(thunder booms)
Weather.
One of the most astonishing forces on Earth.
Capable of both devastating power
and spectacular beauty.
Wherever you are on the planet,
weather shapes your world.
Yet for most of us,
how it works is a mystery,
its secrets hidden out of view.
To really understand how weather works,
you need to get inside it,
uncovering its secrets in a series of ambitious
and revealing experiments
that show weather as it's never been seen before.
All weather, no matter how rare or how unusual,
can be broken down into three simple ingredients:
wind,
water
and temperature.
By combining those three elements in different amounts,
it's possible to create any type of weather at all.
Some of these combinations make perfect sense.
Mixing rain and wind creates a tempest.
Mixing temperature and rain makes ice.
But there are many that are more surprising.
Take intense heat,
add a strong wind,
and you can help create rain.
Here's how it works.
Take one desert,
an area far from water that has been dried out
by the heat of the sun.
Then, add a powerful wind
capable of blowing loose sand and dirt from the dry surface.
The wind whips the dust up into the air
and eventually, it finds its way into the clouds
where water vapor clings onto it
and starts to form raindrops.
Water, wind and temperature
working together.
But it's not quite as simple as that.
They might seem powerful,
but the average dust cloud only travels
between 40 and 80 kilometers before it dies out.
So how does desert dust seed rain
thousands of kilometers away from any desert?
The secret is hidden away inside.
And the only way for scientists to see it
is for them to create a dust storm.
But as Dr. Craig Strong
of the Australian National University explains,
that takes a very special sort of environment.
I'm looking for the landscape that's gonna produce dust,
and I think this stony plain is probably really good.
Because you can see these rocks,
they're acting as a trap for dust,
so I think if we dig down
we'll find that there's plenty of dust,
it's just that means it hasn't blown away yet
because the rocks are locking all that dust in.
And once we get under there,
it's just dust gold.
You know, look at this.
The rocks are doing the job of protecting the soil,
so I reckon we probably should pick up the rocks
and move them out of the way.
That's the first step.
By scraping away the rocks
from this stony plain,
Craig hopes to get all that dust out in the open.
Meanwhile, a second team from Monash University
are working on step two,
assembling an inflatable to help them measure the dust
once it's up in the air.
Okay, beautiful!
The string goes up through here, round there.
And then this will sit on the kite or balloon string,
and then we can sample into the wind no matter what.
These dust collecting samplers
are being attached at various heights under the balloon.
The first at three meters,
the second at eight meters
and the third at 20 meters.
An extra vain at the top carries a miniature camera
to keep a close eye on what's going on.
So Craig has got dust
and he's got a way to track where that dust goes.
Now, he just needs to introduce some wind
and get this dust storm started.
(intense string music)
What's remarkable is how much energy that takes.
The scientists are having to use cars, fans, shovelers
and blowers to try and get the dust off the ground.
This whole experiment is tiny compared to a real dust storm,
but the principles are just the same.
The vehicles and the blowers do the job of the wind,
whipping that dust up off the ground.
There's no way that they can blow it
higher than five or six meters,
but inside the dust storm a hidden process
is starting to take place.
Individual grains are colliding against each other,
propelling the smaller pieces high into the air.
The dust particles aren't merely being blown about,
they're bouncing.
The kinetic energy from one particle
is transferred to another,
causing the smaller piece of dust to shoot skywards.
Hopefully all the way up
to the highest sampler on the balloon.
But there's only one way to know for sure:
check what's in those pumps.
Luckily, Dr. Nigel Tapper has a makeshift laboratory
right on site.
So here we have to lower level, mid level, eight meters,
upper level around 20 meters.
Okay, we'll start with the one closest to the surface
and crack this open, shall we?
So we've actually got quite a bit of material on there.
You can see plenty of dust in that fine material.
So let's now have a look at the mid level.
There we go.
So we've actually got a bit of fine material
at eight meters.
So, it will be interesting to see
if we've actually got material at 20 meters.
And if we do, it will be a tribute
to our dust making activities.
Well look, surprisingly there is a little bit there.
I'm quite impressed with what we were able to see, actually.
I'm really impressed that we were able
to get the dust that far up.
So bouncing is the secret mechanism
that gets dust high up into the atmosphere.
