An avalanche is a mass of snow that rapidly slides down a mountainside, often triggered in seconds and capable of reaching speeds over 130 mph. Most avalanches occur on slopes angled between 30 and 45 degrees, and the majority of avalanches that catch people are triggered by the victims themselves.
How Avalanches Form
Snow doesn’t just pile up uniformly on a mountain. It builds in layers over the course of a winter, and each layer has different properties depending on the weather conditions when it fell. A warm, wet storm creates a dense layer. A cold, calm night can form delicate crystals on the surface that later get buried by the next snowfall. These buried weak layers, sometimes only a few millimeters thick, are where avalanches begin.
Gravity constantly pulls on the snowpack, and as the slope gets steeper, that pulling force increases. When the stress on a weak layer exceeds what it can hold together, the snow above it breaks free. Avalanche risk increases sharply above 30 degrees of slope angle and peaks around 38 to 40 degrees. Counterintuitively, slopes steeper than about 50 degrees are actually less prone to large, dangerous slides because snow sloughs off in small amounts before it can build into massive slabs.
Wind plays a major role. It strips snow from exposed ridges and deposits it on the sheltered, downwind side, creating thick drifts that rapidly add weight to the snowpack below. These wind-loaded slopes are some of the most dangerous terrain on a mountain, especially after fresh snowfall gives the wind plenty of loose snow to move around.
Types of Avalanches
Not all avalanches look or behave the same way. The three main types differ in how the snow breaks loose and how it travels downhill.
- Loose snow (point release) avalanches start at a single point and fan out as they descend, forming an inverted V-shape. They involve only surface snow and are generally smaller and less destructive, though they can still sweep a person off a cliff or into a terrain trap.
- Slab avalanches are the deadliest type. A cohesive plate of snow fractures along a weak layer and slides as a unit. Hard slabs move in one piece, while soft slabs break apart into blocks almost immediately. These are the avalanches responsible for most fatalities.
- Powder avalanches are clouds of snow suspended in the air by turbulence, with no dense core sliding along the ground. They often form when ice or snow falls from extremely steep terrain. Despite looking almost ethereal, the blast of air ahead of a large powder avalanche can flatten trees and structures.
What Triggers Them
Natural avalanches most commonly release during or immediately after storms, when new snow rapidly loads the existing snowpack. Heavy snowfall, rain on snow, and rapid warming can all push a slope past its breaking point without anyone being near it.
But avalanche accident statistics tell a consistent story: the vast majority of avalanches that catch people are triggered by the people themselves. Since 1980, recreationists have accounted for 93 percent of avalanche fatalities in the United States. Skiers, snowboarders, snowmobilers, and climbers make up nearly all of that number. The extra weight of a person crossing an unstable slope is often the final stress that causes a slab to fracture. One finding that surprises many people is that most avalanche victims are highly skilled at their sport. They’re experienced skiers or riders who ventured onto dangerous terrain, sometimes after noticing warning signs like recent fracture lines or the deep “whumph” sound of a collapsing weak layer.
How Fast and Powerful They Are
Large avalanches are extraordinarily fast. Measurements from research sites in Norway have recorded avalanche speeds ranging from about 85 mph to over 130 mph, with volumes varying from a few dozen cubic meters of snow to 100,000 cubic meters. At those speeds, the impact forces are staggering. Researchers at the Ryggfonn test site recorded peak impact pressures of 541 kilopascals, roughly equivalent to 78 pounds of force on every square inch of surface. For reference, a typical wooden house will collapse under pressures far lower than that. Even averaged over a 15-second window, the impact pressure measured 220 kilopascals. A person caught in that kind of force has virtually no chance of survival without being carried clear of the debris.
Survival and Burial Time
If you’re buried in an avalanche, time is the single most important factor. Data from Switzerland spanning four decades shows that survival probability after 10 minutes of burial is about 91 percent. At 15 minutes, it drops to 76 percent. By 30 minutes, only about 31 percent of buried victims survive.
The steep decline happens for two reasons. First, trauma from the initial impact kills some victims almost instantly. Second, those who survive the ride face suffocation. Exhaled breath melts a thin layer of snow around the face, which quickly refreezes into an ice mask that blocks airflow. After roughly 130 minutes, long-term survival rates fall to around 7 percent. That number has improved slightly over the decades thanks to better rescue techniques and equipment, but it underscores how narrow the rescue window really is.
Essential Rescue Gear
Anyone traveling in avalanche terrain should carry three items: a transceiver (also called a beacon), a probe, and a shovel. These three pieces work together, and omitting any one of them seriously compromises a rescue.
A transceiver is a small radio device worn on the body that both sends and receives a signal. Everyone in a group sets theirs to transmit. If someone is buried, the others switch to receive mode and follow the signal to narrow down the victim’s location. A probe, a collapsible pole typically around 8 to 10 feet long, is then used to pinpoint the exact position and depth of the burial. Finally, the shovel is what actually gets the person out. Digging through avalanche debris with your hands is agonizingly slow because the snow compacts into a concrete-like mass almost immediately after it stops moving. The right shovel can significantly reduce the total rescue time, and in a scenario where every minute cuts into survival odds, that matters enormously.
Practicing with this gear before you need it is critical. A transceiver is only useful if you can operate it quickly under stress, and probe and shovel techniques have specific methods that make them far more efficient than just stabbing and digging at random.
Terrain and Warning Signs
Certain features make a slope more dangerous. Convex rolls, where the terrain bulges outward, concentrate stress on the snowpack. Gullies and bowls act as natural collection points for wind-deposited snow. Leeward slopes, the sides of ridges sheltered from the wind, accumulate deep drifts that load quickly during storms.
The snowpack itself gives warnings. Hearing a hollow “whumph” underfoot means a weak layer just collapsed, a direct sign of instability. Shooting cracks, visible lines that race outward from your feet across the snow surface, mean the slab you’re standing on is ready to release. Recent avalanche activity on nearby slopes with similar characteristics is one of the most reliable indicators that your slope could slide too. Clear skies and perfect snow conditions don’t mean the danger has passed. Many fatal avalanches happen on bluebird days when the snow from a recent storm is still sitting on an unstable base.