What to know about avalanches, what causes them and how to stay safe

An avalanche — a mass of snow moving down a slope — happens most often in the wilderness. Watching for forecasts, bringing the proper gear and getting training to prepare for them can help with outings, according to the National Avalanche Center.

Each winter, about 25 people on average die in avalanches in the United States, the center says, adding, “Some days are dangerous and some days are not.”

Here’s a look at avalanches, how they occur and some tips on how to stay safe:

How do avalanches occur?

It generally takes just two ingredients to create dangerous conditions: A slope of 30 degrees or more and layers upon layers of snow.

Extra pressure on top of that snowpack from weight, wind, rain, heavy snow or motion can cause some of the layers to shear off and slide downhill.

Sometimes a slide happens in the form of loose snow, called a sluff. Sluffs account for only a small percentage of deaths and property damage from avalanches, according to the Sierra Avalanche Center.

Other avalanches are made up of slabs, which happen when a large layer of snow breaks away. Those account for most fatalities.

Another kind of avalanche occurs when wind creates a cornice of snow that hangs over a ridge or the edge of a steep slope. The overhang can collapse suddenly, catching anyone underneath or on top of it by surprise, according to the Colorado Avalanche Information Center.

What triggers an avalanche?

Movement, rapidly changing weather, wind — any one of these things can set off an avalanche. But experts say 90% of slides that cause an injury or death are triggered by the victim or a companion.

That means people who ski, snowmobile, snowshoe or enjoy other winter backcountry activities should check the avalanche forecast before they head out and make sure they have the right safety gear.

Most ski resorts have avalanche protocols or mitigation systems. Often that means checking snowpack stability. Remote detonations are also used to trigger slides intentionally and remove risky buildup before skiers are allowed on the slopes, said Chris Lundy, an avalanche specialist with the National Avalanche Center.

How can one avoid an avalanche?

Don’t think for a second that you can outrun one. Dry slab avalanches typically reach speeds up to 80 mph (129 kph) within seconds, according to the Sierra Avalanche Center. Wet avalanches usually travel around 20 mph (32 kph).

By comparison eight-time Olympic gold medalist Usain Bolt set a record of just under 28 mph (45 kph) in the 100-meter dash in 2009 — and that was on a flat track, not a snow-covered slope. Most people caught in avalanches are on the slope when they occur.

The better plan is to make sure you’re not in a place where one is at risk of occurring. Outdoor enthusiasts can check forecasts at regional avalanche centers or www.avalanche.org.

If you do venture out, it’s a good idea to use the buddy system and be versed in wilderness first aid. Experts say three pieces of gear are essential: an avalanche beacon or transceiver, which sends location signals to others; a shovel to test snowpack or dig out companions; and a thin, folding pole to poke into the snow in search of anyone buried. An avalanche airbag backpack that inflates after the skier pulls the trigger giving the person a chance to stay above or close to the surface during an avalanche.

What were some recent avalanches?

On Jan. 5, an avalanche in California’s Sierra Nevada buried a snowmobiler in snow and killed him, authorities said. The snowmobiler was initially reported missing but then was found under the snow several minutes later, the Nevada County Sheriff’s Office said in a statement.

In June 2025, a skier died after being caught in an avalanche on Alaska's Mount McKinley, North America's tallest peak. The Denali National Park and Preserve said the skier and his climbing partner, a snowboarder, triggered the avalanche high on the 20,310-foot (6,190-meter) peak while descending a slope.

That March, a snowmachiner riding on the backside of a popular winter recreation area about 60 miles (97 kilometers) southeast of Anchorage, triggered an avalanche after riding over a weak layer of snow that was buried under newer snow, according to the Chugach National Forest Avalanche Center. He was buried about 10-feet (3-meter) deep in the slide that measured about 500 feet (152 meters) wide.

Earlier that month, three heli-skiers were killed. That accident happened when they were caught in an avalanche near Girdwood, about 40 miles (64 kilometers) south of Anchorage. Heli-skiing is a type of backcountry skiing that involves using a helicopter to access remote areas and slopes of fresh snow.