How Do Avalanches Occur?

skier in avalanche GIF
Is it always snowboarders?

Snow is the key ingredient in an avalanche, but not all snow is created equal. Some heavy snowfall will be much more likely to cause avalanches than other heavy snowfalls; it just depends on the conditions.

The good news is that it’s not a total mystery. An experienced eye can predict an avalanche. They cannot predict the exact details (i.e. timing), but just by looking at a slope and knowing the previous weather conditions, they can determine which ones pose an avalanche danger.

So how do they know?

Where Do Avalanches Happen?

Since you need a lot of snow, the usual locations for avalanches are the Himalayas, the Alps, and the Rockies.

Between 1950 and 2000, over 1,100 people died due to avalanches in the US. On average, 28 deaths are reported annually.

Most deaths by avalanches in the US happen in Colorado, with an average of about six each year. Fatal incidents often take place in quick succession. According to a study by Colorado Avalanche Information Center, 70% of deaths happen within 100 hours of previous events. More people die every year, since more and more thrill seekers are choosing to ski and snowboard in remote, avalanche-prone locations.

While you’re most likely to die in Colorado, you might also experience an avalanche in Washington, Utah, Alaska, or Montana. Seek your thrills wisely!

What Causes an Avalanche?

An avalanche happens when the snowpack becomes loose and slides away. Snowpack is usually stable in the winter, but spring thaws can cause some layers to melt. If an internal layer melts and refreezes, you may have tons of snow on top of ice, which is famously slippery substance.

Snowpack facing the sun may also cause a weight imbalance as it melts. If the snowpack is near a cliff, it may take very little to topple it. Heavier sections—measuring perhaps half a million tons—may also pull or push nearby snowpack when they fall, giving the avalanche more force. It’s just a house of cards, y’all.

Triggers are usually mild storms, small earthquakes, and human activity. Some avalanches stabilize quickly and pose less danger to those nearby, but some gather mass quickly and become very dangerous events.

Types of Avalanches

  1. Slab Avalanches — These are by far the most common and deadly. “Slab” in the name refers to slabs of ice, which can be several feet thick. Being struck by this type of ice can mean instant death or serious trauma. Slab avalanches reach 80mph in about six seconds, and decimate almost everything in their paths.
when do avalanches occur, slab avalanche
  1. Powder Avalanches — Fresh powdery snow moving at frightening speeds (close to 200 mph) makes a powder avalanche the most spectacular to behold. This snow is not frozen in slabs, so much of its mass is airborne it travels. Clouds of snow are likely to reach valleys below, so even the non-skiers should keep an eye out for these. Ahoy, snow volcano!
when do avalanches occur, powder avalance
Powder avalanche with typical cloudy appearance
  1. Wet Snow Avalanches — Powdery snow mixes with rain or melted ice, and forms a slurry of doom. As liquid penetrates lower layers of snow and ice, the slurry gathers and develops a custard consistency. This diabolical custard comes loose and makes its way downhill more slowly than the other types. Delicious.
when do avalanches occur, wet snow avalanche
Wet snow avalanche with a slurry-like appearance
  1. Ice Avalanches — You’ve seen this in every climate change clip. These are the crumbling glaciers we fear so deeply: a solid block of ice calves off at its weakest point and drops into the sea. While they pose a grave existential danger to humanity as a whole, this type of avalanche is less deadly to individuals, since not many people spend time in the Antarctic. You can also see this type of avalanche on land, if you visit the Khumbu Glacier at the base of Mt Everest.
when do avalanches occur, ice avalanche

An ice cornice collapses into the ocean

Now that you know what you’re up against, what should you take with you on your snow adventures?

About the Authors

It takes a village! We are researching, writing and fact checking as a family. Collaboration is the name of the game, whether we’re running from a zombie horde or finding the best way to turn a complex concept into a deliciously digestible set of bullet points.

Katherine Esperanza is a Los Angeles based writer. When she's not conjuring new queer slice-of-life short stories, she's busy watching the newest films, out at queer shows, supporting queer artists, or just checking out the queer community as a whole.

A former international non-profiteer, small business owner, and co-op'er, Katherine is delighted to help introduce more leftist politics into the disaster preparedness/prepper sphere, which is currently far too right-wing.