Why Do Avalanches Strike Suddenly
on Slopes That Look Calm?
A pure white, gently sloping snowfield. Nothing about it looks dangerous. But underfoot, an invisible "fragile layer" can be hiding. Many avalanches are thought to begin when this layer, sometimes just a few tens of centimetres thick, breaks apart.
A hiker is out in the backcountry (an unmanaged mountain slope) one winter day. Ahead lies a quiet, beautiful snowfield, the wind has dropped, and there's not a footprint in sight. It looks easy to walk across.
Step by step, they move across the slope. Then, from underfoot, comes a dull "whumpf", and a crack shoots across the surface. In the next instant, the whole slope starts moving as a single slab.
This is an avalanche. Right up until that moment, nothing looked out of the ordinary.
There are only two reasons avalanches happen
Fallen snow builds up in many layers. One of those layers can be far more fragile than the rest, and when it breaks, everything above it slides off together.
Slopes that are too steep, or too gentle, are actually thought to be less prone to avalanches. The range of 30β45 degrees is the most dangerous.
Let's look at each one in turn.
Reason 1: A "weak layer" hidden in the snow
Snow doesn't all fall at once. It snows, then stops; the surface changes in the sun; then it snows again. Repeated over and over, this builds up a structure like a millefeuille of snow, with many layers of differing character stacked on top of each other.
Within these layers, one can form that is much more weakly bonded and fragile than the rest. This is called a "weak layer." When firm, stiff snow (a slab) keeps piling up on top of a weak layer, that weak layer ends up bearing the load the whole time. Then, at some moment β triggered by a person's weight, vibration, or a change within the weak layer itself β the weak layer breaks. The entire slab above it then slides off as one piece.
Because a weak layer forms inside the snowpack, looking at the surface alone can't tell you whether a slope is dangerous. Experts dig pits to examine the cross-section, or press on the layers by hand to check for weak layers, but this takes specialist knowledge and experience. The scary thing about avalanches is that "it looks calm" does not mean "it's safe."
Reason 2: Slope angle has a "danger zone"
Roll a ball down a slope: on a gentle slope it barely moves, but on a steep one it takes off right away. Snow on a slope works the same way β the steeper the slope, the stronger the force pulling the snow downward.
And yet, avalanches don't happen most often on near-vertical, cliff-like slopes. Statistically, slopes of roughly 30β45 degrees are the most dangerous. There are two reasons. On gentle slopes (under 15 degrees, say), the force pulling snow downward is weak, so even a weak layer rarely breaks. On very steep slopes (60 degrees or more), snow can't keep accumulating in place β it keeps sloughing off in small amounts β so a large slab and weak layer combination struggles to form.
So what should you do?
- Check avalanche reports and slope angle before heading outAvalanche centres publish forecasts, and maps or apps can show you slope angles in advance.
- Don't put several people on a risky slope at onceCross one at a time, spaced apart, so everyone isn't caught at once.
- Carry an avalanche beacon, probe, and shovelThese are tools for finding a buried companion. Owning them isn't enough β practising how to use them matters just as much.
If you're caught in an avalanche, advice suggests "swimming" with your arms and legs to try to stay as close to the surface as possible. Just before you're buried, cupping a hand in front of your mouth is said to improve your chances of creating an air pocket you can breathe from.
The survival rate for someone buried is said to drop sharply as time passes. Since suffocation is the main cause of death, it's crucial that people nearby start searching immediately. Alongside calling emergency services, starting a beacon-and-probe search right away is recommended.
That said, there are cases where rescuers themselves get caught in a second avalanche on the same slope. It's important to check the route from a safe spot and assess whether multiple slopes could slide at once before starting a search.
Summary
Avalanches strike suddenly on slopes that look calm because two things line up: β an invisible weak layer hiding inside the snow, and β‘ a slope angle of 30β45 degrees, the most dangerous range. A calm appearance is no guarantee of safety.
The quieter a slope looks, the less quiet it may be underneath.
Avalanches begin in a layer you can't see.
Being buried in an avalanche also means being unable to move while your body keeps losing heat, which raises the risk of hypothermia at the same time.
- Stack five or six thick books or notebooks, and slip a slick sheet (like a plastic sleeve) in around the third one from the bottom
- Slowly tilt the stack, and at some angle, only the books above the slick sheet will slide off together
The slick sheet plays the role of the "weak layer." The books below stay put while the ones above slide off as a single slab β very similar to how a slab avalanche happens.
Want to know more? β Terms, formulas, and textbook linksWe label which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics"
- HS+Covered in high-school "Physics," or textbook advanced/sidebar content
- Univ.Not taught in high school β university specialist courses (snow/ice science, fracture mechanics)
- ResearchNot yet settled "textbook fact" even at university β an active research question
MSTerms: the vocabulary of avalanches
- Weak layer: A layer within the snowpack that's more fragile than the layers around it.
