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It insulates our planet from
the cold hostility of space...

..and shields us from
the sun's deadly rays.

It brings live-giving water...

..and it's in every breath you take.

It is our atmosphere.

Now, a team of scientists
are going on an expedition...

..to explore this elusive
and precious realm.

We have this dynamic bubble of air

constantly moving,
constantly changing

and that's what we are here
with Cloud Lab to explore.

This unique laboratory,
an airship 200 feet long,

is packed with the latest
scientific instruments.

Scan it up and down vertically
and see if we can hit it.

It actually goes right
up to the sun level.

It will enable the team to carry out
ground-breaking experiments...

This is really good -
now we are sucking in the clouds.

..to discover the many surprising
ways in which the atmosphere

shapes our world.

From the edge of the jet stream...

..to the bottom of the ocean.

Another giant-sized animal. This
whole place is, like, super-sized.

And the ways in which we ourselves
are changing the atmosphere.

We've hard evidence that human beings
are creating their own weather.

An airstrip in south-east Florida,

and the team get their first
sight of the airship.

It's a lot bigger than I thought it
would be. I genuinely thought...

I've been on expeditions in some
pretty extraordinary vehicles

but this has got to beat
the lot, surely?

This is better than my normal
lab, by a long way.

With an expertise in meteorology,

the expedition is being led
by explorer Felicity Aston.

I've spent lots of time looking
at the weather from the ground

and seeing satellite pictures
taken from above the atmosphere,

but with this we're going to be
able to actually go into the clouds

and see the weather from the inside.

So you can't help but be excited
about something like this.

The team will fly the airship
coast to coast across America,

from the Atlantic
all the way to the Pacific.

The extreme range of
atmospheric conditions

this continent encompasses

will enable them to investigate
three distinct themes...

Life. They want to discover the
many complex ways in which wildlife

exploits every level
of the atmosphere,

from close to the earth's surface
to the death-zone of high altitude.

Human impact. They'll explore
the subtle and surprising ways

in which we change the atmosphere.

And weather. The many
extraordinary processes

that generate weather
in the atmosphere.

MAN ON RADIO: Clear for take-off,
remain south, runway 9.

And this is where
their journey begins -

with one of the most beautiful,
transitory and mysterious

of all weather phenomena - clouds.

Fans? Fans are on.

Felicity will examine how clouds
capture and store liquid water

in the skies to form an ocean
of water above our heads.

That's the one I want! That one.

Good for departure. Temperature
pressures. Green for departure.

Andy Torbet will measure
the forces within clouds

that keep this water
floating in the sky.

Dr Chris van Tulleken
will see exactly how clouds

return water to earth

by unravelling one of the remaining
mysteries of meteorology -

what makes raindrops form?

We know about soot, sand and dirt

but I'm looking for something
a bit different.

Which is why, as an infectious
diseases doctor, I'm up in a cloud.

Backing up the team is atmospheric
chemist Jim McQuaid,

who's custom-built the lab.

The instrumentation we've got here
will measure gases in the atmosphere,

pollution, particle measurement.

This is a laser system that will
measure clouds off in the distance.

And then we can measure sunlight
on the top and also on the bottom

of the airship. So, we've got
a really nice little set

of measurements that will allow us
to explore the atmosphere.

Sharing the journey will be
a 15-strong support team,

needed to launch one of the biggest
airships in the world.

The flying capabilities
of the airship

offer the team a unique
research platform,

able to conduct experiments

that would be impossible
in any other aircraft.

They're beginning their expedition
with clouds,

because without them
we simply wouldn't be here.

It's difficult to imagine,

but the skies are home
to a vast ocean of water.

Yet it is beyond our reach,

suspended all around us
as an invisible vaporous gas.

Only once it is transformed into
clouds does it become liquid water.

It's this deceptively
simple transformation

of water from gas to liquid

that ultimately brings water

from the sea to the earth's
land surfaces,

by generating 1.4 trillion tonnes
of rainfall every day.

Yet clouds are as mysterious
as they are beautiful.

How can such delicate ephemeral
structures carry so much water?

To begin to understand exactly
how much water they carry,

Felicity wants to try something
that's never been attempted before.

