Contents
A slab avalanche needs a weak layer under a cohesive slab. Weak layers form from surface hoar, faceted snow near crusts and depth hoar at the base, and some last for weeks. You usually can't see them from the surface, which is why the bulletin names them as the persistent weak layer problem.
The Snowpack Is Not Uniform
The snowpack is a layered structure. Each snowfall, wind event, warm spell, and cold night adds or modifies a layer. Some layers bond well to each other. Others create weak interfaces that can fail under the weight of a skier or the stress of additional snow.
A slab avalanche happens when a cohesive slab sits on top of a weak layer, and that weak layer collapses.
Knowing how the layers form and fail makes the snowpack text in the bulletin readable, and the terrain easier to judge.
How Snow Layers Form
Every snowfall creates a new layer. The characteristics of that layer depend on temperature, wind, humidity, and the intensity of the precipitation. Light, cold snow creates low-density layers. Heavy, wet snow creates dense layers. Wind during or after snowfall transports snow and creates wind slabs: dense, cohesive layers deposited on lee slopes.
Between snowfall events, the existing snow surface transforms. On cold, clear nights, the surface can develop surface hoar: delicate, feathery crystals that form when moisture in the air deposits directly onto the cold snow surface. Surface hoar is one of the most dangerous weak layers because it is widespread, persistent, and very hard to detect once buried.
During warm periods, the snow surface can melt and refreeze, creating a crust. Crusts can be strong or weak depending on their thickness and the conditions that formed them. A smooth crust buried by new snow creates a sliding surface for future avalanches.
Types of Weak Layers
Not all weak layers are the same. The type of weak layer determines how it behaves, how long it persists, and how dangerous it is.
Surface Hoar
Surface hoar forms on cold, clear, calm nights when moisture from the air crystallizes on the snow surface. The resulting crystals are flat and feathery, similar to frost on a window. When buried by subsequent snowfall, surface hoar creates an extremely reactive weak layer.
Surface hoar can persist for weeks or months once buried. It is most dangerous on shady, north-facing slopes where the sun does not break it down. The avalanche bulletin often references "buried surface hoar" as a persistent weak layer problem.
Faceted Grains
Faceted grains (also called facets or sugar snow) form when strong temperature gradients exist within the snowpack. Cold air above, warmer snow below. Water vapour moves upward through the snowpack and recrystallizes into angular, poorly bonded grains.
Facets feel like sugar or sand when you rub them between your fingers. They do not bond well to surrounding snow and create weak layers that can persist all season. Early-season facets buried under later snowfall are a classic setup for persistent slab avalanche problems.
Depth Hoar
Depth hoar is the extreme version of faceting. It forms at the base of shallow snowpacks where the temperature gradient is strongest (warm ground, cold air). The resulting crystals are large, cup-shaped, and have virtually no cohesion.
In the Alps it forms in thin early-season snow and persists longest on shady slopes. Once a depth hoar layer forms at the base, it can persist all season and create conditions for very large, deep slab avalanches.
This is one of the most difficult avalanche problems because it is invisible from the surface and can be triggered from a distance or from the bottom of a slope.
New Snow Instability
Fresh snow does not always bond well to the old snow surface. The interface between new and old snow can be a weak layer, especially if the old surface was cold, smooth, or covered in surface hoar.
New snow instability usually lasts days, as the new snow settles and bonds to the old surface; the bulletin says how the bonding is going. During this period, natural and human-triggered avalanches are common. The bulletin flags this as a "new snow problem."
Wind Slabs
Wind transports snow from windward slopes and deposits it on lee slopes, creating dense, cohesive wind slabs. These slabs often sit on top of softer, weaker snow, creating a classic slab-over-weak-layer setup.
Wind slabs can form rapidly; a few hours of strong wind can create a dangerous slab. They are most common on lee slopes near ridgelines, in gullies, and behind terrain features that create wind loading. Fresh wind slabs are usually sensitive to human triggers and settle over days; the bulletin says when.
Clues: wind-affected snow surfaces (sastrugi, wind crust), recent snow transport visible as plumes or drifts, hollow-sounding snow when walking on it, cracks shooting out from your skis.
Reading the Snowpack in the Field
You can gather information about the snowpack without digging a pit. Observation signs include:
Whoompfing: A collapsing sound underfoot means a weak layer is failing. This is a serious red flag: leave the terrain immediately.
Shooting cracks: Cracks propagating outward from your skis indicate a cohesive slab sitting on a weak layer. Another major warning sign.
Recent avalanche activity: If you see fresh avalanche debris or crown lines on similar aspects and elevations, assume the problem exists on your slope too.
Rollerballs and pinwheels: Small snow rolls on the surface indicate warming and wet snow instability.
For formal assessment, a compression test (CT) or extended column test (ECT) involves isolating a column of snow and tapping it with increasing force to see where and how it fails. These tests are part of standard avalanche safety courses.
How Bulletins Describe the Snowpack
The avalanche bulletin describes current snowpack conditions and identifies the main avalanche problems. Understanding the terminology helps you interpret the bulletin:
Persistent weak layers: Weak layers that have been buried for days to months. These create ongoing avalanche problems that do not resolve with warm weather or new snow.
New snow (storm slab in North American bulletins): a slab of new snow that has not yet bonded to the old surface. Usually days; the bulletin says.
Wind slab: Dense wind-deposited snow on lee slopes. Can form rapidly and is sensitive to triggers.
Wet snow: Snow that is warming and losing strength due to liquid water. Typically a springtime or afternoon problem.
Glide cracks: Visible cracks in the snowpack where the entire slab is slowly sliding on a smooth base (grass, rock). Can release as full-depth avalanches unpredictably.
Check the avalanche bulletins on BEYOND for the current assessment in Switzerland, Tyrol and France; the region pages show the bulletin text where the service publishes it.
Practical Takeaways
What follows from this:
Persistent weak layers (facets, depth hoar, buried surface hoar) are the most dangerous because they last a long time and can produce large avalanches. When the bulletin mentions persistent weak layers, extra caution is required on the affected aspects and elevations.
New snow and wind slab problems are usually short-lived but can be very reactive. The bulletin says when the new snow has bonded; until then keep off terrain over 30° in the named sectors.
Always read the bulletin before heading out. The snowpack section tells you what is happening inside the snow. The danger level alone does not give you enough information to make good decisions. For a step-by-step guide, read how to read an avalanche bulletin.