An avalanche is a mass of snow that breaks free from a mountainside and rushes downhill, sometimes reaching speeds above 200 km/h (130 mph). It happens when the forces pulling snow downslope overcome the bonds holding the snowpack together. What starts as a fracture in a weak layer of snow can propagate across a slope for hundreds or even thousands of meters, releasing enormous volumes of snow in seconds.
How an Avalanche Forms
Snow on a mountain isn’t a single uniform layer. It builds up over the winter in distinct layers, each with different densities and crystal structures depending on the weather conditions when it fell. Between these layers, weak zones can develop where the snow grains don’t bond well to each other. These weak layers are the starting point for most avalanches.
When stress on the snowpack increases, whether from new snowfall, wind loading, warming temperatures, or the weight of a person, cracks begin forming in or around that weak layer. If the crack reaches a critical threshold, it propagates outward rapidly. The snow above the weak layer fractures and loses its grip on the slope. At that point, gravity takes over.
Three factors interact to create avalanche conditions, sometimes called the “avalanche triangle”: terrain, snowpack, and weather. Terrain provides the slope angle. Snowpack determines whether a weak layer exists. Weather controls how quickly stress builds. A rapid change in any one of these, like a heavy storm dumping snow on an already fragile snowpack, sharply increases the risk.
Slope Angle and Terrain
Not every steep slope produces avalanches. A Swiss dataset of over 1,000 avalanches found that 96% released on slopes between 30 and 50 degrees, with most occurring between 34 and 45 degrees. Below 30 degrees, gravity usually isn’t strong enough to overcome the snow’s internal bonds. Above 50 degrees, snow tends to slough off constantly rather than building up into dangerous slabs. The sweet spot for large, destructive avalanches is right in the middle, which also happens to be the angle range most attractive to skiers and snowboarders.
Types of Avalanches
Loose Snow Avalanches
Also called sluffs, these start at a single point on the surface and fan out as they descend, picking up loosely bonded snow crystals along the way. They leave a distinctive inverted “V” pattern on the slope. Sluffs are typically small and rarely cause major structural damage, though they can knock a person off their feet and carry them downhill. Wet sluffs, caused by snowmelt from strong sun or rain, carry more mass and are heavier.
Slab Avalanches
These are the dangerous ones. A slab avalanche occurs when an entire cohesive layer of snow fractures and slides as a block over a failed weak layer beneath it. Slabs can propagate across 10 meters or across 1,000 meters of terrain. Because they release large volumes of dense snow all at once, slab avalanches are far more likely to bury and kill people.
Slab avalanches come in several varieties. Hard slabs, often formed by wind packing snow tightly, remain as solid blocks as they slide. Soft slabs are lower density snow that has bonded over a couple of days on the ground. They tend to break apart more during descent and typically don’t propagate as far. Wind slabs form when strong winds deposit new snow on lee slopes. Wet slabs, most common in spring, release when meltwater or rain seeps through the snowpack and weakens a buried layer to the point of failure.
What Triggers Them
The most common natural trigger is simply new snow loading a slope. Wind can deposit snow on one side of a ridge far faster than snowfall alone, creating dangerous wind slabs in hours. Excessive warming weakens bonds throughout the snowpack. Cornices (overhanging lips of snow on ridgelines) can break off and land on slopes below, triggering release. Rockfall, earthquakes, and even the vibration from one avalanche can set off another.
In fatal avalanches, the trigger is overwhelmingly human. A skier, snowboarder, snowmobiler, or hiker crossing a loaded slope adds just enough weight to fracture the weak layer underfoot. This is what makes avalanche education so critical: the people most at risk are often the ones who cause the slide.
Speed and Destructive Force
Avalanches are fast. Measurements from the Ryggfonn avalanche research site in Norway recorded maximum speeds between 135 and 215 km/h (85 to 135 mph), with volumes ranging from 20 cubic meters to 100,000 cubic meters. To put the force in perspective, the maximum recorded impact pressure at Ryggfonn was 541 kilopascals, roughly equivalent to 55 tons pressing on every square meter. Even averaged over 15 seconds, the pressure held at 220 kilopascals. That kind of force can flatten buildings, snap trees, and toss vehicles.
A powder cloud often accompanies large avalanches, racing ahead of the main snow mass. This airborne component can extend the avalanche’s reach well beyond the visible debris and still carry enough force to cause serious injury.
Warning Signs in the Field
Unstable snowpack often announces itself before a full avalanche releases. “Whumpfing,” a deep, drum-like sound and sudden settling of the snow surface beneath your feet, means a weak layer just collapsed under your weight. Shooting cracks, visible fractures racing away from your skis or footsteps, are an even more urgent signal that the slab is ready to release.
Other red flags include recent natural avalanche activity on nearby slopes, new wind loading (visible as pillowy deposits on lee slopes), and glide cracks: deep, crevasse-like fractures that appear on the snow surface above smooth rock or grass. Glide cracks indicate the snowpack is slowly creeping downhill and could release unpredictably, sometimes days or weeks after the cracks first appear.
Survival After Burial
Time is the defining factor in avalanche burial survival. Swiss data, consistent since the 1990s, shows that more than 90% of buried people survive if rescued within the first 15 to 20 minutes. After that, survival drops steeply to about 35% between 20 and 35 minutes, primarily because victims suffocate as exhaled air saturates the snow around their face with carbon dioxide. A brief plateau follows for those lucky enough to have an air pocket, but after 90 minutes, hypothermia combined with oxygen deprivation causes survival rates to fall again.
This timeline is why companion rescue, your skiing or riding partners digging you out, matters far more than organized search and rescue teams, who almost never arrive within that critical window.
Essential Safety Equipment
The standard companion rescue kit consists of three items: a transceiver, a probe, and a shovel. Every person in a backcountry group wears a transceiver set to transmit a radio signal. If someone is buried, the rest of the group switches their devices to receive mode, which picks up the buried person’s signal and guides rescuers to the location. Once the signal narrows the search area, a collapsible probe (essentially a long, thin pole) pinpoints the victim’s exact depth and position. Then the shovel comes out.
Transceivers have been the backbone of backcountry avalanche safety since the 1970s, when Dr. John Lawton invented the first version using a copper antenna sewn into a parka. The technology has been refined significantly since then, and all modern transceivers operate on a standardized frequency of 457 kHz. Passive reflectors, like the RECCO system, are embedded in some clothing and gear and can be detected by specialized handheld or helicopter-mounted units, but these are search and rescue tools, not a substitute for a transceiver in your group.
The Avalanche Danger Scale
Avalanche forecasting centers across North America use the North American Public Avalanche Danger Scale, a five-level system that communicates the likelihood, size, and distribution of expected avalanches. The levels run from 1 (Low) through 2 (Moderate), 3 (Considerable), 4 (High), to 5 (Extreme). Each level comes with specific travel advice. Most avalanche fatalities occur at levels 3 and 4, not because these are the most dangerous conditions on paper, but because people still venture out, often underestimating what “Considerable” means in practice.
Checking the local avalanche forecast before heading into the backcountry is the single most accessible safety step. Forecasts are published daily during winter months by regional avalanche centers and are free to access online.