White death

Slide into oblivion
Avalanches unveiled
Rotten snow
Avalanche forecasting
Staying safe
Anatomy of a snowflake


Distorted crystal resembles a bread loaf.

Depth hoar crystals form when the temperature changes with depth in the snowpack. Large, faceted grains like this form weak layers that can cause avalanches.

Courtesy Gallatin National Forest Avalanche Center


This car was parked at the wrong time in the wrong place -- under a slide path at Alta, Utah.

Photo by Dan Judd, Courtesy Westwide Avalanche Network.


Slab avalanches slideon a weak layer. What makes the weak layer? * METAMORHISM:- large crystals form when
water vapor moves through the snow, generally near the surface or the ground.  * CRYSTALLIZATION: - frost forms on the snow surface as water in the atmosphere freezes.
Can't get no support
A layer of hoar frost can act like ball bearings under a giant slab of snow. Why would an innocent-looking field of snow suddenly start to slide? Because it's lost its mooring. Something is rotten in Denmark. It can't get a grip. Any number of cliches could apply, but essentially slabs move when they rest on a weak layer in the snowpack. The slab may start moving by itself, but often, an unsuspecting victim shakes the snow to start the slide.

But how does the weak layer form? We figured you'd want to know... When snow falls and lands, it's usually made of the fine-grained flakes we'll check out shortly. As soon as the flake lands, it starts changing. Many flakes undergo metamorhism due to temperature variations.

Three kinds of alterations to snow can cause avalanches:

Depth hoar: Imagine you're a snowflake near the ground. The Earth warms you up, and you start to sweat. (Okay, okay... it's not sweat, but you do pump water vapor into the air spaces between your lovely, fluffy crystals.) This vapor migrates to colder snowflakes and recrystallizes. They get larger, but weaker, and the result is a layer of elaborate but weakly bonded crystals called "depth hoar."

car partly visible under avalanche. Sam Colbeck, studies snow physics at the U.S. Army Cold Regions Research and Engineering Laboratory in New Hampshire. He explains the process this way. "The ground warms the lower crystals, and they're giving up water vapor to the crystals above." The process, he says, is "a form of cannibalism." Neither the density nor the thickness of the snowpack changes, but a weak, unstable and invisible layer forms nonetheless.

In Franz Kafka's mind, metamorhism turned a sleeping man into a giant bug. In snow, metamorhism turns, as Colbeck says, "grains that are round and strongly bonded into grains that are highly faceted, often hollow, and not bonded well." The rate of recrystallization depends on the temperature gradient, so depth hoar forms faster in shallow snow and cold weather.

Near-surface faceting: Apparently a similar phenomenon can occur just under the top of a snowpack. A recent study by Karl Birkeland, director of the Gallatin National Forest Avalanche Center, described the formation of a weak layer called near-surface faceting.

Again, the weak crystals are produced when water vapor forms and recrystallizes under a high temperature gradient. If temperature changes quickly with depth, he says "A significant weak layer of faceted snow formed in just 36 hours," with grains about 1 millimeter across and poor bonding properties.

Was this just make-work for the mountain-lovers who become avalanche forecasters? Apparently not: Birkeland wrote that "Widespread avalanche activity occurred for up to nine days after this layer was buried by 0.50 m of snow," (see "Near-Surface Faceted..." in the bibliography).

Surface hoar: A third type of weakness can occur at the snow surface. If you've ever scraped a frosty windshield on a winter morning, you know water vapor in the atmosphere likes to crystallize on the coldest thing around. Colbeck says a layer of snow gets cold after spending all night losing infrared radiation to the sky.

By morning, a layer of large ice crystals may be deposited on the snow, forming surface hoar that future snowfalls can't bond strongly to. Once surface hoar forms, the slope may be just one good snowfall away from an avalanche.

Any breakthrough on the horizon for avalanche forecasting?

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