Tornado Dangerous Earth

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'll be looking at the latest scientific insights into tornadoes,

the fastest winds on the planet,

unpicking the eyewitness footage that reveals the complex atmospheric

conditions that create tornadoes...

..the radar data, showing how they're formed,

deep within the storm...

..and the experiments investigating the immense destructive forces

they generate.

-MAN:

-The whole house came apart!

And as we observe these monster weather events and record them

in ever more sophisticated detail,

we're really starting to understand and appreciate not just their power,

but also their subtlety.

I don't know what to make of these stringy little features.

Professor Josh Wurman and his team

are from the Centre for Severe Weather Research

in Boulder, Colorado.

Each spring, they spend weeks at a time chasing tornadoes,

trying to get their instruments as close as safely possible

in a bid to try and understand how and why they form.

The reason we're out here studying tornadoes and the reason why we're

driving tens of thousands of kilometres is to make a major step

in understanding how tornadoes form

so that better predictions can be made in

the future. If we can increase the lead time,

people and families will have those several extra critical minutes

to get to better shelters, get to their basements,

or even get to community hardened shelters.

I join them on one such hunt.

We track storms across the States for ten days...

..encountering some of the most astonishing weather

I've ever experienced.

But nothing prepared me for the moment

when I finally came face-to-face...

..with a tornado.

A 2km-high spinning vortex of cloud,

tearing across the countryside.

-We're getting out?

-'It's 3km right now.'

So this is it.

And it's enormous!

I had no idea it would look that big.

It's just amazing. And here it's almost calm.

But over there, those winds are going at hundreds of miles per hour,

pushing stuff right up into the heart of the storm.

I can't stop looking at it, it's incredible!

It actually hits me a little bit here watching that again,

because it was the biggest thing I had ever seen.

It reset the scale of the sky.

We think the clouds are a long way up but until you see a single thing

that connects the ground to the clouds,

you don't realise how big that is and that was all anyone could say,

"How big is that?"

'As surprising as the violence of these things

'is how little time they last.

'This one lasted just ten minutes,

'we drove up, we filmed it, and then it was gone.'

That was it. The trick of studying these things is being there at that

moment, just when all the drama has come together.

Capturing what happens in that moment

is Josh's focus and will bring

scientists one step closer to the ultimate goal -

being able to predict exactly when a tornado will strike.

The key is to work out how the complex flow of winds

in the atmosphere comes together to form a tornado.

Here at Birmingham University,

Chris Baker and his colleagues build small-scale tornadoes,

to study the effects of these unique winds.

What we are looking at here

is the flow visualised by some smoke,

and we can see the swirl on the smoke

and the smoke being dragged upwards.

Tornadoes are swirling masses of air

that move in towards the centre and then move upwards.

In the lab, fans create that rotation and uplift.

The mystery is how this movement is generated in nature.

How these 2km-high vortices can form just through

the interaction of atmospheric winds and pressure differences.

And for that, you need to study them in the real world.

Tornadoes can strike almost anywhere in the world.

This tornado struck a highway in Taiwan.

-WOMAN:

-Oh, my God!

Even the residents of Birmingham...

-MAN:

-No way. There's a tornado!

..have the occasional brush with these fierce weather events.

Look at it, man. It's everywhere!

But there is one place above all

that has become the natural laboratory for studying tornadoes.

The ones that we hear about most often, because they're the largest

and most destructive, is here in North America.

It's not the whole country, it's one specific place,

Tornado Alley, and that is a stripe that goes

from northern Texas through Oklahoma, Kansas,

Nebraska and into South Dakota

and the tornadoes form here because of

a very specific set of conditions that can happen in this region.

This part of the world is witness to devastating weather events.

Nathan Edwards was on a storm-chasing holiday in the US

when he captured these incredible clouds.

Look at that.

-Wow.

-Oh, look at that!

This is a supercell.

A special type of thunderstorm where tornadoes are born.

Pretty darn strong winds coming up just over there.