So, what would be really great,

I don't know if it's going
to be possible or not,

but what would be really great
is if we could weigh a cloud,

see how heavy it is and work out how
much water is in one of those clouds.

But to do that, we've got
to get up there.

So we've got to do
a bit of cloud hunting.

The Florida coastline is the perfect
place to hunt for clouds,

because it's in the ocean

where water's journey
into the atmosphere begins.

Energy from the sun evaporates water
from the sea into the air above,

and when this moist air
is warm enough,

it starts to rise in a column of air
known as a thermal.

As it rises, it gets colder.

And cold air can't hold
as much water as warm air.

So you get to a certain level
when it's cold enough,

that all that water from the sea
starts to rematerialize

as tiny little droplets of water.
That is the birth of a cloud.

OK, we're going to go for a cloud...

Unlike other aircraft,

the airship can travel
slowly enough inside the cloud

to take the crucial measurements
Felicity will need.

OK, these clouds here are
a little bit wispy and broken.

These ones look as if they are
towering a bit too much.

I think that one's lower, over
there, you know. This one here?

OK, that's the one I want. That one.

It'll be really great to go
right through the middle

and right into the heart of it.

This is the airship November
6-1-0 Sierra Kilo...

It will take all of chief pilot
David Byrne's 30 years of experience

to reach the target cloud in time.

6-1-0 is an airship. We'd like
to operate in this area

between 2,500 and 3,000 feet.

MAN ON RADIO: Sierra Kilo,
roger. Proceed as requested.

Small cumulus clouds like this last
on average just ten minutes,

so they'll need to move fast.

Meanwhile, former paratrooper
Andy Torbet

is preparing for
the team's second mission,

researching another
aspect of clouds -

what keeps them in the air?

To do so, he'll be travelling
through a cloud

in a way that the airship can't -
vertically.

The plan is to find a nice cloud,
one that's growing,

one that's sucking moisture up
from the surface of the earth,

and to get out the aircraft
1,000 feet above the top of that,

and then drop just beneath it

and then fly my parachute
just under the cloud.

Whilst a thermal is enough
to give birth to a cloud,

for it to remain in the air
it needs another source of energy.

That energy comes from
within the cloud itself.

As molecules of water vapour
come together in a cloud,

they release the heat
absorbed during evaporation.

And it's this heat energy
that Andy is hoping to detect.

So as he descends through the cloud

he'll record a continuous stream
of temperature readings.

It's an experiment
fraught with hazard.

The powerful air currents that
thrust the cloud upward

also generate turbulence.

Normally with skydiving
you look to avoid clouds.

This is the first time I'll be
going to aim and hit a cloud.

To mitigate the risks, the entire
experiment is being supervised

by a skydive master, Dane Kenny.

OK, Andy, 1,500 feet,
throw the scientific stuff,

we start thinking about landing,
downwind to make the final leg.

Roger. Happy? Happy.

Dane will try to find a route for
Andy through the edges of the cloud,

where he can detect the release of
heat whilst still remaining safe.

Aboard the airship,

they're closing in on the cloud
they've targeted for weighing.

Dr Jim McQuaid primes
the instrumentation.

So we have a...
There's a laser beam here.

So this is one instrument we've
got, it's called a LIDAR.

The LIDAR, a kind of light radar,
will measure the cloud's dimensions

by emitting a laser and analysing
the light reflected back.

So, the time it takes for
the light to go from here

to the cloud and back
will tell us the distance.

A second probe will measure
the exact size and density

of the individual droplets of liquid

as the airship passes
through the cloud.

OK, Jim, are you ready? OK!

So, I'm picking up
cloud droplets now.

The humidity's gone up to 100%.

I don't know how
many times, as a kid,

I wondered what it would feel like
to be up in one of these clouds.

And now I've just gone through one,
so now I know.

It doesn't feel like
cotton wool sadly, but...

It feels really wet and
surprisingly dark in there.

That was great,
that was really perfect.

With the moist, cool air
of the cloud behind them,

they can begin to
figure out the result.

Wow, so that cloud was nearly
a kilometre long.

So, Jim, have you got an idea
of how wide the cloud was?

200 metres across.

So we're going to assume that
it's as tall as it is wide,

because it looked like a fairly
solid elliptical shape,

so we just use a simple formula
to work out the volume of the cloud.

