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

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?

STAYIN" ALIVE

Rescuers must work fast to save a completely buried avalanche victim. Nearly 90 percent of buried victims don't survive more than 30 minutes under the snow. "There are stories of prolonged survival," says Dr. Colin Grissom, of LDS Hospital in Salt Lake City, Utah. Some victims have lasted over 24 hours. But most suffocate (die from lack of oxygen). "It depends on how lucky a person is in getting an air pocket in front of them before the snow stops moving," he says.

Victims can create an air pocket by cupping a hand in front of their face, before the avalanche stops. Without an air pocket, breathing through the snow is more work than seeing through Darth Vader's mask. But even with that oxygen reserve, Grissom says, "The person breathes back in that same air they just expired." Eventually, there's more carbon dioxide than oxygen in the air, and the person passes out.

Fresh snow

Layered snow

Old snow

LOADED LAYERS: Like vanilla cake, snow builds up in layers. By cutting through the pile, you can see how safe the snow is. Weak layers flake off like powder and can signal danger.

FAST FACTS

An avalanche Reaches top Speed within Five seconds After it's Triggered.

True Survivor: In 2000, a 25-year-old German snow-boarders was found alive by snow dogs 20 hours after he was buried.

The shape of a snowflake depends on air temperature and humidity.

A human body is about three times denser than avalanche debris, so people tend to sink unless they swim hard against the snow.

Snowmobilers account for the majority of avalanche victims because the vehicles can cover 100 times more terrain in one day than a skier can.

SNOW DOGS

Olive is a Labrador retriever with the Bridger Bowl search-and-rescue team near Bozeman. Montana. When someone falls victim to an avalanche there, rescuers call in this pup to sniff the person out. "The dog's sense of smell is so keen it's a good 200 times more sensitive than the human nose," says Fay Johnson, dog trainer and ski-patrol director at Bridger Bowl.

But before rescue dogs can become heroes, they must complete at least two years of special training--a dangerous job itself. That's because, in some cases, the dogs can go blind. Bright sunlight reflects off the snow into the dogs' eyes. After spending many hours in the snow for rescue training, UV radiation (form of invisible light) can damage the unprotected eyes and cause snow blindness.

To keep rescue dogs from losing their sight, the German company "Dog Goes" developed doggy sunglasses to block UV rays. The glasses help the stylish dogs safely complete their training to become heroes another day.

Did You Know?

* The air you breathe is 21 percent oxygen and less than 0.03 percent carbon dioxide. But what you expire is about 16 percent oxygen and 4 percent carbon dioxide.

* To study what happens in an avalanche, scientist Ed Adams hides inside a shed perched on a mountainside. Then his research team intentionally sets off snowslides and watches while the shed is buried under snow. Adams takes measurements from inside the shed. And his team digs him out afterward.

* Even if you're traveling on a flat terrain downhill, you can still trigger an avalanche uphill. All snow is connected.

* In the Northern Hemisphere, the south side of a mountain will be sunnier and have stronger snow. And the north, shady side will have more powder--better for skiing, but more likely to trigger an avalanche.

Cross-Curricular Connection:

Language Arts: Many Hollywood flicks, such as the PG-13 rated XXX and Die Another Day, show avalanche scenes. Choose a scene to watch and compare it with the facts presented in the survivor science article. Did Hollywood get it right or wrong? Write up a short comparison.

Take it Further: Researchers have invented several devices to help avalanche victims survive. Research one and write a short description of the science behind how it works.

[ILLUSTRATION OMITTED]

Resources

Novices and professionals alike use this Web site to find up-to-date avalanche forecasts, accident statistics, and general information. Visit: www.avalanhe.org/

For an in-depth diagram of how avalanches work, see Science World, March 12, 2001, p. 12

NOVA has a documentary series on avalanches accompanied by a Web site with classroom resources. Check out: www.pbs.org/wgbh/nova/avalanche/

This fun article describes Ed Adams's avalanche research at Montana State University: "A Snow Job," People, January 12, 2004.
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Article Details
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Title Annotation:Physical/earth: forces/weather
Author:Tucker, Libby
Publication:Science World
Date:Mar 8, 2004
Words:1628
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