Winter Sports · General · Snow and mountain science
How Does Snow Form? The Science of Snow Crystals
Snow is made of ice crystals that grow as water vapour turns directly into ice in clouds. Cloud droplets do not freeze straight away at 0 °C; they stay liquid in a supercooled state. According to Caltech physicist Kenneth Libbrecht, most droplets freeze at around −10 °C thanks to an ice nucleus such as a speck of dust in the air; below −40 °C no droplet can stay liquid. The frozen crystal grows by drawing the surrounding vapour onto itself. Temperature determines the crystal's shape, and humidity determines how complex it becomes: thin plates form between about 0 and −4 °C, columns and needles between −4 and −10 °C, and plates again between −10 and −22 °C; the classic six-armed star (dendrite) grows at around −15 °C in humid air. Once on the ground, snow keeps changing with temperature and wind; the snow you feel on the slope is the result of this transformation.
Cloud droplets cool. Clouds are made of water droplets only 10–20 micrometres across. According to Libbrecht, these droplets do not freeze at 0 °C; they stay liquid in a supercooled state.
A droplet freezes. Freezing usually needs a tiny particle to get started: this is called an ice nucleus. Dust particles in the air start freezing at around −10 °C. According to Libbrecht, most droplets freeze at around −10 °C, and below −40 °C no droplet can stay liquid.
The crystal grows. The small frozen piece of ice draws the surrounding water vapour onto itself directly as ice. Because the vapour pressure over ice is lower than over water, the crystal grows at the expense of the supercooled droplets around it. Once it is heavy enough, it starts to fall.
Why six-sided?
The water molecules inside an ice crystal are arranged in a hexagonal pattern. As the crystal grows, this internal order shows on the outside: snow crystals grow as six-sided plates, six-armed stars or columns with a hexagonal cross-section.
Temperature and humidity: the shape of the crystal
The Japanese physicist Ukichiro Nakaya, followed by Kobayashi, grew crystals in the laboratory at different temperatures and humidities and drew up a habit diagram. As Libbrecht sums it up: temperature determines whether the crystal becomes a plate or a column, and humidity (supersaturation) determines how complex it becomes.
Temperature (approx.)
Crystal shape
In humid air
0 to −4 °C
Thin hexagonal plates
Branched plates
−4 to −10 °C
Columns and needles
Hollow columns, long needles
−10 to −22 °C
Plates again
Star (dendrite) crystals; most pronounced at around −15 °C
Below −22 °C
Columns and mixtures of plates and columns
Clusters
The boundaries differ by a few degrees between sources; the International Classification for Seasonal Snow on the Ground, for example, places dendrites at roughly −12 to −16 °C. So read the ranges in the table as approximate.
As a snowflake falls, it passes through layers of different temperature and humidity. That is why some crystals have mixed shapes, such as a column with a plate on its end. Crystals can stick together as they fall and form large snowflakes (clumps); at temperatures close to 0 °C in particular, snow falls in large, wet flakes.
Snow on the ground transforms
Snow keeps changing after it reaches the ground. The International Classification for Seasonal Snow on the Ground (Fierz et al. 2009, UNESCO-IHP and the International Association of Cryospheric Sciences) divides snow grains on the ground into nine main classes. The ones that matter for skiers:
Class
What does it mean?
On the slope
New snow
Freshly fallen snow with recognisable crystal shapes
Light and soft; powder
Decomposing snow
Snow whose crystal arms have broken with wind and time
Starts to settle
Rounded grains
Fine grains that have compacted and bonded together
Firm, groomed-slope feel
Faceted crystals and depth hoar
Weakly bonded grains formed by a large temperature difference within the snowpack
Invisible; can be a weak layer for avalanches
Surface hoar
Feather-like crystals growing on the snow surface on clear, cold nights
Becomes a weak layer if buried by snow
Melt-freeze grains
Coarse grains that melt by day and freeze at night
Icy in the morning, "corn snow" in the afternoon
Faceted crystals and surface hoar can act like a slippery base for a layer of snow on top of them; most avalanches start in weak layers of this kind. The details are in our guide How Do Avalanches Form?.