How wide was it, 200 metres?

20 million. 20 million...
..cubic metres.

That's a small, compact cloud,
20 million cubic metres.

To calculate the cloud's weight,

they factor in the size and density
of the water droplets within it.

The weight per cubic metre
is about...

Say the average is 0.2.

0.2g per cubic metre.
0.2g per cubic metre.

OK, so we times 0.2
by 20 million. Yes.

4,000kg.

Yeah. So that small cloud
weighs four tonnes. Yeah.

That's incredible. It is.

And that was a small one.

Well, I think we can congratulate
ourselves. We've weighed a cloud.

We know it weighs four tonnes.

I don't know if anybody
has ever done it before.

I'm not sure anyone's
going to believe us,

that cloud weighs four tonnes, but
it does. All the figures are there.

Your machine did good.

Felicity's experiment has
revealed that even a small

cumulus cloud converts large
amounts of vapour to liquid water.

It also begins to explain how,
despite being fleeting,

delicate structures, clouds can
deliver all the earth's water needs.

The average cumulus is 50 times
larger than the one the team

have measured, so it carries
around 200 tonnes of water.

Even the most diffuse cloud,
a wispy, high altitude cirrus

of the same volume,
would weigh two tonnes.

But the greatest water bearers
are cumulonimbus clouds.

Up to ten times more dense than
a cumulus cloud, and measuring

on average 1,000 times larger,
these can weigh one million tonnes.

At any one point in time,
the world's clouds

hold an astonishing 129 billion
tonnes of water in the sky.

So, given clouds carry
vast amounts of water,

they must also generate
vast amounts of energy

in order to defy gravity
and remain aloft.

Dane and Andy are seeking to measure
this process, as it occurs,

by detecting the heat energy
generated by a cloud.

They've climbed to 10,000 feet
amongst a cluster of cumulus clouds.

Happy? Happy. Your handles?

Happy? Happy.

But the clouds are building fast.

Too fast for Dane's liking.

We're going to descend to 8,000 feet

because there's a lot of turbulence
up here, and I want to make sure

I put Andy out in the right
place at the right time.

Dane needs to position the aircraft
above a cloud, so that they can

descend through its fringes
and then beneath it.

That will enable Andy

to take the stream of
temperature readings he needs.

With a suitably isolated
cloud in sight,

Dane times the run-in.

Five, four, three, two, one, go!

They free-fall to 7,000 feet
to reach the cloud tops.

Now Andy's instruments
can set to work.

They've managed to fly into
the edge of the cloud,

and are soon met by
its powerful updraughts.

I can feel the turbulence.
There's a lot of activity here

and it's throwing my canopy about.

I'm now at 3,800 feet and getting
good readings on the Flytec.

Excellent, mate. Good job.

They've reached 3,000 feet
and the cloud base.

They navigate beneath it to record
the way the temperature changes

now they're out of the cloud.

Andy, I want you to head
towards the drop zone.

You should be able to see
the drop zone.

Head towards the sun. Roger.

Yeah, I can see the drop zone,
so that's good.

All Andy needs to do now to complete
his data set is reach the ground.

Yee-ha! Dane to Andy -
down safe, mate.

Andy has measured
the air temperature

from the top of the cloud
all the way to ground level.

It's now up to Felicity to see
if they've managed to detect

the generation of heat energy
within the cloud.

I've just been having a look at the
data that came back from Andy's jump

and they're perfect. They're
exactly what we wanted.

The data reveals that the atmosphere
cools at a predictable rate,

called the lapse rate, from
ground level to the cloud base.

But then the rate of cooling slows.

So the cloud is clearly
generating heat.

And why that's really lovely to see
that is because what we know happens

is that when water
condenses out of air

it releases a huge amount of energy,

and that energy
warms the air around it

and that creates big bursts
of energy inside the cloud.

And that's why clouds have big,
uneven fluffy tops.

So this is exactly what helps
to keep the cloud afloat.

This energy, released by
water vapour as it condenses,

is called latent heat,

and it is possible to work out

how much energy is delivered
to a cloud by this process.

A typical cumulus cloud, similar
to the one Dane and Andy measured,

generates enough heat energy to
power the average home for 17 years,

or about 300 tonnes of TNT.

