Weather Explainers

Weather Words: Avalanche

An avalanche is a sudden, fast-moving slide of snow down a mountain slope, triggered when weather conditions weaken the snowpack.

Chances are you've heard of an avalanche, and if you're a skier, you might even fear them. But do you know what actually causes them and how dangerous they can actually be?

An avalanche (sometimes also called a snowslide) is the name for a mass of snow rapidly descending a mountain slope. Snowpacks are made up of many layers, each formed under different weather conditions. When these layers don’t bond well, a slope can become unstable, setting the stage for an avalanche. Snowslides can move downhill faster than the fastest human skier in the world, descending at up to over 200 mph.

Weather plays a major role in avalanche formation. Heavy snowfall can quickly overload a slope, while strong winds can move snow from one area to another, creating dense slabs that rest on weaker layers below. Warming temperatures, rain or even strong sunshine can also weaken the snowpack by adding weight or reducing the strength of the bonds holding it together.

Annually, avalanches kill 150 people around the world and nearly 30 in the U.S. Larger avalanches can move at speeds of 200 mph and weigh as much as 1 million tons.
(Avalanche.org)

Once an avalanche begins, it can accelerate rapidly. Some avalanches start small but grow as they sweep up additional snow, ice, rocks and debris along the way. Others release as large slabs, breaking free all at once and carrying enormous force downslope. Large avalanches sweep currents of cold air in front of them called avalanche winds that can add to their destructive force.

Avalanches are most common in mountainous regions during winter and early spring, but they can occur any time snow is present. Understanding how weather affects the snowpack is critical for forecasting avalanche danger and helping people who live, work or recreate in snowy terrain stay safe.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.