Backcountry Snow
Wind slab: the avalanche problem that builds while you sleep
You do not need new snowfall to get a fresh avalanche problem. Wind alone will move enough snow to build slabs, and it does it on the aspects people most want to ski.
A common misconception is that avalanche danger tracks snowfall. Big storm, big danger; clear skies, safer. It is a reasonable heuristic and it misses one of the most frequent problems in mountain snowpacks: wind slab, which forms on clear, cold, blue days as long as the wind is blowing.
How wind builds a slab
Wind picks up loose surface snow, breaks the crystals into small rounded fragments as they tumble, and deposits them on the sheltered side of terrain features. Those broken fragments pack together densely and bond quickly — much more densely than snow that falls straight down.
The result is a stiff, cohesive layer sitting on top of whatever was there before, which is often something soft and weak. A slab on a weak layer is the basic recipe for a slab avalanche.
Wind can transport a great deal of snow. A moderate wind over a few hours can deposit several times the depth on a lee slope that fell as new snow, which is why forecasts frequently talk about wind-loaded terrain being far more dangerous than sheltered terrain in the same area on the same day.
Where it ends up
On the lee side of anything: ridges, gullies, rolls, clumps of trees, small terrain breaks. If wind is from the west, expect loading on eastern aspects — but cross-loading also happens, filling in gullies and depressions from the side, which is why an east-facing gully wall can be loaded even when the general aspect is not.
Cornices are the visible signature. Where there is a cornice, there is loading on the slope beneath it, and the cornice itself is an additional hazard because it can break off and trigger the slope below.
Reading it on the ground
Textural clues. Wind-affected snow feels and sounds different underfoot — firmer, sometimes with a hollow drum sound, often with a slightly chalky or matte appearance. Sastrugi, ripples and scoured patches on the windward side tell you wind has been working, which means something has been deposited somewhere.
Pillowy, smooth deposits in the lee of features, especially where the surface looks rounded and filled-in rather than following the underlying terrain.
Cracks shooting away from your skis — the clearest possible signal that a slab is present and reactive. This is not something to investigate further. It is a reason to leave that slope immediately, moving back along your track.
Whumpfing. The sound of a weak layer collapsing under you. Same response.
Snow actively blowing off ridges — plumes off summits mean loading is happening right now, and the problem is being built while you look at it.
Why it is dangerous in a specific way
Wind slabs are usually shallow, often in the range of tens of centimetres. That sounds harmless and it is not, for three reasons.
They form exactly where people want to ski: on lee slopes, which hold the softest, deepest snow, in the terrain that looks most appealing.
They are easy to trigger from a distance and from below, because the slab is cohesive and can propagate a fracture across a slope.
And they are frequently found in terrain with traps — gullies, above cliffs, in narrow chutes — where a shallow slide has serious consequences. Depth of burial in a gully after a shallow slide can be several metres.
Timing
Fresh wind slabs are most reactive in the first day or two. They then stabilise relatively quickly compared with persistent problems, as the slab settles and bonds. This is genuinely different from persistent weak layers, which can stay dangerous for weeks or months.
The practical rule: give a wind event a day or two before skiing loaded terrain, and even then, test the transition carefully at the edge of the slab before committing.
Travelling around it
Prefer windward aspects and ridges, which are scoured rather than loaded. The snow is worse and the risk is much lower, which is the trade you are making all winter.
Stay off and out from under cornices, and be aware that cornices break further back than they look — the fracture line is often well behind the visible lip. Give them a wide margin on ridge travel.
Cross suspect slopes one at a time with the group watching from safe ground, and enter at the top edge rather than the middle where possible.
And read the wind before you plan. Wind speed and direction from the last twenty-four to forty-eight hours tells you which aspects were loaded, and that is the single most actionable piece of weather information for a day tour.