The incredible structures we can see here

reveal the dangerous combination of air masses and wind patterns

that make this place such a breeding ground for tornadoes.

David Schultz, from the University of Manchester,

is a meteorologist and an expert on the conditions

that lead to such incredible storms.

So, imagine that we're standing here on the Rockies,

looking out over

the Great Plains to the east.

What we see down there is warm, moist air,

flowing up from the Gulf of Mexico.

Up here, where we are, in the Rockies,

we have dry air that flows out over this moist air.

The boundary between these two air masses

produces what we call the lid.

The lid is essential for making the storm so powerful because it traps

that energy,

that warm, moist air, underneath the lid until it's ready to explode and

then we can have the development of this supercell thunderstorm.

This amazing, almost circular, disc-shaped cloud

is where the air is forcing its way through the lid.

Once through, the rising air accelerates upwards

and spreads into the turbulent clouds above,

drawing air in and up into the storm,

in tremendous flows called updraughts.

The lid enables a supercell to build up huge amounts of energy.

And the incredible upward surge of air fuelling the storm

creates epic rain and hail.

Wow!

'It's something I experienced myself when chasing the storm.

'And that was just a taste of what these storms are capable of.'

HAIL CLATTERS

The powerful updraughts of air can keep hail trapped within the cloud,

building up layers of ice until they become so big and heavy that

the storm can no longer hold them.

-MAN:

-Man, look at that massive thing.

My God!

It can create downpours of hail the size of baseballs.

HAIL THUDS

Not a time to be caught in the open.

That intense rain and hail would also bring about the end

of a normal storm.

An ordinary storm only lasts about 20 or 30 minutes.

That's because the air flowing into the storm rises up,

forms the cloud but then, if there's any precipitation that forms,

it falls back down into the updraught and that kills the storm.

But not in the case of a supercell storm.

Because a supercell rotates.

Because the storm is spinning,

all that intense rainfall and hail

is no longer happening directly over the rising air.

The supercell ensures its longevity because the rotation of the storm

separates the updraught from the downdraught.

You have the ascent, the updraught,

circling around the storm, rising up,

you have the descent, separated from that, coming in on the other side,

circling around,

and it's the separation between these two that ensures

the longevity of the storm.

Now we have a powerful, long-lived storm, primed to produce a tornado.

All thanks to the mesmerising rotation

captured in Nathan's footage.

What creates this rotation is once again

the specific conditions at work in the atmosphere.

There's lots of things to see on this footage but there is one very

important thing that you cannot see

and that is the direction of the wind.

Up at height, in this image,

winds are going sideways like that,

but down at the ground they are going back the other way and this is

called wind shear,

when the wind changes direction with height and you can

see that if the winds are going to the right at the top and the left at

the bottom, you start a rotation like this

and the consequence of the wind shear

is that you get a roll of air that is rotating around this way.

If we look at the storm,

that's not what we see in the storm we've got.

Here it's rotating like that.

So there has to be an extra step to generate rotation that goes around

this way.

If we think about that roll of air that the wind shear made,

like this tube of pipe, so the wind is rotating around like this,

the other thing we can't see in this storm is the updraughts, and that's

air that's coming from the ground,

pushing up into the storm, and if an updraught acts on our roll of air

here, it can draw this upwards

and so you are left with a single column with

the air rotating around it like this and, just like we can see in the

video, THIS is what starts the rotation of the supercell.

This storm has all the ingredients needed to produce a tornado.

But it never did.

Why one storm produces a tornado

when another very similar storm doesn't,

is one of the main reasons that predicting tornadoes

is so difficult.

That's one of the big mysteries with the supercells.

75% of these severe rotating supercells don't make tornadoes,

only about a quarter do, so when we go out to observe them,

what we're trying to learn

are the real subtle differences between the ones that do

and the ones that don't.

The basic process of how a tornado forms in the lower part of a storm

is known, but when it first develops,

a tornado is very different to the ones

we see tearing across the country.

