Snow explained

What modelled snow conditions mean, and what they don't

Snowvy shows modelled snow conditions. Here is what that means, why it can be wrong, and why it never replaces the avalanche bulletin.

What a snowpack model is

A snowpack model is a physics simulation. It takes the weather a slope has had (snowfall, temperature, wind, sun, humidity) and works out, hour by hour, how the snow on that slope builds up, settles, warms, melts, refreezes and changes its grains. The result is a modelled stack of layers, each with a history: when it fell, what happened to it, and what it is like now.

National avalanche services use models like this too, as one input among many. Snowvy uses published model physics (from Crocus and related research) and runs it on each slope class of a built area.

What it is not

• Not an avalanche bulletin. Bulletins are written by trained forecasters who combine models with field observations, reported avalanches, snow profiles and experience, and who issue a danger level on the five-level European or North American scale. Snowvy issues no danger level.
• Not a measurement. Nobody dug a pit on the slope you are looking at. The layers are calculated.
• Not a go/no-go answer. Snowvy never says a slope is safe, and a missing concern does not mean there is no problem.

Why modelled conditions can be wrong

• Weather input. The model is only as good as the weather it is given. Snowfall amounts in mountains are hard to estimate, and an error there flows into every layer.
• Wind at small scales. Wind moves snow around ridges, gullies and boulders at scales finer than any model grid. Wind slabs can be thicker, thinner or somewhere else than modelled.
• Terrain detail. Snowvy groups 30 m cells into slope classes. A small rib, a tree well or a rock band inside that is not resolved.
• People and machines. Grooming, skier traffic and snowmaking change the snow on pistes and popular slopes. The model does not know about them.
• Validation is ongoing. Snowvy is being checked against measured snow depths and against layers reported by forecasters. Until those results are published, treat the model as an informed explanation.

How to use it well

1. Start with the bulletin. Read the danger level and the avalanche problems for your region and day.
2. Use the model for the why. If the bulletin mentions a persistent weak layer, the model's layer history can help you picture when it formed and on which aspects and elevations it is likely.
3. Notice disagreement. Where the model and the bulletin differ, trust the bulletin, and treat the difference as a question to answer with your own observations.
4. Look and test in the field. Recent avalanches, shooting cracks, collapsing ("whumpf") sounds and rapid warming are signs to take seriously whatever any model says.

Why Snowvy explains instead of scoring

A colour or a score hides its reasoning. Snowvy writes every explanation from recorded weather episodes, with dates and numbers you can check, so you can judge it against what you see. See how the explanations are made.

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
• 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
• 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