Snow explained

How sun and aspect change the snow

Two slopes on the same mountain, on the same day, can hold completely different snow. The biggest reason is which way they face.

Aspect decides how much sun a slope gets

Aspect is the compass direction a slope faces. In the northern hemisphere the winter sun stays low in the southern sky, so south-facing slopes take most of the direct sunshine. Steep north-facing slopes can get little or no direct sun for weeks in midwinter, especially when a ridge to the south blocks the horizon. East-facing slopes warm in the morning and west-facing slopes in the afternoon. In the southern hemisphere the same pattern applies with north and south swapped.

Sun matters because it is the snowpack's main source of heat on a clear day. The difference between a sunny and a shady slope is often larger than the difference made by a few hundred metres of elevation.

What happens on sunny slopes

• Settling and rounding. Warmth speeds up settlement, so new snow on sunny aspects tends to bond and compact faster.
• Melt-freeze crusts. A warm sunny day wets the surface; a clear cold night refreezes it into a crust. Repeated cycles build the firm-then-soft "corn" snow of spring.
• Wet snow later in the day. As the surface warms through the day, wet snow becomes more likely. It tends to start on east-facing slopes, then south, then west, following the sun.
• Buried crusts. When new snow falls on a sun crust, the crust stays in the snowpack as a smooth, hard layer that later snow can sit on poorly.

What happens on shady slopes

• Cold snow lasts. Powder stays light for longer because nothing warms it.
• Faceting. In a shallow, cold snowpack the ground stays near 0 °C while the surface is much colder. That temperature difference drives water vapour upward and grows angular, poorly bonded crystals called facets, and depth hoar near the ground.
• Surface hoar survives. Feathery frost that grows on clear, calm nights is destroyed quickly by sun on sunny slopes, but can survive on shady ones until the next snowfall buries it.

Because facets and buried surface hoar are persistent weak layers, shady slopes often keep them longer than sunny ones. That is a general tendency, not a rule: a region's weather each season decides what actually forms where.

Elevation, season and time of day

Air cools with height, typically by around 0.5 to 0.7 °C per 100 m, although cold air often pools in valleys during calm high-pressure spells and turns that around. Later in the season the sun climbs higher, so by March even north-facing slopes start to feel it. And the same slope can change within hours: a crust that is firm at 9 am may be soft by noon.

How Snowvy models it

Snowvy splits the terrain into virtual slopes by elevation band, one of eight aspects, steepness and forest cover. For every 30 m cell it computes the sky view and the horizon, so a slope shaded by a neighbouring ridge is treated as shaded. The weather history is then adjusted to each slope's elevation and sun, and the snowpack is run hour by hour. That is why a north face and a south face in the same area get different layers and different explanations. Read how the model works.

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

• 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
• 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