Snow explained

Surface hoar: the frost that becomes a weak layer

Surface hoar is beautiful to ski through and notorious once it is buried. It is one of the persistent weak layers avalanche forecasters watch most closely.

How it forms

On a clear night the snow surface radiates heat to the sky and becomes colder than the air just above it. If that air is humid and the wind is light, water vapour deposits directly onto the cold surface as ice crystals, the same way frost forms on a car windscreen. Night after night of clear, calm, humid weather can grow feathery, sparkling crystals several millimetres to a few centimetres long.

What destroys it

Surface hoar is fragile while it is on the surface. Wind breaks it, sunshine and warm air melt or round it, and rain ruins it. That is why it tends to survive best in sheltered, shady terrain and in calm spells.

Why it matters once buried

When snow falls gently on surface hoar, the crystals can be buried standing up. They form a thin layer that is very weak in shear, like a layer of tiny stilts under the new snow. Because the crystals change slowly, the layer can stay weak for weeks, long after the snowfall that buried it. Forecasters often name it by its burial date, for example "the 5 February surface hoar".

A buried surface hoar layer is often patchy. It may be present in one clearing and absent on the open ridge next to it, which makes it hard to predict from a distance.

How Snowvy tracks it

The model grows surface hoar on a slope when nights are clear, calm (wind at or below about 3.5 m/s) and humid, destroys it with wind, sun or melt, and buries it as its own layer when enough snow falls on crystals that have grown large enough. The explanation then names the layer by its burial date and says how deep it now lies and what has been deposited on top. "Buried surface hoar" is one of the concern types Snowvy reports. It is a modelled indication, not an observation, so compare it 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
• Wind slab: how wind loads a slope
• 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