So, a tornado doesn't start life looking like that,

it actually is part of a parent storm

and it has to grow from the base of that cloud.

To get a tornado started, you need wind shear.

Now, this is different from the wind shear that we needed in order to

produce the rotation associated with the supercell.

That occurs from the surface to 6km or 7km above the Earth.

For the tornado, we just need that over the lowest 1km.

Wind shear at the lowest 1km

gives you a sense of rotation like this.

But we know a tornado has a sense of rotation like this.

The fledgling tornado needs something

to tilt it downwards and make it vertical.

And a clue to what that might be was captured in a chance recording.

Mike Lapera was recording what looked like a normal storm

outside his shop.

CHILD SCREAMS

-MAN:

-Go, go!

The instantaneous burst of 160kph wind was not caused by

a tornado, but by a micro burst.

An extreme example of a downdraught, falling air within the storm.

Brian Snyder was recording a storm...

..when he captured this...

A rarely seen micro burst in action.

Plummeting air, crashing to the ground.

And it's the action of downdraughts

that's the final step in producing a tornado.

How do we get from here to here?

We need the downdraught,

that tilts this vorticity into a vertical orientation.

-MAN:

-The tornado on the ground.

But once a tornado is formed, what exactly are you looking at?

It's not as simple a question as it sounds.

This is forming right over our heads.

One stormy afternoon,

Jim Kenefick had an unwanted visitor.

Oh, I want it gone.

I don't want that thing touching down here.

His footage captures the rare moment

when a tornado first descends from the clouds.

Just break up.

And that gives a unique insight into why we can see tornadoes.

Just break up.

It's all thanks to what happens to air

from the surrounding atmosphere as it's drawn into the tornado.

I'm coming, I'm coming, I'm coming.

Right at the core of a tornado, there is a region of low pressure,

driving the whole system. And the reason you can see the tornado

is all to do with what happens when warm, moist air

that's moving along the ground

reaches that area of low pressure.

In this bottle here,

I've got warm, moist air and I've pumped up the pressure inside

and so when I release the top,

we'll see what happens when you suddenly release the pressure.

HISSING

SHE LAUGHS

And suddenly you get a cloud.

And the reason for that is that as the pressure drops,

the air cools and, suddenly,

that means that all the water vapour that is inside there

starts to condense.

And when it condenses, it makes little particles and so you can see,

very quickly, a cloud formed inside the bottle.

As the warm, moist air is continuously pulled

into the low pressure of the tornado,

the water vapour within it cools and condenses...

..forming a cloud that traces out the spinning column of air.

What we're seeing is a tornado's own internal cloud.

Where the tornado touches down, the wind speeds can be ferocious.

Yet the rotation of their parent storm is relatively slow.

So where do the intense wind speeds come from?

Josh Wurman and his team gained an unexpected insight into the true

strength of these winds.

A tornado!

-In front of us?

-No, I can't see

-BLEEP!

Hunting a tornado at night is an even more dangerous business

than usual.

And one evening, whilst chasing through the dark streets of Russell,

Kansas...

Wow!

..they found themselves directly in the path of the tornado.

It got hit.

The windows are broken.

-RADIO:

-'We are OK, we are upright and we are...

'Our radar has stopped.'

This chance encounter became one of the very few times

that the wind speed

has ever been measured directly from the edge of the tornado.

-I can still see the funnel.

-It's still on the ground here.

At just six metres above the ground,

the wind was travelling at over 250kph.

'OK, we are coming up towards you.'

The clue to where these tremendous winds come from

is found in the shape of the tornado itself.

As it moves down out of the cloud,

the column of spinning air is stretched and compressed...

..forcing the tube to become narrower.

Look at that! Look at that!

That change in shape concentrates

more of the energy of the tornado into its spin...

Holy cow!

..making it rotate much faster and driving the wind speeds

up to hundreds of kilometres per hour.

It's a tornado! Look at that!

But the destructive ability of the tornado

comes from more than the effect of such incredible wind speed.