A large dust storm can move 15 million tons of sand
in a single go.
And when that dust has bounced high enough,
it gets caught in global wind patterns
which move it around the planet.
And that is how it travels so far
from where it started.
Eventually, some of that dust finds itself in a rain cloud
where temperature and water combine
to form water vapor.
But how do those water droplets come together?
Well it's impractical to look into a cloud to find out.
But it is possible to get an idea of what's going on
by looking in a puddle.
(driving piano music)
As the raindrop hits,
part of it is attracted to the water.
What bounces back up is a smaller droplet
about half the size.
When that droplet hits,
the same thing happens again.
Around half of it stays in the puddle.
Now, imagine that in reverse and upside down.
The puddle is the cloud.
A water droplet doubles in size
by attracting other water droplets
to the tiny seed of dust.
These stick on in a process scientists call coalescence.
It increases again and again
until it's so heavy, it falls away.
So down the dust comes,
carried in its own little water drop,
out of the sky and onto the ground.
And that is basically how rain is formed.
There are about 13 trillion tons of water
being moved around in the atmosphere.
And every day, temperature and wind
cause a tenth of that to come crashing back down to Earth.
Sometimes, these storms are incredibly intense.
The quickest on record dumped 12 centimeters of water
in just eight minutes.
The heaviest managed nearly four meters in 72 hours.
(intense music)
To give some idea of just how heavy that is,
this bucket is being filled up with water
to a depth of four meters.
It's then going to be dropped
onto an area the size of this car.
(loud crash)
(dramatic music)
Four cubic meters of water
actually weighs four tons.
Four times heavier than the car underneath it.
(loud splash)
Luckily, this could never happen with real rain.
Not even in a tropical storm, where sometimes it feels
that the heavens have literally opened,
partly because raindrops fall
the moment they get heavy enough.
And partly because they are falling
from such a great height.
As the water falls, it meets air resistance,
and the larger the lump of water,
the more resistance it experiences.
That friction breaks the water up into smaller pieces,
sometimes inflating the drops like parachutes
before blowing them apart.
The further they fall, the smaller those drops become.
Until finally they are so small
that the air has little effect on them
and they land as rain.
If that digger had been just a few meters higher,
then the car might well have survived.
Because they break down like this,
the average raindrop ends up measuring
about two millimeters across.
Air resistance and friction prevent them
getting much bigger than that.
So despite water's heavy weight,
any damage rain causes is not down to the force of impact.
But add a variation in temperature
and all that can change.
This process usually happens in thunderclouds
because they are much higher
and therefore colder than normal clouds.
That drop in temperature causes some of the water droplets
inside the clouds to freeze.
(thunder rumbles)
The heavy ice crystals are held aloft,
supported by the rising air,
collecting more and more water around them,
with each new layer freezing on in a separate shell.
(crackling)
Until eventually, there are so many layers
that the balls of ice are just too heavy to be supported
and they fall to the ground as hail.
Suddenly, impact is an issue.
Hail causes over a billion dollars worth of damage a year
in the United States alone.
But is hail so destructive just because it's frozen
or because of the unique way that it's formed?
Solving that riddle requires a very unusual piece
of apparatus.
It's inventors, engineers Jim Stratton
and Craig Zehrung, from Purdue University,
had a bet to see just how fast
they could get an ordinary ping pong ball to fly.
By firing hailstones instead of ping pong balls,
it might be possible to test
whether the layered ice in hail is harder than ordinary ice.
(cracks)
But first, Jim and Craig need to create some hailstones.
Instead of a speck of dust,
they are using this small plastic bead
to act as the seed that starts the hailstone growing.
The bead is dipped into dry ice
to lower its temperature dramatically.
Then, it's plunged into cold water.
Immediately, a thin layer of ice freezes onto the surface.
The bead is removed
and the whole process is repeated.
It takes Jim and Craig 10 minutes of patient dipping
but finally their homemade hail is big enough
to fire from the barrel of their machine.
They are done.
So they can compare hail to ordinary ice,
Jim and Craig have also frozen plain water
using a few of their spare ping pong balls as molds.
The two engineers have got lots of different sorts of wood,
and they're going to take two shots at each piece.
First with plain ice,
then with their homemade hail.
