The Five-Minute Avalanche Challenge

Snow falls from the sky, pulled by gravity, and lands on the ground. Snow falls from the sky, pulled by gravity, and lands on the ground. On the ground, gravity keeps pulling on the snow, but the ground holds the snow in place.

Sometimes, like on the side of a mountain, the ground is at an angle, and that angle reduces the ground's ability to hold the snow in place. When the force of gravity pulling on the snow is greater than the friction holding the snow in place, the snow slides down the hill.

We call that an avalanche.

Avalanches can happen on their own, spontaneously. We call those naturally-triggered or just "natural" avalanches. Avalanches can also be triggered by a human being. We call those "human-triggered" avalanches.

Whether triggered naturally or by humans, avalanches can be divided into two general categories.

The first is what I call "loosely-coupled" or loosely-organized avalanches. You may know these as wet slides, or loose-snow avalanches, or sluffs. In this type of avalanche, the snow crystals act like grains of sand—they aren't well-connected to each other.

This "disorganized" avalanche happens when one snow crystal shifts and tumbles, bumping into its neighbor, which sets it in motion, which bumps into a new neighbor, which sets it in motion, and so on, and so on, creating a cascade of loose snow that is typically slow and predictable.

Loose or wet snow avalanches can get quite large, but on their own they don't tend to kill people. To become hazardous, they often require another factor, like a cliff or a tree to act in combination.

The opposite of the loosely-organized avalanche is what I call "tightly-coupled." You probably know these as "slab" avalanches. They are the boogeymen of the backcountry—the avalanches, overwhelmingly, that bury and kill people.

In a tightly-coupled avalanche, the snow crystals are entangled not just with their immediate next-door neighbors, but also with distant snow crystals over a wide area. Instead of sand, think of the slab avalanche as being like a plate of glass: when you strike it with a hammer, the entire plate shatters.

Slab avalanches can set truly immense amounts of snow in motion virtually all at once, as if the entire mountainside were shearing off. This makes them extremely destructive and extremely dangerous.

There are two important things you need to know about slab avalanches.

The first is that the snow does not have to look like a slab to act like a slab.

It is extremely important you understand this.

That glorious cold-smoke powder beneath your feet, so light and fluffy, can act like a slab depending on what's happening in the layers beneath it. Loose snow is not inherently free from slab danger. In fact, if it's cold and dry, the better the skiing, the more likely it is you're in danger.

The second thing I have to tell you about slab avalanches is the most important thing of all—the reason, really, that I came up with this five-minute challenge:

Because of their dependency on the interplay of friction and gravity, slab avalanches can only form in a very narrow window of slope angles. 37.5 degrees appears to be the critical angle for optimal slab formation, and this is true no matter where you go.

Slab avalanche danger peaks on slopes around 37.5 degrees in steepness, and dissipates rapidly as the slope angle lessens. Slopes below about 30 degrees, for example, are rarely capable of producing slab avalanches.

This means you can choose to opt-out of being killed by a slab avalanche.

By learning to read the mountain (identifying slope angle and likely slide paths) and by avoiding exposure to critical-angle terrain, you can virtually guarantee you will never be caught in a slab avalanche even if you ignore every other factor in the snowpack.

That is a choice, to be sure.

Lots of people ski complex midwinter snowpacks in the backcountry on critical-angle slopes (or within their runout zones), trusting their skills, experience, and judgment to keep them safe. You too can play that game—or you can opt-out.

Instead of assessing 37° pitches for snowpack stability (and betting your life on the result), you can ski as much lower-angle and ridgeline terrain as you want at a fraction of the hazard.

It's up to you.

And that's what you need to know about avalanches, in five minutes. There is much more you can learn about avalanches and snow science, and I encourage you to do so, via AIARE classes or other resources.

But never forget: your exposure to avalanche hazard is a choice.

→ Next: Learning to Read the Mountain

— December 1, 2023

Andy Lewicky is the author and creator of SierraDescents

Andy December 2, 2023 at 8:56 am

Andy here: this is one of two posts I will be making on the subject of avalanche safety that I think is both critically important and also very likely to be misinterpreted. Expect a lot of revision to occur here, and if you have comments/suggestions you'd like to share, please feel free to post them here, or email me if you'd rather do so privately. lewicky@gmail.com

Joseph J Gregory December 5, 2023 at 1:01 pm

Love it.

geo mellon December 6, 2023 at 9:48 pm

What is the best way/simplest way to determine slope angle?

Andy December 7, 2023 at 7:17 am

Geo, experienced skiers tend to come with this ability built in--the steepness of a typical North American black ski run's headwall is usually right around the 33-37 degree mark.

So you can reference that as a starting point.

There are smartphone apps that turn your phone into an inclinometer: lay your ski pole on the snow to create a straight edge, then put your phone on the pole, and that will give you a spot reading of the angle.

You can also do this with many good compasses, which often have inclinometers--look for these at REI.

Wherever you are, start assessing slope angles and then check your guess with an app or a compass. Start identifying slope angles in Google Earth so you know *in advance* how steep the area you're skiing is going to be and where likely slide paths are going to be.

Develop this as a skill and you'll get good at it.

Ray December 18, 2023 at 9:24 pm

At Mammoth- what would be above 30 degrees - Cornice Bowl? Face of Chair 3 and 5? Climax?

Bill December 21, 2023 at 4:08 am

I kinda think the headwall of San Gorgonio is less than 37 degrees, no? Where did that big avi that cut the path through the old-growth forest on the way up the South Fork trail come from? (Its momentum carried it well past the steeps - which highlights your point: It's not just the slope you're on but the slope you're under).

Andy December 22, 2023 at 8:44 am

I probably should stress that 37-38 degrees is where the crown fracture is going to be, and even just a small rollover (convexity) can create that delicate balance point. Things get most dangerous when there's a large area at and around that angle, which means a lot of potential fuel.

It's remarkably consistent where slab avalanches are concerned. Take a look at this photo of an avalanche on Mammoth's Monument ski run. The mountain is literally telling you where that critical angle is.

Bill, I think you're talking about the slide path underneath Alto-Diablo Peak's northeast face. That's actually a really large slide path. Take a look at it in Google Earth from the northeast. You should be able to identify where the likely trigger zones were.

Matt D December 22, 2023 at 10:23 pm

I like this approach. It reminds me a lot of the hierarchy of hazard controls which holds that the best control is eliminating the hazard (not going out) followed by substitution (skiing lower angle terrain), engineering controls, administrative controls (digging snow pits), and finally at the bottom PPE (avalanche backpacks).