- Slab: A firm, stiff sheet of snow sitting on top of a weak layer. When the weak layer breaks, it slides off as one piece.
- Surface avalanche: An avalanche where only the layers near the surface slide off.
- Full-depth avalanche: An avalanche where the entire snowpack slides off the ground beneath it. These are said to be common in early spring, when temperatures rise.
HSChecking with formulas: how slope angle affects force
For an object on a slope, only part of gravity acts in the "sliding" direction. This can be explained using the high-school physics idea of resolving forces.
Force along the slope = Gravity Γ sin(slope angle)
| Force along the slope | The size of the force pulling the snow down the slope [N] |
| Gravity | The gravitational force on the mass of snow (mass Γ 9.8) [N] |
| Slope angle | The incline from horizontal [Β°] |
The larger the angle, the larger the sine value β and so the larger the force along the slope (the force pulling it downward) is thought to be.
| Gravity acting on the snow | 100 Γ 9.8 = 980 N |
| At 15Β° slope | 980 Γ sin15Β° β 980 Γ 0.26 β 253 N |
| At 35Β° slope | 980 Γ sin35Β° β 980 Γ 0.57 β 560 N |
| At 60Β° slope | 980 Γ sin60Β° β 980 Γ 0.87 β 849 N |
β» Sine values are approximated to two decimal places.
Speed = β(2 Γ 9.8 Γ drop height)
| Falling a 50m drop | β(2 Γ 9.8 Γ 50) = β980 β 31.3 m/s β 113 km/h |
| Falling a 100m drop | β(2 Γ 9.8 Γ 100) = β1960 β 44.3 m/s β 159 km/h |
This is an ideal upper-bound estimate that ignores friction and air resistance. Real avalanches move slower than this, but dry, powdery avalanches have reportedly been observed moving close to these speeds. That means a wall of snow can bear down on you at a speed equal to or faster than a car on a motorway. It's not a speed a person can outrun.
HS+Why does "too steep" also lower the danger?
Calculations β and β‘ show the sliding force increasing monotonically with slope angle. But the fact that actual avalanche frequency peaks at 30β45 degrees is because of a separate factor: not just the size of the force, but how much snow the slope can actually hold. On very steep slopes, snow keeps sloughing off in small amounts before it can build up, so a large weak layer and slab combination struggles to develop. "The mechanical force pulling it downward is strong" and "large avalanches actually happen often" are not necessarily the same thing.
Univ.Why does a slab break as a single "sheet"?
The failure of a weak layer is thought to be a phenomenon called fracture propagation, in which failure spreads rapidly across an entire plane rather than at a single point or along a line. Once a crack forms in the weak layer, it can travel through the layer at speeds close to the speed of sound, spreading failure over tens or even hundreds of metres almost instantly. This relates to particular mechanical properties of snow as a material (a balance of brittleness and toughness), and is studied in university-level snow and ice science and fracture mechanics.
ResearchWhat's still unclear
- Accurately predicting in advance when, and under how much force, a weak layer will break is still said to be difficult. The internal structure of snow varies greatly from place to place, and even a few metres apart on the same slope, a weak layer might be present or absent.
- Precisely simulating how far and how fast a fracture will propagate is still an area of ongoing research. The challenge is how accurately the complex mechanical properties of snow as a material can be captured in a computational model.
- How climate change will affect future avalanche patterns is a subject researchers still disagree on. It's been suggested that changing patterns of snowfall and temperature could alter how readily weak layers form, but detailed regional predictions are still an active area of research.
Links to textbooks (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science β how forces work | The basic picture of snow sliding on a slope |
| HS | Physics Basics β resolving forces | The whole of calculations β and β‘ |
| HS+ | Physics β mechanical energy, statistical view of phenomena | Estimating fall speed, the link between angle and frequency |
| Univ. | Snow/ice science, fracture mechanics | The mechanism of weak-layer fracture propagation |
| Research | Avalanche science, climate science (ongoing research) | Fracture prediction models, simulation, effects of climate change |
| β | Disaster prevention, outdoor safety education | Checking avalanche reports, beacon/probe/shovel, what to do if buried |
- Explanatory material on avalanche mechanisms and safety measures from avalanche specialist organizations, including the Japan Avalanche Network (ζ₯ζ¬ιͺε΄©γγγγ―γΌγ―).
- General accounts of weak layers, slab avalanches, and fracture propagation from snow and ice science textbooks.
- General accounts of resolving forces on a slope, free fall, and conservation of energy from physics textbooks.
- Material on mountain rescue and outdoor safety education (general accounts of what to do when buried, and search equipment).
β» Figures such as mass, angle, and drop height are approximations and assumptions used to illustrate the mechanism. The actual speed and scale of avalanches vary greatly with snow conditions, terrain, and weather.
β»This article is a general-audience science explainer. For actual safety decisions while hiking or skiing, follow the guidance of specialist organizations such as avalanche centres and the instructions of experts. The figures given are approximations and assumptions meant to illustrate the underlying mechanism.