The first target is chipboard.
Three, two, one.
The ice goes straight through,
as does the hail.
So they try a slightly thicker piece of chipboard.
The results are the same.
Next, plywood.
The ice barely dents it.
Will hail be any stronger?
Three, two, one.
Well, there is a difference.
(crash)
That hail has splintered the back of the plywood.
They decide to try a slightly thinner piece.
This time, the ice hardly makes it through.
The hole it makes is far smaller
than the projectile itself.
Can hail succeed where the ice pellet failed?
Woo, nice.
That's awesome.
It does.
So although they're both made from frozen water,
it does appear that the layers in hail
make it measurably stronger.
(glass breaking)
That information could be invaluable
in helping scientists design better protection
for aircraft and buildings exposed
to hail's destructive power.
But temperature can cause water to change
into something even more dangerous.
Naturally quicker than hail,
with a mightier punch then hail.
And what it is might well surprise you.
It's snow.
This is how most people are used to seeing snow move.
Delicate flakes floating gently down to Earth.
(magical music)
Floating so gently that a snowflake can take nearly an hour
before it finally reaches the ground.
It travels at just six and a half kilometers an hour,
little more than walking speed.
And yet snow can be the fastest
and arguably most dangerous form of water that there is.
(rumbling)
Because when it's moving in an avalanche,
snow can reach 130 kilometers an hour
in six seconds flat.
And then, it just keeps on accelerating.
The fastest one ever recorded,
on Mount St. Helens in America,
clocked a staggering 400 kilometers an hour.
But how is it possible that snow can move down a mountain
faster than water can?
Walter Steinkogler,
of the Institute for Snow and Avalanche Research,
is trying to find out.
He aims to examine the movement of an avalanche
by starting one of his own.
(ominous music)
Walter has already found a suitable slope,
and he and his team have equipped it
with instruments, sensors, and slow motion cameras.
Now, he is just waiting for a gap in the weather.
So the deal is at the moment,
the helicopter will be ready to start at the heli-base,
ready to go.
But now we are waiting, as you can see,
that the weather clears a bit more.
There is some blue sky over there,
but it's coming and going,
moving in moving out, so we're just waiting for the weather
to bring the helicopter in, yes.
Even though Walter is creating
a weather event, the real weather still has to be right.
30 minutes later, he gets his window.
(dramatic music)
He needs the helicopter
to fly these fences up onto the slope.
By placing them an exact distance apart,
the team should be able to time
how long it takes the avalanche to travel between them.
That way, they can work out its speed.
Walter has already predicted where he expects the avalanche
to fall, so the first fence is positioned right in its path.
The second is positioned exactly 100 meters behind it.
More than this and the avalanche might miss it
and the distance will be too short to time.
With the fences in place,
Walter and his team wait for the avalanche to be triggered.
(explosive booming)
To the naked eye,
the resulting avalanche is incredibly fast.
The fences are destroyed within an instant of each other
and the whole thing is over in seconds.
Luckily, the slow motion cameras have captured everything,
enabling Walter and his team to analyze the entire event
starting with the moment the dynamite is dropped
from the helicopter,
causing this explosion at the top of the mountain
that releases the slab of snow.
Immediately the central core is surrounded by a powder cloud
made up of 1% snow and 99% air.
This is a dry powder avalanche,
the fastest and most dangerous avalanche of them all.
The avalanche accelerates down the steep incline
until it reaches the first fence,
though not exactly at the angle Walter expected.
The leading edge hits the first one,
now.
And that particular part of the snow front
reaches the second fence,
now.
Mere moments after the first fence is destroyed.
No wonder it seemed so quick.
In fact, the data shows that this avalanche
only reached a velocity of 40 kilometers an hour,
just a tenth of the speed of the fastest one ever measured.
But 40 kilometers an hour is still faster
than that slab of snow could have achieved
if it had just been dropped from a great height.
So how is that possible?
Well like all weather phenomena,
there is more going on inside it than meets the eye.
Walter and his team take to the slopes
to investigate the aftermath
of the avalanche more thoroughly,
and he explains what happens to the snow
underneath the powder cloud as the avalanche starts to move.
First, the snow cover that releases and gets eroded,
it will break into pieces and it gets rounded a bit
and it also gets compressed so it can form snowballs.
