Could you survive an avalanche? Learn the science behind surviving one of Earth's deadliest natural disasters

Science World, March 8, 2004 by Libby Tucker

It starts with a muffled "whoomphf"--the sound of snow collapsing deep under your feet. Somewhere below the white blanket is a crack that spreads in an invisible wave through the snow all around you. Before you realize what's happening, an entire slab (thick top layer of snow separates from the ground and slides downhill.

You can't escape. The snow spills into your mouth and nose, and carries you downhill like a white-water rapid at a velocity (speed in one direction) near 45 meters per second, or 100 miles per hour. You're tossed like dirty laundry. And the cloud gathers more snow as it accelerates (increases velocity)--reaching a mass or weight of up to 10,000 tons. As it finally stops, snow piles around you and solidifies like cement. Buried alive in this cold, dark place, all you can do is wait for help.

Is this nightmare really caused by the fluffy wet stuff used to build snowmen? You bet. Snowflakes accumulate in layers, called the snowpack, which increase in density (mass per unit of volume) as the snow settles over time (see photo, right).

"A cubic meter (35 cubic feet) of snow that's been sitting on the ground for a while would weigh about 300 kilograms (661 pounds)," says avalanche scientist Karl Birkeland, of the National Forest Service. "And when it gets going in an avalanche it can go through buildings."

An avalanche is a natural disaster that can happen anytime snow builds up on a steep slope. Depending on the weather, snow can collect in such a way that heavy slabs lie on top of weak layers (bottom layers where snow particles aren't bonded well). Sooner or later the pile will collapse.

The good news? There are ways we can prevent triggering the so-called white death.

WARNIHG SIGNS

About 95 percent of human-triggered avalanches are set off by the victims. That means most deadly snow slides can be prevented, if you know the warning signs. "The best survivors are the ones who try to understand everything they can [about what triggers avalanches]," says engineering professor Ed Adams, of Montana State University in Bozeman. "They don't get caught."

An avalanche happens when the force of gravity (pull toward Earth's center) is stronger than the forces that cement snow particles together between a slab and a weak layer. Once weather conditions set up weak layers in a snowpack, all it takes to start a slide is a little extra pressure (force applied over an area) to crush the weak layer and send the slab sliding off the top.

Can loud noises set off a slide, as Hollywood would lead you to believe? "Sound is a pressure wave, but the pressure probably doesn't get large enough (to break the snow)," says Adams. Likewise, simply dropping your sunglasses on a fragile bank of snow is unlikely to set off an avalanche. How much pressure does it take? "Somewhere between the weight of your sunglasses and the weight of your body, most likely," says Adams. The trick is to avoid places where your weight causes too much pressure on the snow.

SPOTTING DANGER

Here are just a few ways to spy spill-prone slopes. First, always call ahead for local weather conditions. "Most avalanches tend to happen during or right after storms, new snowfall, or wind," says Birkeland. Fresh snow adds more mass to the snowpack, making a weak bottom layer more likely to collapse when a person crosses it.

Next, when you arrive at the slope look for snow heaps where an avalanche has already tumbled. Chances are another one will happen in the same place. "It's the biggest bull's-eye indicator of unstable snow," says Birkeland.

Experienced backcountry travelers, who take unmarked paths, also evaluate a spot's danger with a slope meter, an instrument that measures the angle of the mountainside relative to a vertical line. Avalanches mostly happen on slopes between 30 and 45 degrees steep. At these angles, gravity and the snow's sticky forces tend to balance out, Any angle larger than 45 degrees will cause gravity to pull the snow downhill. At a smaller angle, the snow won't slide. With careful planning and observation, you can avoid danger.

HIDE-AND-SEEK

In most cases, an avalanche victim's only chance for survival are his ski partners. Show is heavy and once a person is buried they're stuck like a tongue on a frozen metal pole. "You can't even move a toe in your boom," says Adams. It's almost impossible to dig yourself out. Victims can also become disoriented as they're tossed around--they don't know which way is up.

Avalanche tools, such as locator beacons and a shovel, help your friends find you and dig you out. A beacon is a transceiver (device that sends and receives signals) that creams an electromagnetic field (energy in the form of electric and magnetic waves) around itself. Each skier wears the beacon under the top layer of clothing so it stays in place during a fall. If a member of the party is buried, those who escaped turn their beacons to receive mode, which picks up the signal from the buried beacon. The receiver beeps louder as rescuers get closer to the electromagnetic field's source. A shovel comes in handy when the signal is located. Can you dig it?