Wild Weather with Richard Hammond

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.

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.

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.

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.

Four cubic meters of water

actually weighs four tons.

Four times heavier than the car underneath it.

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,