These snowballs might be the secret
to how an avalanche can move so fast.
There are studies that say,
it's the ratio between the bigger balls to the smaller balls
that can significantly influence the speed
and the motion of the avalanche.
Put something on wheels
and it can accelerate quicker than if you simply drop it.
These snowballs may be the wheels of a dry powder avalanche.
(rumbling)
Snow is the softest, lightest way
that water can fall to Earth,
but an avalanche can move faster
than any other type of water.
Four times faster
than the fastest flash flood ever measured.
But, as we have seen, weather is not about water alone.
One of the other three key components can move even faster.
It's a powerful force that's completely invisible.
That force is wind.
In many ways, wind is the most dramatic
of all the three main ingredients,
because it drives most of the planet's extreme weather.
Dust devils,
water spouts,
tornadoes.
All are spinning winds of the most intense kind.
Even hurricanes and cyclones are large scale winds
with that same spiral shape.
To see how those spirals come about
means examining perhaps the most unusual spinning wind
of them all.
It's called a fire whirl.
And because it's made entirely of flames,
it should be easier to watch the twisting structure at work.
(intense music)
Australia has more known fire whirls
than anywhere on the planet.
But it's very rare to actually see one.
Which is why the best way to examine a fire whirl
is to build one.
Dr. Kazunori Kuwana
and engineer Kozo Sekimoto
are going to do exactly that.
They have spent the last 10 years looking at how
and why fire whirls spin.
But they've never managed to capture the moment
where that cyclical motion is triggered.
They're hoping today will change all that.
What they do know is
that the fire needs to be an asymmetric shape,
which is why they're laying these baking pans out
in the pattern of a giant letter L.
Next, they take a highly flammable liquid called heptane
and fill the pans with it.
Heptane burns fiercely
and should create enough heat
for a fire whirl to start sucking in oxygen.
And that is crucial.
The principle can be demonstrated by lighting a cooker.
Hot air rises from the flames,
making the air above them less dense.
And because it's less dense,
it makes the air pressure there lower.
But the cold air around the cooker
is still at normal pressure,
so it rushes in to fill the gap,
turning these children's windmills.
Using the smoke from this match,
we can see that movement of air.
The flames are sucking the oxygen in.
The colder, high pressure air is rushing in
to fill the gap left by the warmer, low pressure air.
And it's the air rushing in opposing directions
that trigger the firewhirl.
That's what Kazu and Kozo's L shape is all about.
But is it actually going to work?
At first, the experiment doesn't look too promising.
But as cold air rushes in, it feeds the flames.
And then, quite suddenly, they start to spin.
The spin seems to intensify the fire even more.
The flames continue to grow
until they are more than 10 meters tall.
A real life fire whirl.
By introducing smoke, Kazu and Kozo hope to make
that spinning wind visible.
The crosswind is so strong,
but on the far side it blows in a pretty straight line.
But on this side, the smoke is being pulled
in two directions at once, towards both arms of the L.
That creates those little green swirls
and ultimately, the fire whirl itself.
Natural L shapes occur when two separate fire fronts meet,
each creating their own opposing winds.
And other types of spinning weather
start in much the same way.
Two or more winds meeting at different angles and speeds.
Some rising warm air,
and cold air rushing in to fill the gap.
That formula is so potent
that it's capable of creating the most powerful
and deadly wind of them all:
the tornado.
Tornadoes aren't the fastest wind across the ground.
Their top speed is only 110 kilometers an hour.
But the speed that they can spin is formidable.
It is this spin that gives tornadoes
their devastating power.
So it's surprising to learn
that despite decades of intense study,
nobody knows exactly how fast they can spin.
Here in Boulder, Colorado,
the Center for Severe Weather Research has long been
at the forefront of tornado research.
Its president, Josh Wurman,
explains why spin speeds are still such a mystery.
Scientists have gotten very good at measuring the winds
above the ground in the tornado.
Maybe from 50 meters above the ground
up to a couple kilometers.
But the strongest winds in the tornado are below that.
The strongest winds might even be below 10 meters.
Using remote sensing with radars,
we can get up close, we can scan back and forth,
but unfortunately, objects block us.
There's debris, pieces of houses, cows, whatever
flying around in the tornado,
and that's the one place where we are the most blind.
