Snow explained

Persistent weak layers: facets, depth hoar and a weak base

Most snow bonds and strengthens within days of falling. Some layers don't, and can stay weak for most of a season.

What makes a layer persistent

New snow usually settles and bonds: its grains round off and stick together. A persistent weak layer is made of grains that do this slowly or not at all, so the layer stays weak for weeks or months. Three kinds matter most:

• Facets. Angular, poorly bonded grains that grow when the temperature changes sharply across the snow, typically in a thin snowpack during cold, clear weather, or next to a crust.
• Depth hoar. Large, cup-shaped facets near the ground, grown over weeks in a shallow, cold snowpack. They can be several millimetres across and feel like sugar.
• Buried surface hoar. Frost crystals that grew on the surface and were buried intact. See surface hoar.

Why the start of the season matters

A thin early-season snowpack followed by a long cold, dry spell is the classic recipe. The ground stays near 0 °C, the air is far colder, and the temperature difference across a few tens of centimetres of snow is steep. That turns the whole thin pack into facets and depth hoar. When the big storms finally arrive, they bury it, and the weak base can stay in the snowpack for the rest of the winter.

Why they are tricky

Persistent weak layers can be buried deep enough that a slope seems fine for days and then releases, sometimes from a thin spot, a rock or a shallow edge. They are often patchy across a slope. Avalanche bulletins describe them as a separate avalanche problem because ordinary signs of stability are less reliable over them.

How Snowvy finds them

For every modelled layer, Snowvy checks six structural warning signs used in avalanche research on simulated profiles: large grains, a big jump in grain size or in hardness against the next layer, a soft weak layer, a persistent grain type, and a depth within reach of a skier. It reports up to the three strongest buried concerns, names each by the date it was buried, and says what lies on top of it. When the snowpack is still thin but already contains facets, depth hoar or a crust, it says so too: on its own that snow is too thin to form a slab, but it can become a weak base under the next big snowfall.

These are modelled indications from weather history, not observations from a snow pit, so compare them with your avalanche bulletin.

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
• Wind slab: how wind loads a slope
• Crusts and facets: why a hard layer can make a weak one
• 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