Snow types on the slope
We explain how these transformations feel on the slope, and the differences between powder, wet, icy and artificial snow, in our guide to snow types.
Seeing snow science on the slope
Spotting star crystals on your jacket sleeve on a cold morning and watching the snow change as it warms up in the afternoon becomes possible once you stay in the mountains for a few days. An activity holiday spent for several days at the same resort is the easiest way to learn snow science by experiencing it on the slope. Doğadayız winter packages include accommodation, ski pass and equipment. When "what was the snow like today?" becomes the group's shared topic over dinner, that is a social adventure holiday in its natural form. For the programmes, see the winter activity holidays page.
Doğadayız Packages
Doğadayız packages for your level
Winter activity holidays with accommodation, ski pass and equipment included, shared with a group.
The easiest way to learn to ski: a winter activity holiday
An activity holiday puts a sport at the heart of your trip: transport, accommodation and meals are planned, so you can focus on learning on the slopes. On Doğadayız winter activity holidays, accommodation, ski pass and ski or snowboard equipment are included in every package; course packages include basic lessons with instructors affiliated with the Turkish Ski Federation, while experience packages let you practise on the slopes at your own level.
What makes the programme different is the group: sharing the slopes with people at a similar level, dinners and nights out together, and a group leader by your side at all times. Solo travellers become part of this social adventure holiday from day one. That is why we describe our programmes as a social adventure holiday.
One of the supercooled water droplets in a cloud freezes thanks to an ice nucleus; the resulting ice crystal grows by drawing the surrounding water vapour onto itself directly as ice, and falls once it is heavy enough. According to Libbrecht, most droplets freeze at around −10 °C.
Why are snowflakes six-sided?
The water molecules inside an ice crystal are arranged in a hexagonal pattern. As the crystal grows, this internal order shows on the outside, which is why snow crystals grow as six-sided plates, six-armed stars or hexagonal columns.
At what temperature do star-shaped snowflakes form?
The classic six-armed star (dendrite) grows at roughly −10 to −22 °C in humid air; it is most pronounced at around −15 °C. The boundaries differ by a few degrees between sources.
The snow on the slope is not the same every day. According to the definitions of MeteoSwiss, the Swiss meteorological office, powder is fresh snow that has not yet been compacted; hard snow is snow compacted by melting and refreezing or by wind, and its most extreme form is crust; corn (spring) snow is moist snow whose surface thaws in the daytime sun, typical in spring. Artificial snow (machine-made snow, the name scientists prefer), according to the WSL Institute for Snow and Avalanche Research (SLF), is much denser than natural snow (fresh natural snow averages ~100 kg/m³, dry machine-made snow ~400 kg/m³) and consists of small, round grains of 0.1–1 mm. That is why machine-made snow creates a more durable but harder slope.
Artificial snowmaking means snow guns spraying water into very small droplets and freezing them in the air; no additives are used, only water and air. According to the slope preparation handbook of the Swiss Institute for Snow and Avalanche Research (SLF), the water is broken up at a pressure of about 12–60 bar into droplets of 0.1–1 mm; the droplets freeze in the air and bond together on the ground to form snow. Snowmaking depends more on humidity than on air temperature: according to the SLF, at a wet-bulb temperature between −2 and −4 °C snowmaking can only just begin. In practice, at most about 2.2 cubic metres of snow are obtained from one cubic metre of water. In Türkiye, Erciyes, for example, has 154 snow guns.
An avalanche is snow on a slope suddenly sliding and flowing downhill. The great majority of fatal avalanches are slab avalanches: there is a weak layer inside the snowpack; the cohesive layer of snow above it (the slab) fractures on this layer under a load, and a wide area breaks away at once. According to the Swiss Institute for Snow and Avalanche Research (SLF), more than 90% of avalanche deaths are caused by slab avalanches. A dry slab avalanche generally needs a slope of at least 30°; the slope angle where human-triggered avalanches start averages around 38–39°. The main factors that increase the risk are new snow, wind-drifted snow, temperature changes and persistent weak layers in the snowpack (surface hoar, faceted grains, depth hoar). The European Avalanche Warning Services (EAWS) group these situations into five typical avalanche problems.
