Snow explained

Wind slab: how wind loads a slope

Wind moves more snow around a mountain than most people expect. Where it drops that snow, it can build a slab.

From windward to lee

When wind blows over loose snow it can lift and bounce the grains along the surface. On the windward side of a ridge snow is stripped away; on the lee side, where the wind slows down, it is dropped. The grains are broken into small pieces as they travel, so they pack together tightly and bond into a denser, more cohesive layer: a wind slab.

• Top-loading fills the slopes just below a ridge on its downwind side, and builds cornices.
• Cross-loading happens when wind blows across a slope and fills the sides of gullies and the lee of ribs.

It doesn't need new snow

A strong wind after a snowfall, or even days later if the surface snow is still loose, can build wind slabs under a blue sky. The amount of snow the wind can move depends on how loose and light the surface is, and on how strong the wind is.

What it looks like

Wind-deposited snow often looks smooth, rounded or pillow-shaped, and can feel chalky or sound hollow underfoot. Shooting cracks that run away from your skis are a clear sign of a slab. These are signs to take seriously, but their absence does not mean there is no slab.

How long it lasts

Wind slabs often bond to the snow below within days, faster in mild weather and more slowly in cold weather. If the slab sits on a persistent weak layer such as facets or buried surface hoar, the problem can last much longer.

How Snowvy models it

For each slope and hour, the model estimates how mobile the surface snow is and how strong the wind is, and marks snowfall that fell or was moved in strong wind as wind-packed: denser, with smaller grains. Slopes are classified by how sheltered or exposed they are to the prevailing wind. "Wind slab" is one of the concern types Snowvy reports, with the dates of the wind episodes behind it. Wind at the scale of a single gully is finer than any model resolves, so read the avalanche bulletin and look for the signs above.

Modelled, not an avalanche forecast. Snowvy explains the snowpack; it never says a slope is safe. Read your regional avalanche bulletin and make your own decisions.

Official avalanche bulletins: EAWS (Europe), avalanche.org (USA), Avalanche Canada, and every other official service we know of.

More explainers

• How sun and aspect change the snow
• What modelled snow conditions mean, and what they don't
• Surface hoar: the frost that becomes a weak layer
• Crusts and facets: why a hard layer can make a weak one
• Persistent weak layers: facets, depth hoar and a weak base
• Wet snow: how warmth, sun and rain change the snowpack
• New snow: what a storm does to the snowpack
• Regional estimate or detailed area: what Snowvy covers where
• Surface snow and wind: what your skis will meet

See it on your slopes