Scale that up to a million tonne
cumulonimbus, and you're looking at

the heat energy equivalent
to a nuclear warhead.

It's really great that
we've managed to detect

that release of latent heat,

because it is so important

to all the different weather
systems that we see.

It's the fundamental driving force,
it's the energy source

of every single weather system.

So the formation
of a cloud is not just

the transfer of massive amounts
of water to the skies,

but massive amounts of energy, too.

And that means clouds not only have
the power to nourish our planet,

delivering rain to the earth,

but if that energy is released
quickly, wreak destruction.

It's a theme the team will examine
further into their mission,

but first they want to complete
their study of clouds.

The team is heading west
across the Florida peninsula,

towards the area known as
the Panhandle.

Having investigated how water
arrives in our skies

and is held aloft,

the team have come here
to examine how water

is returned to earth
in the form of rain.

At the heart of this question

is one of the most radical ideas
in meteorology today -

that some clouds are alive,

and as a consequence,
behave differently to others.

There's a hint of a wee bit of rain
there. To the north-west...

INDISTINCT RADIO CHATTER

It all comes down to the
little-understood process

that causes raindrops to form.

What we are looking for is
the stuff that makes rain.

Rain doesn't form easily,

which people in the UK
and frankly, people in Florida,

are going to think is a bit odd
because it rains a lot.

But you need a little catalyst,
a nucleus, to help raindrops form.

It's a bit like a grain of sand
at the heart of a pearl.

Molecules of water vapour
need a surface to collide with,

and condense onto, to form a liquid.

Normally, tiny particles
like dust or sea salt

suspended in clouds do the job.

But a new idea has emerged

suggesting the presence
of something quite different,

which could be causing some clouds
to produce rain

while others don't...

Life. In the form of bacteria.

It's a theory Chris and Jim are
seeking to find evidence for.

What we are trying to find out is,

is there bacteria in
the droplets of water?

Do they have stuff in them that could
act as a nucleus to help form rain?

As a microbiologist, Chris
is used to examining bacteria

that live within the human body.

He's now hoping the airship's
ability to enter clouds

with minimum disturbance will
enable him to see if clouds, too,

could be alive with microorganisms.

I'll just have a word with the pilot,

without falling out of the airship!

Dave, the deeper we are
into thick cloud the better.

I know there are limits
to what you can do

but that's what I'm looking for.

The tricky thing is distinguishing
between bacteria

and other tiny particles
like soot and dust

which have been swept into
the atmosphere by the wind.

So Jim has rigged the airship
with a particle analyser

known as a WIBS machine.

The WIBS machine uses lasers
to detect soot, particles,

to look for signs of life.

You know when you go into a room and
there's ultraviolet light,

like a nightclub? Bits of dandruff,
and your teeth,

and even if you have a cup
of urine - bit unlikely,

but that would glow under
the ultraviolet light.

So there are these
fluorescent molecules,

that when you shine particular
light on them, they glow.

And that's essentially what this
machine is going to look for.

It's going to shine a laser at all
the stuff that comes into it

and if something glows,
it's probably biological.

As they enter the cloud, an inlet
pipe draws in air for analysis.

This is great. This is the thickest
cloud we've been in, I think.

This is the thickest cloud
I'VE been in.

So the first question is,
are we detecting any signs

that there might be microscopic life
up in cloud vapour?

Yeah, I'm actually seeing
some response now.

This top channel is one of
the fluorescence channels

and it responds to proteins.

And anything above this baseline
is actually fluorescence,

and that's exactly what
you are wanting to see.

So you can see that we are getting
fluorescence from material going in.

These particles here,
what size are they?

The size is up to five microns,

nothing particularly big,
quite small. Smaller than pollen?

Oh, much smaller than
pollen, yeah, yeah.

And potentially the right size
for bacteria? Yes, yes.

So this is quite...

This is quite a big deal.

To me, this is a really big deal.

We've got evidence here that
we've got bacteria in clouds

and that's right at
the cutting edge of science.

Having established that some
clouds are alive with bacteria,

Chris now wants to know
whether those microorganisms

could be helping clouds
to produce rain.

