Snow explained

Crusts and facets: why a hard layer can make a weak one

A crust feels solid underfoot. The trouble is often what grows next to it.

How crusts form

• Melt-freeze crust. Sun or warm air wets the surface; a cold night refreezes it into a hard layer. Common on sunny aspects and at lower elevations.
• Rain crust. Rain falling on snow soaks the top layer, which freezes when the temperature drops. Rain crusts can cover all aspects up to the elevation the rain reached.
• Wind crust. Strong wind can pack the surface into a thin, firm skin.

Why crusts and facets go together

Once snow falls on a crust, the crust becomes a layer inside the snowpack. In cold weather, temperature can change sharply across a thin layer of snow next to the crust, and the crust can also hold moisture. Those conditions favour the growth of facets: angular, poorly bonded crystals. The result is a weak layer of facets sitting on or under a smooth, hard bed. Avalanche researchers call this a crust–facet combination, and it can persist for a long time.

Where to expect it

Crust–facet layers are common after a warm or rainy spell followed by cold, clear weather and then new snow. Rain crusts tend to be widespread up to the rain line; sun crusts mostly on sunny aspects. A shallow snowpack makes faceting more likely, because the temperature difference between the ground and the surface is spread over less snow.

How Snowvy models it

The model turns any wet layer that refreezes into a melt-freeze crust, keeps it a crust when it is re-wetted and refrozen, and tracks the temperature gradient around it hour by hour. Where faceting grows next to a buried crust, it reports the "crust–facet combination" concern with the dates of the warm or rainy spell and of the snowfall that buried it. A typical explanation reads like: rain on 8 Jan made a crust, a cold clear week grew facets on top, and the storm of 15 Jan covered both. Compare it with your avalanche bulletin, which reports what forecasters have actually observed.

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
• 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