The alternative approach has been
to try to get measuring instruments inside a tornado.
But that has proved equally difficult.
There are two main challenges with measurements.
The first is how to get something inside the tornado.
The tornado's moving down the fields
and we don't know exactly how it's going,
it's an unpredictable path.
So getting something in front is very hard.
Challenge number two is what happens when we succeed.
And that is, the tornado runs over the object
and destroys it.
So unfortunately, the place that we most need to know about
is the place that's hardest for us to see.
We need to understand what's happening
in those strongest levels of the tornado
which are causing the damage.
If we can understand that better,
then engineers will be able to build better buildings,
we'll be able to have better shelters,
and fewer people will get injured and die in tornadoes.
But work is being done
to try and measure the speed of tornadoes near the ground.
This is the wind engineering energy
and environment research institute, or WINDEEE for short.
And it's the only place on the planet
capable of duplicating the real life dynamics of a tornado.
The whole structure cost $23 million.
Its director, professor Horia Hangan,
explains how it works.
We have a bunch of fans on the periphery here,
so there are six walls and on every one of those walls,
there are eight fans.
So you blow air through the fans at the base,
and you set loouers at different angles in front of the fan,
so that gives them a twist.
The air comes in the chamber with a twist.
And then you have this bell mouth at the top,
and another couple of fans, six larger fans
which are above the ceiling level,
and you run those in reverse
so they create a suction.
So, twist plus suction, that's a manmade tornado.
Professor Hangan is going to attempt
to measure the speed of that manmade tornado
right next to the ground
using tracking technology
and a big box of pink foam squares.
These squares should be large enough
for the trackers to follow,
but still light enough for the tornado to draw them out.
We're gonna start the fans.
But as yet, this technology
is entirely untested.
We'll go a little bit up with the updraft at 35%.
(dramatic music)
We'll just go up to 40.
That's it.
They're up.
The pink foam squares are flying successfully.
But can Horia and his team measure how fast they are flying?
They turn on the tracking technology.
The computer follows individual squares one after another
so it can create an average speed
from all the different flight paths.
And it works.
According to the calculations, the tornado is spinning
at a shade over 35 kilometers an hour.
The first time one has ever been measured
this near the ground.
So the concept of tracking is a good one.
But a real tornado is about 100 times bigger
and much, much faster.
Meaning it's still a problem getting near enough
to put something inside one.
However, a team of meteorologists from Oklahoma
think they may have found the answer
with this.
This bizarre vehicle is the Dominator 3,
a hand-built, tornado-proof armored car.
And it's equipped with a roof cannon
that can fire tracking devices straight into a tornado.
Once the probe is at peak height,
a parachute pops out,
allowing it to be sucked into the tornado.
So using the Dominator, can the Oklahoma team
actually manage to measure the speed
of a tornado at its base?
They intend to find out.
It takes them six weeks into tornado season,
but finally, meteorologist Reed Timmer and his crew
are hot on the heels of a real life twister.
The trick now is to get as close as they dare.
Close enough to fire a probe straight into its heart.
But that is no simple task.
The path of a tornado is notoriously unpredictable.
Reed and his team are determined.
If they can crack the problem of measuring tornado speed
in the wild, then it could revolutionize the way we deal
with these massive winds.
But as they get closer,
the dangers involved become all too evident.
They prime the Dominator for action.
There it is, tornado on the ground.
Straight ahead!
Gun it!
Straight ahead.
Go!
Oh, that's huge.
It is huge,
about 100 meters across and at least a kilometer tall.
They get the Dominator as close as the road will take them.
Stop!
Perfect.
Fix it.
Oh my god.
The tornado is coming straight for them.
Get ready to shoot!
It's perfect.
Deploy!
Drop it down!
Unfortunately, as they try
to make the vehicle secure, Reed's window fails.
Roll your window up.
Tell me when.
He has to resort to pulling it out by hand.
(loud winds)
It's in.
They got the probe inside.
I saw it make one full revolution
then I lost visual on it,
so I know it at least went around one time.
But that's only half the challenge.
Now they need to retrieve the probe
to find out what it recorded.
They wait for the storm to pass,
then set off out through the trail of devastation
in search of the probe.