Surprisingly, most rain
starts as ice crystals,

because high up inside clouds

temperatures are often
well below freezing.

Those crystals of ice act like
a magnet, attracting water vapour

and growing rapidly.

When they are big enough
and heavy enough, they fall,

and as they fall they melt
to become rain.

There is a theory that
water freezes more easily

around some types of particles
than others.

If you do the water and the mineral
dust, I'll do the bacteria.

So Chris is mounting an experiment

to find out which is best
at producing ice.

Is it dust or bacteria?

We've got three rows of drops here.
We've got the first row near me

is pure water,

and then the second row

has mineral dust in it,

and the third row has bacteria
that we know does live in clouds.

And we are just going to drop
the temperature on this plate

and see which freezes more easily.

And if a bacterial protein helps
water turn into ice more easily

than the mineral that we know is the
most common reason that rain happens,

that's really significant.

You know, if that process
is happening then bacteria

might be making their own rain.

This is the temperature
of the plate cooling down.

So it's just above freezing.

So this is the pure water,
this is the mineral dust,

this is the bacteria.

So we are below freezing.

It's funny, isn't it? We
talk about freezing as zero,

but it's actually really hard
to get water to freeze.

In fact, pure water doesn't
freeze until well below zero.

There needs to be impurities
in the water for it to freeze

at higher temperatures.

The water in your tap at home
you make ice cubes from

is full of all kinds of minerals

and particles and dust
and some bacteria.

That means when you put it
in the freezer it'll freeze.

So we are at minus 3.9.
So this is a cold day in there.

That's minus 4.5 now.

It's minus 8, almost minus 8.5.

Nothing's frozen yet.

There you go. There, there.
There you go.

The whole lot just went. You just
saw bang, bang, bang, bang, bang.

Everything just froze.

It's not a gradual thing.
Once there's one ice crystal -

bang, they all go.

But that was only the bacterial ones.
None of the mineral ones froze.

Only when it is two degrees colder

does the mineral dust
finally start to freeze.

Those... Yeah. Almost minus 11.

Some of the mineral ones are going.

Not only has the experiment
demonstrated

that ice forms around bacteria,

but that it does so at a higher
temperature than around dust.

So the bacterial protein is more
efficient than the main mineral

that we think causes rain.

And to me the key thing is here,
bacteria have evolved a protein,

they've made something that helps
water freeze, that helps ice form.

For Chris, this result
challenges the whole way

we understand how
weather is created.

It raises the intriguing possibility

that living clouds will rain more
readily than clouds that aren't.

So knowing whether a cloud
is a home to bacteria or not

could help forecasters predict
if it's going to rain.

But the real enigma
of living weather

is why bacteria are in
a cloud at all.

Bacteria like moist environments.

If you have rain, you have
vegetation, that's food for bacteria.

You know. Could it be that simple?

That it's not just their way
of getting out of the clouds,

it's their way of creating an
ecosystem

in which in which they can live?

That changes the whole way
you've got to think about

how weather happens on the planet.

The team have come to the Florida
Panhandle in search of rain,

but now it's about to find them.

Rain's coming.

I can see it moving towards us.

A powerful northerly wind
has brought a cold front

1,000 miles long
to the edge of the Gulf.

The airship is lighter than air,

its envelope filled with helium,

and in these conditions it's
hard to keep it under control.

For a brief moment, Cloud Lab
is at the mercy of the wind.

Stranded on board is
Dr Chris van Tulleken.

That was terrifying.

HE LAUGHS

Here I am just sitting
in the driving seat

and the whole thing
just turns on its end.

Does that happen a lot?

That's the first time I've seen it
go like that. Oh, really?

I assumed you knew
what you were doing!

With the airship finally
secure, there's nothing to do

but sit the storm out.

You can hear it beating down
on the top of the airship

and it makes the hairs on the back
of your neck go up.

You can feel the energy
in the air around you.

It's absolutely fantastic.

It's brilliant.

In just a few hours, close to
30,000 tonnes of water

are unleashed on this
one small airfield alone.

In the surrounding area,
a staggering 2.8 million tonnes.

With the passing of the cold front
comes the opportunity

for the team to pursue
a new theme...

..the relationship between the
atmosphere and the life forms

that make it their home.

For all creatures that fly,
the atmosphere is vital.

