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Transcript Edit

Text reads: The Mysteries of Life with Tim and Moby

Tim is skiing down a slope and stops. Moby is skiing behind him and smacks into Tim. They fall into a snowbank.

TIM: Watch where you're going.

Tim reads from a typed letter.

TIM: Dear Tim and Moby, My family is going skiing for vacation, but I'm afraid of avalanches. Help me! From, Zeena. Okay, an avalanche happens when a mass of snow, rock, soil, or other debris falls suddenly down a slope. There are lots of different kinds of avalanches, but the ones people usually think of are slab avalanches. They're the most dangerous type.

Images show types of avalanches.

TIM: They happen on mountainsides where wind and other weather conditions pack snow into dense slabs. As long as a slab isn't disturbed, it'll stay in place.

An animation shows a slab forming from wind-blown snow piling up on a mountainside. MOBY: Beep?

TIM: Well, packed snow on a mountainside is basically in a war between stress and strength. See, gravity is pulling the slab of snow downwards, that's the stress, but the strength at the top, bottom, and sides of the slab keeps the whole thing in place.

An image shows a slab of snow. Arrows indicate gravity pulling downward and creating stress. The areas of strength hold the slab in place.

TIM: But if that balance is disturbed…

MOBY: Beep.

TIM: Yep, the whole thing can go, almost like a plate sliding off a table.

A crack appears in the slab and a piece breaks off and slides away.

TIM: Slab avalanches are dangerous, because it's hard to tell where an unstable slab is. Often, you won't know it's there until it's already too late.

MOBY: Beep.

TIM: Well, there are three variables to keep an eye on: terrain, snowpack, and weather. Images show the three variables Tim lists.

MOBY: Beep.

TIM: Well, first of all, snow will only avalanche on the right kind of terrain, or land. Avalanches happen when the land's slope is between twenty-five and sixty degrees: any steeper, and the snow won't pile up into a slab; any flatter, and the snow just won't slide.

An image shows the type of terrain favorable for avalanches, between twenty-five and sixty degrees.

TIM: And rocks or trees on a slope can act as anchors.

An image shows rocks and trees on a slope.

TIM: 'Course, you can't have a snow avalanche without snow. A snowpack grows through the whole winter as snow falls, rain turns into ice, or wind blows snow around.

An image shows snow, ice, and wind creating three layers of a snowpack.

TIM: Usually, a snowpack has both strong and weak layers. A strong layer of snow is very dense, with small, round crystals of snow packed tightly together. A weak layer of snow is much less dense, with granular snow that doesn't bond together. If a weak layer of snow keeps strong layers from bonding together, the snowpack can get really unstable.

An image shows the snowpack's layers. Close-up images show a dense strong layer compared with a weak layer, as Tim describes. An animation shows the weak layer moving in between two strong layers.

TIM: Finally, we have weather. Sudden snowfalls or rains, quick thawing, and heavy winds can all destabilize the snowpack.

Images illustrate the weather conditions Tim describes.

MOBY: Beep.

TIM: Well, snow is what's called viscoelastic, which means it has some properties of a liquid and some properties of a solid. Honey is a good example of something that's viscoelastic. It pours like a liquid, but some of the honey will flow back into the jar if you turn it upright. Most of the time, these substances act differently depending on how hot or cold it is.

Moby demonstrates what Tim describes. First, Moby pours honey out of a jar onto the kitchen counter. Then he compares how hot and cold honey pours. Honey in one jar flows out slower than the other.

TIM: I hope you're planning on cleaning that up.

Moby frowns.

TIM: Because snow is viscoelastic, it can adjust to lots of different conditions without collapsing. But if too much stress happens at once, look out!

An animation shows snow on a slope breaking apart.

MOBY: Beep.

TIM: It's hard to imagine soft, fluffy snow doing any real damage, but you have to remember that avalanches can involve millions of tons of snow, moving at sixty miles per hour.

An animation shows snow breaking off and sliding down a slope.

TIM: And mixed up in all that snow are rocks, trees, and other debris. People caught in the path of an avalanche can get seriously injured or killed from the impact of all that stuff. Or they can get buried in the snow and not be able to dig their way out. About ninety percent of avalanches that injure or kill someone are triggered either by the victim or someone traveling with the victim. Still, the likelihood of getting caught in an avalanche is pretty small.

An image shows rocks, trees, and debris sticking out of the snow.

TIM: Places like ski resorts take special precautions to avoid avalanches.

An animation shows people riding on a ski lift at a ski resort.

TIM: Sometimes they even trigger explosives to prevent large snowpacks from building up.

An animation shows sticks of dynamite exploding in the snow.

TIM: So, if you use common sense and stay off of dangerous slope, you shouldn't have much to worry about. I'd still look out for Moby, though.

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