Winter Sports · General · Snow and mountain science
How Do Avalanches Form? Snow Layers, Weak Layers and Slope Angle
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.
A cohesive layer of snow fractures on the weak layer beneath it, and a wide area breaks away at once
According to the SLF, more than 90% of avalanche deaths
Loose snow avalanche
Starts from a single point and widens into a pear shape as it descends
Generally on slopes of around 40° and steeper
Wet snow avalanche
Warming or rain saturates the snowpack with water and it loses its strength
Spring and afternoons
Glide-snow avalanche (glide)
The entire snowpack slides over smooth rock or grassy ground
According to the SLF, cannot be triggered by people; its timing cannot be predicted
The slab and the weak layer
The snowpack is made up of layers that pile up with each snowfall. Some layers are dense and well bonded, others loose and weak. According to the 2003 review by Schweizer, Jamieson and Schneebeli, a slab avalanche has three components:
Slab: A cohesive layer of snow that moves as one. In human-triggered avalanches the average thickness is about 0.5 metres.
Weak layer: The poorly bonded layer of snow beneath the slab.
Trigger: An additional load. This can be a skier's weight, new snow or wind-drifted snow.
According to the same review, if there is no weak layer, new snow or wind does not upset the stability of the snowpack. In other words, the key to an avalanche is the weak layer, which is often invisible to the eye.
Persistent weak layers
Some weak layers stay in the snowpack for weeks, or even the whole season:
Surface hoar: Feather-like crystals that grow on the snow surface on clear, cold nights. Once covered by new snow, they become a buried weak layer.
Faceted grains: Poorly bonded grains that form when there is a large temperature difference within the snowpack.
Depth hoar: Large, angular crystals near the ground; they develop especially in a thin snowpack and in cold weather.
We explain how snow crystals change once they are on the ground in our guide How Does Snow Form?
Slope angle: 30–45 degrees
Slope angle
In terms of slab avalanches
Below 30°
Dry slab avalanches are rare (but if you are below a steeper slope, you may be in an avalanche's path)
30–45°
According to Avalanche Canada, the great majority of avalanches start in this range
Average 38–39°
Average starting slope angle of 809 human-triggered avalanches (Schweizer et al. 2003)
Above 45°
Slab avalanches are less frequent; loose snow avalanches, on the other hand, are typical on these steep slopes
Some sources extend this range up to 50°. Slope angle is hard to judge by eye; experts use an inclinometer or a topographic map.
Factors that trigger avalanches
Schweizer et al. explain avalanche formation with five contributing factors: terrain, precipitation, wind, temperature and the layering of the snowpack.
New snow: According to the review, 30–50 cm of new snow can be critical; in unfavourable conditions even 10–20 cm is enough. The rate of snowfall also matters: a lot of snow falling in a short time gives the snowpack no time to adjust.
Wind: After new snow, it is the most important factor. Wind carries snow to the leeward side of a ridge, where it builds thick, dense wind slabs and cornices.
Temperature: Rapid warming, sun and rain weaken the snowpack and lead to wet avalanches. Long cold spells, on the other hand, help weak layers such as faceted grains to develop.
Aspect and altitude: According to the SLF's Swiss data, the risk on north-facing slopes is about twice that on south-facing ones, and at 2,700 metres it is more than four times that at 1,900 metres.
The EAWS's five typical avalanche problems
The European Avalanche Warning Services (EAWS) describe the situation in avalanche bulletins using five typical avalanche problems:
Avalanche problem
What does it mean?
New snow
The danger created by freshly fallen snow
Wind slab
Snow carried and deposited by the wind
Persistent weak layers
Weak layers that remain in the snowpack for a long time; the most treacherous problem
According to the "Ways to Protect Yourself from Avalanches" (Çığdan Korunma Yolları) leaflet of the Turkish State Meteorological Service (MGM), avalanche risk in Türkiye is highest in the north-east and east of the country, mostly in areas above 1,000 metres. For the risks of leaving the pistes at ski resorts and examples from Türkiye, see our guide Avalanche Risk When Skiing Off-Piste.
Open pistes and off-piste
Open, marked pistes are protected as part of the ski resort's avalanche safety work. Avalanche risk mainly applies on closed pistes, outside the piste boundaries and in off-piste areas. Only people who have had avalanche training, carry avalanche equipment (transceiver, probe, shovel) and go with a local guide should go off-piste.
Skiing with an understanding of the mountain
Knowing about avalanches means understanding the mountain better, even for a skier who stays on marked pistes. On an activity holiday spent on the mountain for several days, you see for yourself why closed-piste signs and piste boundaries matter. Doğadayız winter packages include accommodation, ski pass and equipment. Following the rules together and looking out for each other when skiing as a group is the foundation of a social adventure holiday. For programmes, see our 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.
The great majority of avalanches are slab avalanches: the cohesive layer of snow above a weak layer in the snowpack fractures on this layer under a load, and a wide area breaks away at once. According to the SLF, more than 90% of avalanche deaths are caused by slab avalanches.
On what slope angle do avalanches happen?
A dry slab avalanche generally needs a slope of at least 30°. According to Avalanche Canada, the great majority of avalanches start between 30° and 45°; in 809 human-triggered avalanches, the average starting slope angle was around 38–39°.
How much new snow increases avalanche risk?
According to the review by Schweizer and colleagues, 30–50 cm of new snow can be critical; in unfavourable conditions even 10–20 cm may be enough. After new snow, wind is the most important factor.
Marked and open slopes are secured against avalanches; off-piste terrain is not, and skiing there is entirely at your own risk. Even in Türkiye, skiers who went off-piste have lost their lives in avalanches: at Erciyes in 2022 and at Palandöken in 2024. If you are thinking of going off-piste, there are three conditions: reading the avalanche bulletin (on the European scale of 1–5; about half of deaths happen at level 3), everyone in the group carrying a transceiver, probe and shovel, and having learned to use them through training. A buried person's chance of survival is above 90% in the first 10–18 minutes but drops sharply after that; the rescue is up to the friends you are with.
An avalanche bulletin is a daily forecast showing the avalanche danger in a region on a scale from 1 to 5, on which aspects and elevations it is expected and with which avalanche problem. In Europe the scale is set by the European Avalanche Warning Services (EAWS): 1 low, 2 moderate, 3 considerable, 4 high, 5 very high. The level is based on how unstable the snowpack is, on how many slopes this is found and the size of the largest expected avalanche. An important detail: according to data from the Swiss Institute for Snow and Avalanche Research (SLF) and the EAWS, about half of avalanche deaths occur at level 3 (considerable), because this level is issued often and people underestimate the danger. The bulletin is a regional forecast; it cannot tell you the situation on a single slope and does not replace an on-site assessment. In Türkiye, no daily regional avalanche bulletin with levels 1–5 is published; for Bansko, the Bulgarian Avalanche Association publishes a bulletin.
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.
