Avalanche 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.

TRANSLATION FROM GERMAN:

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

when just a small section of the weak layer is disturbed.

And the place to watch is this bit.

This is a big block of snow

and there's a chainsaw from the other side

that's cutting into the weak layer here.

And the saw gets further and further along.

Suddenly, the whole top block starts to slide.

So what happens is, the scientist has cut away at the weak layer

and that has failed and that has made the next bit fail

and so all the way along the weak layer here has suddenly broken

and all of that top layer is sliding off.

So to trigger an avalanche, you just need this weak layer to fail.

And even though an avalanche can have

hundreds of thousands of tonnes of snow,

just the weight of one skier can be enough to start it off.

But in rare cases,

something entirely different can trigger an avalanche.

And this is hardly ever caught on camera.

Oh, look at that! Look at that! Look at that!

Straight ahead.

Do you want to go in the tents or what?

In April 2015,

a huge earthquake struck in Nepal...

Whoa! Whoa!

..triggering a fatal avalanche

that engulfed the base camp on Mount Everest.

Tragically, 19 people lost their lives,

making it the deadliest disaster on the mountain.

-MAN:

-Stay together. Stay together.

But despite the devastation,

the ground only got covered in a few centimetres of snow.

In this avalanche, it didn't seem like it was the snow that killed.

Avalanche expert Perry Bartelt

simulates the forces behind avalanches.

Avalanches are basically symbols of chaos

and it's this chaos that makes avalanches so dangerous

and why modelling avalanches is so particularly difficult.

Perry wanted to find out exactly what happened on Mount Everest.

When we heard the news

that there were deaths at the Everest base camp,

we thought the avalanche must have been immense.

Using satellite pictures and photographs

taken immediately after the event,

Perry set out to calculate the size of the Everest avalanche.

What we have here are photographs before the event

and after the event

and we've studied the photographs and what we immediately saw

was that there was a large region here of ice

located at about 6,000 metres high

that was missing in the after photograph.

-MAN:

-The ground is shaking!

It was this missing chunk of ice

that caused the destruction on Everest.

But at 50,000 cubic metres, it was surprisingly small for an avalanche.

We were extremely shocked to see that such a small avalanche

could cause so much damage.

But because Everest is extremely steep,

the avalanche accelerated to about 200km/h

in just ten seconds.

And it's what happened next that made it so deadly.

Because it was running on ground which is very, very rough,

it allowed the avalanche to take in huge amounts of air,

probably millions of cubic metres of air,

and grew to an incredible size.

-MAN:

-Do you want to go in the tents or what?

This created an enormous powder cloud.

Nearly 200 metres high, it inundated base camp.

-MAN:

-Do you want to go inside?

And because it was so fast-moving,

it brought with it a strong pressure wave

that would have blown the mountaineers against the rock face.

What you see here is the calculated impact pressure of the powder cloud.

The red zone here signifies pressures

that are dangerous to human beings standing outside.

Down here you have the Everest base camp

and you see that it is clearly in the red zone.

If the cloud hits them,

there is a good chance that they are going to be killed.

Such a lethal powder cloud is very unusual.

In most avalanches,

it's not the powder cloud that causes all the devastation.

The really deadly and destructive part of an avalanche is its core.

It's the core where the mass of snow and ice sits

and that annihilates everything in its path.

But it's often hidden under a powder cloud,

making it difficult to investigate.

Which is why the pioneering scientists

from the SLF have built the world's largest

avalanche laboratory in a steep Swiss valley.

Here, they artificially trigger avalanches with dynamite.

EXPLOSION

MAN SHOUTS IN FRENCH

But when they first started their experiments in the 1990s,

they set off an avalanche much bigger and more powerful

than they'd bargained for.

We were really surprised by the force

by which the avalanche hit the shelter.

We did not expect such a big thing to come down.

This is a very massive and solid bunker.

I think the walls are about 40 centimetres thick.

Nevertheless, you could feel

the vibrations of the whole building.

LOUD CRASHING AND BANGING

We heard a strong noise.

That was because the door broke open and the snow came in,

then the pressure in the shelter rose enormously.

It was like diving into two metres of water.

After the avalanche had hit us,

we first had to try to get out

because the shelter was completely covered by snow

and so we started digging a tunnel out and then we had to work hard.

It was really very compact snow.

From this experiment,

Dieter and his team were able to gain a better understanding

of how the avalanche core behaves as it races down the mountain.

Today, they're able to use far more sensitive equipment

to get the most accurate picture of the avalanche core.

And some of the best results come from radar.

What we've got here is radar data

from one of these large-scale experiments

and what it shows is time going on along the bottom

and then distance from the bottom of the mountain up the side

and what you can see is here's the start of the avalanche

coming down the slope as time comes on.

But then it's followed by another strong line here

and then another one here.

What this is telling us is that the avalanche is coming down in stages.

It's surging.

And the only reason that we know

that this is happening inside the avalanche core

is that we've got radar data like this

that lets us see past the powder cloud.

This new data on avalanche surges

also gives us a better understanding

of what exactly happened to Kristoffer

as he tumbled down the mountain.

You can see from Kristoffer's footage

that there was a point where he almost stops.

It looks like he's safe

and then another surge comes

and carries him on further down the mountain.

These surges are common in avalanches and they happen because

the snow grains travel at different speeds at different heights,

so the ones at the top tend to go faster

and that means the top layer can overtake the bottom one.

And then, while all of this is going on,

the avalanche is incorporating more snow,

so it's getting bigger

and the thicker the avalanche core and the higher up it is,

the more of these surges you tend to get.

These new insights from models, experiments and lab analyses

have given us a profound understanding of how snow changes...

..what triggers an avalanche...

..and how it develops as it hurtles downhill.

And they're being used to improve the way we forecast risks

and protect people.

But despite these significant advances in avalanche science,

there's still one thing that we can't predict.

Exactly when and where one will strike.

What can be done, however, is to stop

uncontrollably large and life-threatening avalanches

by triggering them while they're small enough

to be safely neutralised.

In the Alps alone,

around 50,000 controlled avalanches need to be set off every year.

After several days of snowfall,

the risk of avalanches here in Grindelwald becomes extreme.

It's ski patrolman Martin Matthis' job

to trigger an avalanche before any more snow falls.

TRANSLATION FROM HIS OWN LANGUAGE:

He's taking 50 kilos of dynamite,

enough to blow up several city blocks.

Martin reaches the summit and sets the charge.

EXPLOSION

The explosion creates an air pressure wave in the snow pack,

triggering a mini avalanche.

But...it's not enough. He needs to go again.

EXPLOSION

Martin succeeds.

This time, it's enough to cause the weak layer to fail

and for the snow above to tumble downhill.

This is the avalanche he needs to make the mountain safe.

We associate avalanches with chaos and destruction

as though an entire landscape is temporally out of control.

But we can see now that they do have internal structure

and predictable patterns.

This is nature at its grandest but it's not random.

We're not going to stop avalanches happening,

but all this new science

will let us understand and co-exist better with these gigantic events.