The Basics in Becoming Avalanche Savvy: Part I
Add to Del.icio.usAn Introduction to Winter's Natural Hazard
by Alex Sepulveda
Photo: Ethan Green
Who causes avalanches?
On Monday, January 6 a man triggered an avalanche in the Wasatch Mountains of Utah, slid 800 feet over a rock band, and came to a frozen halt under 5 feet of snow. Fortunately, a couple of fellow backcountry skiers witnessed his harrowing experience and were able to dig him out before he suffocated.
This season has seen unprecedented avalanche danger in many parts of the West, including northern Utah, the Tetons, and Central Idaho. In the East, a couple of climbers were killed on Mt. Washington, New Hampshire when a slide ripped through Tuckerman Ravine in late November.
With so much attention surrounding a particularly dangerous season, it’s enough to make you never want to travel in the backcountry. Is it ever really safe? How do you know? Trying to educate yourself can be overwhelming with the loads of info out there and hardcore backcountry skiers constantly chastising clueless beginners, usually out of genuine concern but often just to toot their own horns. It’s intimidating.
So let’s take it from the top, and start with the basics.
What causes avalanches?
Photo: Bruce Tremper
What is an avalanche? There are a couple of basic kinds, loose snow and slab, but slab avalanches are the true danger. Think of the snow pack as a series of layers, akin to a stack of pancakes. The earth would be the plate supporting the stack.
The first storm of the season deposits a couple of feet of snow, your first pancake. It sits a few days, but then another storm comes along and deposits the next layer of snow, your second pancake, and so on for the rest of the season until you end up with a large stack. If you were to tilt your plate of pancakes, eventually some of them would naturally slide off due to gravity. Some would slide sooner than others depending on how much friction there were between the individual pancakes.
Slopes steep enough will exhibit the same behavior; that is, gravity will cause some of the snow layers to shear and slide depending again on the friction (the bond, in the case of snow) between the individual layers. When a layer of snow slides, you have an avalanche.
When do avalanches occur?
Snow can sit in the sun for many days like when evil high pressure moves in over your favorite range of mountains and it seems like it will never snow again. When this happens, the surface crystals exposed to the air and sun will begin to change, melting and refreezing; rotting into loose, granular shards; and morphing into feathery crystals known as surface hoar.The result is a surface that’s about as stable as a layer of champagne glasses. A storm comes along and deposits a fresh layer of snow on this slippery slope, and the only place it wants to go is down. It doesn’t take much to trigger this movement, and all too often it’s an unsuspecting backcountry skier, boarder, or ’biler.
To continue with our analogy, if you pour syrup over the top of the stack and then throw another pancake on top of it and tilt the plate, which pancake do you think will slide first? The one on the slickest surface, right? The snow that rots into a weak surface becomes like the syrup on your stack, slicker than the dickens.
So one time to avoid the backcountry is after a storm following a dry spell because the old surface has had time to develop into a terribly unstable, weak layer.
Where do avalanches happen?
How far do you have to tip your plate until the pancakes come a tumbling? The average intermediate run at a resort is 30°, and an advanced 35°. Avalanches generally occur on slopes between 32°-45°, but are certainly not limited to this range[1]. Lower angle slopes are safer since gravity’s effect will be diminished. Very steep slopes, because they are so steep, are also less prone to slide since snow tends to slough off naturally never having the chance to form a slab. (Steep terrain still slides frequently, however, and is way more dangerous than low angle.)
At the resort, ski patrollers perform “control work” to eliminate avalanche hazards by throwing hand charges (explosives) along a series of routes to initiate any potential avalanches. Another tactic in control work is to strategically shoot the surrounding mountains with a gun called an Avalauncher, a modified Howitzer capable of delivering a blast from afar. The disturbance is more than enough to bring down anything poised to slide. For many, waking up to the sound of distant explosions means it’s going to be a beautiful day of powder skiing at the resort.
Photo: Jason Mitchell
In other words, so long as you’re skiing inbounds, respecting run closures, and heeding posted signs, you shouldn’t have to worry about an avalanche catching you at the resort (though it happens on occasion).
In the backcountry, you and your partner(s) are on your own. No control work done there.
Why do avalanches happen?
But surely in a season there are many extended periods when it doesn’t snow, so why aren’t weak surfaces developing and avalanches ripping out all the time? The answer is because old snow will typically bond to new snow given enough time. If you let your stack sit for a while, the syrup will harden up and “glue” the pancakes together.
To that end, the vast majority of avalanches occur within 48 hours after a storm. Delayed action avalanches, those released after 48 hours, are less common because the snow has usually had enough time to bond to the new layer. (This certainly doesn’t mean that 48 hours after a storm, it’s a free-for-all in the backcountry. Quite the contrary; there’s that much more snow that can avalanche.)
Surface hoar (pictured below) is one the most notorious types of weak layer[2], and develops when moisture from supersaturated air condenses and freezes onto a cold snow surface. It is the winter equivalent of morning dew, or in the case of our stack o’ pancakes, the syrup you pour over it. But unlike syrup, surface hoar doesn’t always bond (harden) to the layer (pancake) above it. The result is a vulnerable, weak slab that is ready to give at the touch of a ski.
