Whether you’re hunting for deep powder to ski or just out exploring the backcountry, avalanches are one of the biggest winter dangers you’ll face in the mountains. Learning the basics of avalanches—what causes them and how to assess snow conditions, for example—will go a long way toward keeping you safe as you explore. Anyone considering a backcountry trip should complete a proper avalanche safety course beforehand, but the info below will give you a good primer on what you need to know.
There are three necessary conditions to create a slide—this is called the Avalanche Triangle.
Avalanches can only happen in certain types of terrain, and the most important factor is pitch or steepness. Most avalanches happen on slopes with an incline between 30 and 45 degrees. Steeper inclines won’t accumulate enough snow to create an avalanche, and snow on shallower slopes typically won’t slide.
Keep in mind that areas surrounding avalanche-prone slopes can be at risk of avalanches, too. If you’re on flat ground but there’s a steep hill above you, for example, you might be at risk of snow sliding down on top of you. Use a device called an inclinometer (traditionally a bubble level that tells you the degree of the slope) or even an app on your phone to check the angle of a slope (iPhones come with a built-in Measure app; lay your phone on the snow to check the angle). Always be aware of what’s around you.
In addition to the slope aspect, look out for “terrain traps.” These are terrain features that can exaggerate the effects of an avalanche. The main ones to know are gullies (where snow can pile deeper), cliffs (which an avalanche can push you over), or trees (which sliding snow can throw you into). Each one can make an avalanche much more dangerous. For example, even a small slide that dumps into a terrain trap like a gully can bury someone in the gully under several feet of snow.
Terrain Red Flags
Steep Slopes: Avalanches typically happen on slopes between 30 and 45 degrees.
Terrain Traps: Cliffs, gullies, trees, and other obstacles
Prior Slide Paths: These are areas where avalanches have occurred previously. Look for sections of downed trees or other debris that indicates a past slide.
Avalanches require unstable snowpack. Instability is the result of a variety of factors, but, in general, slides occur when a layer of heavy snow rests on top of a layer of weaker, looser snow. A weak layer can start as loose snow formed by cold or wind, a firm slippery layer formed by frozen rain, or countless other scenarios. Weather conditions have a big influence on snowpack stability (see below), but combined with steep terrain, an unstable expanse of snow can be ripe for a slide.
Snowpack Red Flags
Cracks: If you’re walking across a slope and see a lightning bolt crack shoot across the surface of the snow, that’s a sign of weakness deep in the snowpack and of the potential for an avalanche.
Postholing: If you experience a deep, loud collapse in the snowpack as you cross it, that’s a major red flag. The snowpack can’t take your weight and is likely unstable.
The weather plays a major role in creating the circumstances that can lead to an avalanche. Along with precipitation, wind and temperature changes (even changes that happened weeks in the past) can create unstable snow.
Weather Red Flags
Heavy Snow: Big storms with lots of precipitation put more weight on the snowpack. If a heavy layer of fresh snow falls on a weak layer, it could lead to an avalanche.
Temperature Swings: Any big change can destabilize the snowpack.
High Winds: Strong winds can move as much snow (or more) as precipitation. The wind picks up snow from one place and dumps it in another, putting more weight on the snowpack.