Slide into oblivion
Photo by P.Chernouss, Courtesy Westwide Avalanche Network.
Avalanches unveiled
Rotten snow
Avalanche forecasting
Staying safe
Anatomy of a snowflake
Stand back! This avalanche, released by a 160-millimeter mortar in Russia's Murmansk region in 1998, had a volume of 30,000 cubic meters.
Sno joke
Avalanches are rapid snow slides, and you'd have to survive one to know their true terror. We've not had the pleasure, but here's what we hear it feels like:
You're tumbling down a slope.
You may smash into trees or rocks.
Your eyes, nose and mouth can get plugged with snow.
You don't know which end is up.
In a shroud of ice
If, as instructed, you manage to carve out a little air space around your face as the slide grinds to a halt, heat from your breath will soon ice up that lifesaving air cavity. This impenetrable "ice mask" may asphyxiate you within half an hour. Asphyxiation is the major cause of death in avalanches.
Fewer than half of those who are totally buried survive, and nobody who's been buried deeper than seven feet has lived to tell about it.
Here's a painful irony: Almost all avalanches that bowl people over are set off by the victims themselves, or by people in their party.
Don't believe avalanches cause snow to become "as hard as concrete"? Here's how the Anchorage Daily News described efforts to clear slides that cut Gridwood, Alaska, off from the world this February: "State workers and contractors attacked the Seward Highway avalanches with dynamite and iron Thursday..." [Emphasis ours].
Gridwood, we should note, had already been isolated for four days. As the Daily News reported, "Officials were optimistic they could safely work through the night and end the siege of Gridwood by noon today. If they do, it wouldn't be soon enough for the people trapped there with dwindling supplies of food, medicine, fuel, beer and diapers." [Emphasis, well you understand...]
Avalanches come in two essential flavors:
A loose snow avalanche starts at a point on the slope and widens as it travels, releasing more snow.
The far more dangerous slab avalanche occurs when a hunk of snow -- already bonded by something like the wind -- starts sliding downslope. As the slab slides, it breaks into great chunks of hardened snow that trap or injure victims. After a slab avalanche, a clear fracture line usually marks the point where the slab cleaved away from the slope.
As an increasing number of visitors head into the winter backcountry, avalanche "productivity," if we may use that term, is up. In the winter of 1998-99, 32 people died during snowslides in the United States, compared to only 15 in 1983. Traditionally, the victims have included skiers and climbers. Today, an increasing number are back-country snowboarders and gasoline-crazed snowmobilers.
(While we're talking deadly snow, a growing cause of death in areas with deep snowfall is simply getting stuck in loose snow (see "Eight Non-Avalanche..." in the bibliography).
Starting point
To start an avalanche, you need a slope of roughly 30 degrees to 45 degrees. On significantly steeper slopes, the snow generally "sloughs" off before it can build into an avalanche. On shallower slopes, the snow has little reason to move -- although avalanches often extend beyond the slope onto flatter "run-out zones."
Snow, the other requirement, is a much more complicated subject. The more dangerous slab avalanche requires a big hunk of snow that suddenly lets go. Slabs form when snow is disturbed -- either by melting, or more commonly by wind. That's why lee (sheltered) slopes, where snow builds up, can be particularly dangerous.
But a build-up is nothing without a mechanism to release it, and it's explaining this suddenly-lets-go part that we dread. Any time we start talking crystal structure, The Why Files is out of its depth.
Read all about the crystalline crisis.
