. Recognised avalanche forecasts around the world are built on a small set of avalanche problem types. This is not academic tidiness. A danger rating tells you roughly how much care to take; the problem type tells you what to actually do, because different problems live in different terrain, punish different timing and heal on completely different clocks. Two days rated the same can demand opposite plans. A storm slab day might send you into sheltered low angle trees; a persistent slab day might make those same trees fine and the open alpine bowl unacceptable at any hour. Learn to read the problem first and the rating second.
2. Storm slab is new snow that has bonded to itself into a cohesive layer but has not yet bonded to the surface it landed on, or contains a weak interface within the storm snow. It is created by heavy, fast snowfall, especially when the snow starts light and cold and turns denser and warmer as the storm goes on. It is reactive during the storm and in the hours after it. It punishes anyone who goes straight to steep open terrain on the first clear morning. Storm slab is usually the friendliest problem in one respect: it tends to settle and strengthen over hours to a few days, so patience genuinely works on it. That is also why it lures people, because waiting for it to settle feels like watching the powder go stale.
3. Wind slab is snow that the wind has picked up, broken down and packed into a dense, cohesive slab on the sheltered side of terrain. Wind moves far more snow than snowfall alone, and it can build slabs on a clear day with no new snow at all. Look for it on lee slopes below ridge lines, in the lee of rolls and knolls, and cross loaded into the sides of gullies where the wind blows along a ridge rather than over it. It feels hollow and drum like underfoot, and it often sits over softer snow. Wind slabs are usually localised, which is a gift, because it means you can often ride the same run on a different aspect and avoid the problem entirely. They typically strengthen over days, more slowly in cold weather.
4. Persistent slab is the problem that kills experienced people. It is a cohesive slab sitting over a weak layer that does not heal quickly: buried surface hoar, faceted grains, or facets formed against a crust. It is created by a period of weather that builds a weak surface, followed by snowfall that buries it before it can be destroyed. It behaves badly in every sense. It can stay reactive for weeks. It can fail well away from where you are standing, so you can trigger a slope from flat ground or from the trees below it. It propagates, meaning a small failure can run a long way across and take the whole feature. It is stubborn and then suddenly it is not, which is why people ride a slope repeatedly and then one party does not come back. When a persistent problem is listed, the honest response is to change your terrain, not your technique.
5. Deep persistent slab is the same mechanism at the bottom of the snowpack, usually on a weak base built early in the season when the snow was shallow and cold. It is hard to trigger, because the slab above it is thick, and it is enormous when it goes. The trigger point is often a shallow, rocky spot where a person's weight can still reach the weak layer, so a rider crossing thin snow can bring down the entire slope above. It can stay in the snowpack for months, and it wakes up during heavy loading, rapid warming, or when a rider hits exactly the wrong metre of ground. There is no test that clears it and no smart way to manage it on the slope. The management is terrain: stay off and out from under big, unsupported features while it is listed.
6. Wet slab and the loose problems belong to the melt side of the season and the warm hours of the day. Wet slab happens when liquid water from rain, sustained warming or the first strong sun of spring reaches a weak layer or an interface and destroys its strength. It is hard to forecast, ugly to be caught in, and it demands you get off and out from under solar aspects before the snow goes soggy. Loose wet avalanches start at a point and fan out as they run, entraining heavy saturated snow, and while they are often small they carry enormous weight for their size and will push you into whatever is downhill. Loose dry avalanches, or sluff, are point releases in unconsolidated cold snow. Sluff is usually harmless on open ground and lethal above a cliff or a gully, because it does not have to bury you to kill you, it only has to knock you off your feet in the wrong place, and on very steep terrain it can catch and outrun you.
7. Cornices and glide avalanches are the two problems where timing feels random. A cornice is a mass of wind drifted snow overhanging the lee side of a ridge, and it fails under its own weight, under warming, or under a person who walked too close to an edge that is further back than it looks. A falling cornice is both a hazard in itself and a very effective trigger for the slope beneath it. Do not linger on top of one, do not stop under one, and stay well back from ridge crests where you cannot see the edge. Glide avalanches are the whole seasonal snowpack sliding on the ground, usually on smooth rock or grass, announced by glide cracks that open and then sit there for days or weeks. Nobody can tell you when a glide crack will release. The only sound advice is to avoid travelling under them and never to camp, regroup or eat lunch below one.
8. Attach four questions to every problem you read in a forecast. What creates it, so you know which weather makes it worse. How does it behave, so you know whether it is localised or likely to propagate. What terrain and timing does it punish, so you know which aspects, elevations and hours to remove from the plan. How long does it stay, so you know whether patience is a strategy or a fantasy. Storm and wind slabs reward patience. Persistent and deep persistent slabs do not, and treating them like storm slab is one of the most common fatal errors in the sport.
9. Expect more than one problem at a time and plan for the worst one listed. A day can hold fresh wind slab in the alpine, a persistent layer at treeline and a wet loose issue on sunny aspects by early afternoon. Your terrain choice has to satisfy all of them at once, not the average of them. Work the list from the top down: remove the terrain that the worst problem punishes, then remove what the next problem punishes, and ride what is left. If nothing is left, the day is a tour, a low angle lap or a coffee.

Common Mistakes

Reading the danger rating and skipping the problem description. Fix: make the problem type, the aspects and elevations it lives on, and its likely size the first three things you write in your plan.

• Treating a persistent slab like a storm slab and waiting two days for it to settle. Fix: check the forecaster's wording about how long the layer has been reactive, and assume weeks rather than days until told otherwise.
• Assuming no new snow means no new slabs. Fix: treat wind as a loading event in its own right and look for fresh drifting on lee and cross loaded terrain after any windy period.
• Dismissing sluff as harmless because it is small. Fix: judge loose avalanches by what is below you, not by their size, and never let a sluff push you above a cliff, a gully or a creek bed.

The Drill

Take the current forecast for any range you follow and rewrite it as a table without looking at the rating: one row per listed problem, with columns for what created it, which aspects and elevations it lives on, which hours are worst, and how long you expect it to last. Standard: from your table alone, a partner should be able to strike out the unacceptable terrain on a map and name the one thing that would make you change the plan mid day.

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