What's key is not just the physical impact of the winds

but also the lift,

as Scott McPartland and his fellow storm chasers witnessed.

Oh, my God! Yeah, there's a house being destroyed over there.

The house is coming apart!

-Oh, these poor people!

-Oh, my God!

-Oh, my God!

The whole house came apart!

What's surprising is that the house is lifted

BEFORE the main part of the tornado reaches it.

Chris and his team are trying to work out

exactly why tornado winds are so destructive.

Within their chamber,

they've placed a model building covered in sensors

that can measure what happens when it encounters their tornado.

At the moment, the generator is being used

to measure the loads on the building,

the pressures on the surface of the building,

as the building's at different positions

relative to the centre of the tornado.

The key is the low pressures created by the fast-flowing winds.

As the tornado moves across the building, it will go quickly,

particularly over the eaves and the ridge -

you get very low pressures at those areas.

And the pressure inside the building can stay quite high and,

if you've got a high pressure inside, a low pressure outside,

that will either lift the roof or,

if the roof is attached firmly and the foundations aren't,

it will lift the whole building.

The combination of the low pressure of the tornado

and its forward motion

means that the place with the strongest winds and greatest uplift

is actually in front of the tornado as it approaches,

just where this house is unlucky enough to be.

In the face of such powerful forces,

the amount of warning time is critical,

giving people crucial minutes in which to take shelter.

Funnel, right across the road!

Right across the road!

On May 22, 2011, a tornado hit Joplin.

I have a large, destructive tornado!

It measured over a kilometre across.

Coming on the ground right here!

Get the sirens going! Get the sirens going!

-I'm telling you!

-Back up!

I am!

And with wind speeds of over 300kph,

the impact was brutal.

Stop, stop!

It was on the ground for less than 40 minutes

but during that time,

it tore a path of devastation through the heart of the city.

Joplin Middle School was directly in the firing line.

Thankfully, the school was deserted at the time.

But across the city, the tornado took the lives of over 150 people.

Many more would have died if not for the warning that was issued before

the tornado hit.

INDISTINCT RADIO CHATTER

To improve that critical warning time,

scientists like Josh are trying to capture

the exact wind conditions that create the tornado...

Sandra, when you get there,

you may have to cut sort of through it, somehow.

..by peering deep into the hidden heart of the storm.

We are in the Doppler on wheels.

A Doppler radar is able to send a beam of microwaves out

through a tornado and scan back and forth

and make three-dimensional maps of the winds and watch them evolve.

And we're basically watching the process of tornado formation.

And then, equally importantly,

we can measure growth of humidities, the types of precipitation,

which are really forcing those winds,

they are causing the updraughts and downdraughts which may or may not

evolve into a tornado.

So the radar lets us measure the rain and hail,

and that's what we're seeing here in red.

This red is a hail area and surrounding this...

Look at that go!

At the tip of the hook, where we have this ball,

is where the tornado is beginning to develop.

And, eventually, we see it spinning there and we even see a clear eye.

That green spot is the eye of the tornado.

Capturing these moments, revealing the intricate details,

has already improved warning times from only five minutes

in the 1980s to nearer 15 now -

minutes that save lives.

But the drive to improve that continues.

If we can increase the lead time from its current 13 minutes

to about 20-minute warnings or even 25 or 30-minute warnings,

people, families, who live in Tornado Alley

can live here feeling safer,

they can live here being safer and can weather the storm.

The explosion in footage of tornadoes

reveals a beauty and power that is

both awe-inspiring and terrifying.

The insights into the unusual combination of weather that creates

tornadoes is improving warning times.

But the exact conditions that lead to these violent storms

are still elusive.

Tornadoes are a reminder of the amount of energy there is in

our atmosphere and the amount of destruction that it can cause

if only a tiny fraction of it is focused in just one place.

We are unlikely ever to be able to control these phenomena but,

as we learn more and understand better, hopefully,

we will be able to predict them.