It's a place to find food,
to hunt and be hunted.

But it's also a domain
of wildly diverging

and rapidly changing habitats,

from gentle breezes
to powerful thermals,

which to us, remain mostly unseen.

So the team want to examine the
extent to which life actively

exploits the many different
characteristics of the atmosphere.

This is Gulf Shores, Alabama.

It's an important staging post

for a number of different
migratory bird species,

all of which are trying to escape
the approaching American winter.

They are resting up here before the
most perilous part of their journey

to South and Central America,

the 600 mile flight across
the Gulf of Mexico.

Andy is joining
a group of scientists

tracking the migration patterns
of the birds that depart from here.

They're on a dawn raid
to catch and then tag some.

And you've got to check them
every...? 30 minutes. OK.

The question they're
trying to answer is

do the birds time their departures
to take advantage

of favourable atmospheric
conditions?

With the passing of the front, now
is an ideal time to test the idea.

Three? So what's this? This
is a Swainson's thrush? No.

No, it's a, er...

It's actually a warbler. OK.

It that a migrant bird? It is.

They will come down all the way from
Alaska and migrate through here.

I really wanted to see a hummingbird.
They just seem so delicate.

Yeah, they're very, very delicate.
That's why we put them in the bags

instead of the boxes,

just for that reason.

The birds are caught between
two conflicting pressures.

On the one hand, winter is coming

and they have to move
before food becomes scarce.

On the other, if they
get their timing wrong,

they may find themselves
fighting headwinds.

So, all the ones we are catching
here, are they night-time migrants?

Every one of all these
birds are night migrants...

Cloud Lab's biologist,
Dr Sarah Beynon,

has just joined the expedition.

She's meeting the project's lead
scientist, Professor Frank Moore.

His team have caught
more birds than usual.

Do you think this is due to the
weather that we had yesterday?

I suspect that what happened was
the birds that flew last night,

they encountered that weather on
the coast and they stopped here.

And this is the last place
they could stop

before the Gulf of Mexico.

Amongst the many species
they are studying,

some have migrating patterns that
are still not fully understood,

such as hummingbirds.

He's making a spot there
to attach the transmitter.

The birds are fitted
with radio transmitters

to track their departure,

and see if it is related to any
particular weather conditions.

How much does that weigh?

It weighs about 4%
of the bird's body mass.

It may look invasive,

but the procedures have been
honed over many years.

Where will you be picking up the data
from that transmitter?

From the towers that we have
here on the peninsula,

that will pick up the signal
when the bird departs

across the Gulf of Mexico.

Do you want to let him go?
Oh, yes, please.

OK, how do I hold him?
Open your hand.

OK, and then hold the wings?

If you just let your hands go

he'll fly off, or maybe
with a little encouragement. OK.

Good luck, little one. There he goes.

Wow! Pretty impressive.

That was brilliant.

Andy is discovering that some birds
find it harder to leave than others.

OK, this is not going well.

Oh, man, you don't want to be
the one bloke who kills

a hummingbird on TV, do you?

Phew! That was a close one.

I'll have a little lie down now.

Now all they can do is wait and see
if the birds use the better weather

to make the crossing that evening.

On the airship, they're
heading west along the Gulf

so that Felicity can
rendezvous with Sarah.

Along the way, she'll gather
more meteorological data

to cross-reference with
the bird tagging data.

We put radio tags on some birds

to see whether they actually
made it across the Gulf,

and three of the birds that
we tagged made the journey.

And it took them between
16 and 24 hours. Wow!

But it just showed that they
were able to make that journey.

The tagged hummingbirds and thrushes
departed that same evening

and reached their destination.

The passage of the cold front led
to an improvement in the weather,

and delivered a tailwind that
the birds seem to have exploited.

And the data Felicity has gathered
suggests they're not the only birds

that take advantage
of a change in the wind.

This is National Radar Data.

So, any of the green, red
and yellow signals you can see,

that's bad weather that was
sitting right on top of you

and pinning all those birds down.
But then as that front moves across,

there's a sudden explosion of these
sort of rosette blue colours.

And nobody knew what
they were at first,

but now they know that it's
biological matter showing up

on the radar. So that is the birds
leaving, it shows up on the radar.

