Dangerous Earth

It's a tornado!

Look at that!

Our planet is home to some spectacular natural wonders.

Yet exactly how and why they form is still a mystery.

But now new camera technologies

are revealing their inner workings in stunning detail.

My name is Dr Helen Czerski

and I'll be looking at how these extraordinary images

are transforming our understanding of the natural world.

In this programme, we uncover the latest scientific insights

into the devastating power of avalanches.

The scale and grandeur of an avalanche are gigantic

and yet, many of the details needed to understand them

lie in the world of the really tiny.

Now, detailed CT scans are showing how microscopic changes in snow

can cause an avalanche at the lightest touch.

The latest computer models

are revealing why the 2015 Everest avalanche was so deadly.

And extraordinary eyewitness footage is giving vital clues

about how avalanche snow can seize up like concrete.

It's new findings like these

that allow scientists to peer deeper inside the anatomy of an avalanche

than ever before.

Snow draws millions into the mountains each winter,

but snow can also be deadly

and avalanche scientists are trying to understand why.

For ski guide Kristoffer Carlsson, the morning of February 28th 2011

started like any other.

This day we woke up really early, I think at 6:30am in the morning,

because we had seen on the weather forecast

that it was supposed to be a really beautiful day

and the past three or four days it had been snowing quite heavily.

But on this particular day

we were just so happy about the sun being out again

and we were just looking forward

to one of the greatest ski days of the season.

The day was so perfect,

Kristoffer decided to record everything on his helmet camera.

His footage reveals one of the strange properties of snow

that makes avalanches so deadly.

My friends, they went down on the left side

and I chose going a bit more to the right.

I saw that there was a lot of fresh snow on that route

and then everything just happened in a second.

Kristoffer triggered an avalanche

that carried him 200 metres down the mountain.

There was a huge amount of snow taking my skis away.

It felt like being in a washing machine,

just tumbling down the mountain.

He knew he had to try and stay on top of the snow at all costs.

The only thought I had was, "Don't get buried, don't get buried,"

because, if you get buried, your chances drop drastically.

Kristoffer ended up buried under two metres of snow,

feet pointing upwards and unable to move.

HE GROANS AND STRAINS

The moment that I realised that I was completely buried,

I remember that I was quite shocked about how hard the snow was.

It really turned into something that felt like concrete

in an instant, really.

HE GROANS

I couldn't move.

HE CRIES OUT

The pressure of the snow on my chest made it really hard to breathe.

HE CRIES OUT

The only thing that I could do was trying to stay calm.

HE CRIES OUT

As if entombed in concrete, Kristoffer couldn't move.

HE CRIES OUT

And he only had a small pocket of air around his mouth to breathe.

HE CRIES OUT

HE CRIES OUT

VOICES

SHOUTING

It took a terrifying five minutes

until his friends managed to dig him out.

This is so amazing.

Kristoffer was incredibly lucky.

Only half of all people buried in an avalanche like this survive.

It's impossible to dig yourself out

and Kristoffer's footage reveals why.

Kristoffer describes the snow setting like concrete

when it stops moving. And that's weird.

How does something as light and fluffy as snow become a solid?

Well, there's a clue in the footage.

As he starts to fall and the snow comes rushing past him,

the snow grains are rushing over each other, bumping into each other,

and there's lots of friction and that is generating heat.

So the outside of the snow grains are starting to melt slightly.

And once you get this thin layer of water around a snow grain,

it behaves in a peculiar way.

These ice cubes

behave like snow grains in the avalanche once it's stopped.

They're really close to their melting point,

so their surface is like a thin layer of water.

Those molecules are really mobile.

And if I push two of them together

and then take my hand away, they stick.

And what happened was that thin layer of water,

when it was stuck between the two ice cubes,

the ice cubes stole its heat away

and so it re-froze, gluing them together.

This is called ice sintering

and this is what happens to the snow grains in the avalanche

and it's what makes the snow pack go solid.

And it's because of ice sintering that Kristoffer was unable to move,

let alone dig himself out,

as he was buried deep under the snow.

More than a million avalanches

happen throughout the world every year.

In an average winter, about 500 people die in avalanches.

The largest can destroy whole towns and kill thousands.

So understanding them is crucial

for protecting people's lives and livelihoods.

What's surprising about avalanches is that all their destructive force

comes from simple snowflakes

and the way they change at a microscopic level.

All snowflakes start off in the heart of frozen clouds.

They begin life as an ice crystal, a six-sided shape a bit like this.

As water molecules land and freeze on to the crystal, it grows.

But they don't always hook on in the same way.

Minute changes in temperature and humidity

stamp their identity on the snowflake.

By the time it hits the ground,

each snowflake has been through a unique growth history.

But it's how snow melts and re-freezes when on the ground

that leads to avalanches.

To examine how snow transforms, you need to make your own.

Plenty of it.

At the SLF,

the Institute for Snow and Avalanche Research in Davos, Switzerland,

they study snow crystals and what happens at their melting point.

From a geological point of view,

snow is a high-temperature material

and that sounds very strange for most people

because snow is almost a symbol for cold.

But because snow is always very close to the melting point,

it behaves as a high-temperature material.

It's like a metal at several hundred or even a thousand degrees.

And that makes it one of the fastest changing

natural materials we see at all.

Fresh snow can melt and re-freeze within the snow pack

and it's this change of structure that can lead to an avalanche.

To study how the snow changes in more detail,

Martin designed a special CT scanner,

a machine more commonly used in medicine

to examine bones and tissues.

Snow is a very elusive material.

That made it very hard to really get a complete picture of the snow.

And that was the state until about ten years ago,

when we started this tomography.

When we could really visualise snow in 3D,

then we started to see snow in a very different way than before.

The machine has enabled him to build up a 3D sequence of images,

revealing how the snow structure evolves

in the previously hidden detail.

First I thought that must be great for everybody

because now people can understand snow.

But it doesn't look like the nice hexagonal,

perfectly symmetric snowflakes.

It looks simply strange.

Martin then used the CT scanner to analyse snow

taken from the mountainside immediately after an avalanche.

So this sample is from an avalanche site.

This block is only four millimetres wide.

We see in this block the essential features.

This big blob is re-frozen snow.

So it got warm, but only a little bit.

It created this huge crystal and that's the interface, you could say,

between the hot upper layer and the weak layer.

And the avalanche forms now somewhere in this weak layer.

It's this weak layer that's at the root of most avalanches.

The bonds between

the large cup-shaped snow crystals in this layer

are only weak and they break easily.

And if such a weak layer sits in the middle of the snow pack,

it becomes an avalanche waiting to happen.

Once you've got a weak layer,

all you need to start an avalanche is a trigger

and over 90% of the deaths that are caused in avalanches

happen in events that were triggered

by the skier or the snowboarder themselves.

And you can see it happening in this footage,

which is amazing and dreadful in equal measure.

This is a skier in Alaska.

And you can see that he has triggered an avalanche.

And if we look at it here, we can see that the snow pack

has failed along this line.

And you can see it even more clearly in this clip here.

Scientists at the SLF devised an experiment

that shows precisely what happens