Snow explained

New snow: what a storm does to the snowpack

A storm is what skiers wait for. It is also when the snowpack changes most quickly.

Fresh snow needs time

New snow falls as delicate crystals with little bond to each other or to the snow below. Over the following days the crystals break down, the snow settles and the layer gets stronger. Cold weather slows this down; mild weather speeds it up. That is why the first day or two after a big snowfall are usually the most sensitive.

How much is a lot

Bulletins look at how much snow fell over the last few days, together with wind and temperature. A common rule of thumb in avalanche education is that around 30 cm or more of new snow within three days deserves attention, and less is needed when it comes with strong wind. The exact amount that matters depends on the region and the conditions, which is what the bulletin is for.

What it lands on

New snow bonds well to a soft, rough surface and poorly to a smooth or weak one. A sun or rain crust, surface hoar or faceted snow underneath can keep the storm snow from bonding for days. Snow that falls warm and then turns cold, or cold and then turns warm, can also form a weak boundary inside the storm snow itself.

Wind makes it more

Wind during or after a storm moves the new snow into lee slopes and gullies, where it can be several times deeper than on open ground. See wind slab.

How Snowvy models it

Each storm becomes its own layer in the model, with its date, density and whether the wind packed it. When about 30 cm or more of new snow has fallen in the last three days, Snowvy reports the "new snow" concern with the amount. It also checks the old surface under the storm snow: if the storm slab is thick enough and the surface underneath is a crust, persistent grains or much harder or coarser snow, it reports that interface and the date the storm began. Compare it with your avalanche bulletin, which reflects what forecasters have seen in the field.

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
• Wet snow: how warmth, sun and rain change 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