So how far ahead do you think it is?
It's probably about three miles, I would say.
For some reason,
they're not picking up its GPS signal.
So they're reduced to searching on foot.
When I launched it, I saw it go out
over the road that way,
spun around like this, all the way around
and it descended either behind these trees
or these tress right here,
so we're within a couple hundred feet of it right now.
Okay so it's gotta be somewhere over this way over here.
Against all the odds, they spot it.
(group cheers)
Reed and his team have accomplished something
that no one has ever done before.
High fives!
They've managed to get a flying probe
into the base of a tornado.
Today's the first time we've recovered one
that we know was inside a tornado,
so this is a huge success for our science mission
and I'd say this is definitely a stepping stone
for things to come in the future.
It's a proud moment.
But this is real life.
The probe was damaged in its trip around the twister.
As yet, they haven't managed to retrieve any of its data.
So they're planning on doing it all over again.
Most of us will never have to live through a tornado.
But there is one type of extreme weather
we've all experienced.
And it's a fitting conclusion.
Because it requires all three of the key ingredients
that we've looked at in this program.
Temperature,
water,
and wind.
When heat makes the ground intensely warm
and the air is heavy with water vapor
and strong winds mold the clouds, you create:
a thunderstorm.
Most of us are familiar with thunder and lightning.
We know lightning is a powerful bolt of electricity
and thunder is the noise that accompanies it.
(thunder crashes)
But in its own way,
thunder is every bit as destructive as lightning.
This is one of the few places in the world
capable of creating full blown thunder.
They do it by firing 200,000 amps of electrical current
down this narrow copper wire,
exactly the same amount of current
that's in a real lightning bolt.
Chief scientist Dan Morgan explains how they go about it.
This is controlling our lightning generator
so we can produce a simulated bolt of lightning
and the corresponding thunder.
So first of all we raise the earth switch,
earth is the whole system, so we can now raise the dumps
and now in a position to charge.
We're nearly there.
We're storing energy in the crust
for the duration of a minute or so
and then releasing all that energy very quickly.
(brief boom)
That short, sharp bang
was actual thunder,
even though it's not the sound we're used to hearing.
Because a bolt of lightning is around a kilometer long,
some of that sound gets to us quicker than other bits.
So what we normally hear is multiple echoes
of that initial short bang.
But thunder is not just about sound,
as Dan is about to demonstrate
using slow motion cameras
and a line of lit candles.
(loud beeping)
(explosion)
In the aftermath of the discharge,
the candles are all blown out.
(explosion)
And if you watch carefully,
you can see that they are blown out one by one.
So what is going on?
Well, it's all to do with temperature.
We get our lightning arc
and we have a huge temperature increase.
That heats the air up surrounding the arc
and that moves faster then the speed of sound
which causes a shockwave which we hear as thunder.
And that's what we've just produced there.
Because lightning is so ferociously hot,
it explodes the air around it,
causing it to rush outwards.
What we see with the candles is that air moving away
from the lightning bolt in a shockwave.
The scientists here at Cobham are interested
in the effects of this shockwave.
They're trying to measure just how powerful it can be.
For instance, is it strong enough to shatter glass?
We're starting to think
that the inner ring of lightbulbs will go,
I'm not quite so sure about the outer ring.
One way to find out.
(glass breaking)
The bulbs have been destroyed
by the sheer force of the hot air exploding outwards.
In other words, the thunder.
Notice it's not the lightning destroying the lightbulbs.
The arc never even touches them.
It's the shockwave after the flash that does the damage.
Notice also that the sound of thunder happens even later,
after the flash
and after the bulbs have exploded.
By using an even more specialized camera,
the team can actually attempt
to capture their shockwave on screen.
Not the effects,
but the actual shockwave itself.
(electric buzz)
That is the air exploding away from the hot lightning bolt
at over 1100 kilometers an hour.
So there's the secret.
You can actually see thunder.
Weather is endlessly fascinating.
A stunning display of magic and spectacle
performed in front of us every single day.
Even when conditions are wet and miserable,
there are amazing events going on
just behind the scenes.
And though it may seem
that only extreme weather is worthy of our attention,
the weather around us every day is equally
full of wonder.
Wild weather
in all it's glory.
(triumphant music)