Snow explained

Wet snow: how warmth, sun and rain change the snowpack

Water changes snow faster than anything else. A slope can go from firm to slushy in a few hours.

How snow gets wet

Snow becomes wet when it holds liquid water. That happens in three main ways:

• Sun. On a clear day, sunshine warms the surface of sunny slopes to 0 °C and melts it, from the top down.
• Warm air. Mild air, especially with cloud cover at night that stops the snow cooling, warms and melts the snow on every aspect.
• Rain. Rain on snow adds water directly and carries heat into the pack.

Why it matters

A little water helps grains round and bond. More water weakens the bonds between grains, and water running down through the pack can collect on a crust or a layer of coarse grains and weaken it. That is why wet snow avalanches tend to come in clusters when it warms quickly, rains on fresh snow, or warms for the first time in spring.

The daily cycle

On clear spring days the pattern is regular: a hard, refrozen crust in the morning, softening as the sun reaches each aspect (east first, then south, then west), and wet, heavy snow in the afternoon. A clear, cold night refreezes it. A cloudy, mild night may not, and the next day starts wet. How well the surface refroze overnight is one of the most useful things to know on a spring morning.

Rain on snow

Rain falling on dry, cold snow can weaken it quickly, and the rain line in the mountains can move a long way during one storm. When the weather turns cold again, the wet layer freezes into a rain crust that can later become a sliding surface. See crusts and facets.

How Snowvy models it

The model tracks the liquid water in every layer hour by hour: how much each layer can hold, how much drains down, and how much refreezes as the snow cools. When a layer within the top 60 cm has become noticeably wet (liquid water of about 1 % or more) in the last 24 hours, Snowvy reports the "wet snow" concern, with how deep the wet snow goes and the cause it recorded: rain, warm air (with the maximum temperature) or strong sun. It also reports a refrozen surface crust when there is one. The bulletin is still the place to check the danger of wet snow avalanches for your day.

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