And if I just let this play, you can
see that over the whole country

as fronts move across,

behind the fronts you'll see this
sudden explosion of birds leaving.

After the passage of a front, many
millions of birds take to the skies

in an attempt to reduce the energy
required to make their migration.

It really just shows how important
these weather fronts are

for the birds.
They have to fly in the air

that's following these
cold fronts along.

And just seeing it on this level
shows that these weather fronts,

you know, they are vital
for movement,

not just on a small scale
but on a global scale.

The team are heading further west to
begin exploring their third theme -

the relationship between the
atmosphere and ourselves.

Humans have been changing
the atmosphere for millennia.

In recent years we've witnessed
the depletion of the ozone layer.

Today our carbon emissions
are changing the atmosphere

on a global scale.

The team want to explore one newly
emerging and surprising consequence

of our relationship
with the atmosphere -

the apparent increase
in the frequency

and intensity of hurricanes.

They've arrived at New Orleans.

In 2005, this was the scene of
the deadliest hurricane

to hit the United States
in more than half a century.

GEORGE W BUSH: Hurricane Katrina
is now designated

a Category 5 hurricane.

We cannot stress enough the danger
that this hurricane poses

to Gulf Coast communities. I urge all
citizens to put their own safety

and the safety of their families
first, by moving to safe ground.

The city still bears
the scars to this day.

A lot of the damage
is still so evident.

There's foundations with nothing
on them and roofs shaken to bits,

and I can see a lot of houses
where they're just destroyed.

It really brings home how powerful
this flooding must have been.

Hurricanes have battered these
shores since long before

there were human settlements.

It's a consequence of the particular
geography in this area.

As the team have learnt,
the journey of water to sky

releases vast amounts of energy
through the action of latent heat.

In the warm, shallow waters of the
Gulf, that process takes place

with such intensity
it can help to generate a hurricane.

But Katrina is evidence
of a new and disturbing trend

towards an increase in the number
and intensity of storms.

We already know that
the sea surface temperatures

drive the hurricanes,
they're the hurricane fuel.

And so if we look at a graph
of sea surface temperatures,

we can see that there's
a very obvious upward trend.

So temperatures are getting warmer
and warmer, decade after decade.

And that's what's driving not only
more hurricanes but worse hurricanes.

So what I'd like to know now

is what's driving that upward trend
in temperature?

The most likely cause
for the ocean warming is us.

But Felicity suspects it may not be
in the way we might at first expect.

There's a newly emerging idea
that the temperature of the Gulf

may be influenced by pollutants
in the atmosphere.

To test the idea, Felicity is taking
the airship on the eight-hour,

300 mile journey to one of America's
most industrialised cities -

Houston, Texas.

So, we've come to an area that has
a lot of heavy industry

and also one of the busiest
shipping lanes in the US,

because here we're likely to see
what impact that's having

on the clouds that are
forming in this area.

Clouds have an important
effect on sea temperatures

because of the way they
block out the sun's heat.

But the extent to which
they block the sun

depends upon what they're made from,

because polluted clouds
have different properties

compared to clean clouds.

What we'd now like to do

is to try and get into some
of these clouds over here.

We're looking for a dirty cloud.
Dirty?

Something that's either
over this shipping channel

or over the oil refineries. OK.

She first needs to confirm
whether the cloud is polluted.

OK, we're in.

Jim detects methane
and carbon dioxide -

important markers for
other pollutants.

So, can we tell whether that
was a dirty cloud or not?

We can measure the
cocktail of pollutants.

So, what we've got here,
we're getting these

increases in concentration.

So these lumps are where
we went through clouds,

and it's a peak in methane
and carbon dioxide.

The high levels of pollution mean
that there are more particles

on which the cloud
droplets can form.

And that has an important
knock-on effect.

This is the size distribution,
and the average is about six microns

and that's quite small.
Whereas in the cleaner clouds,

which we've flown through
in Florida,

the average size is more like ten.
Right.

So we're seeing more small droplets
that you would in a clean cloud? Yes.

In dirty clouds you have
more and smaller particles,

so they are going to be denser
clouds, there's more droplets.

The consequences of this
are far-reaching.

The more water droplets
a cloud contains,

the more sunlight it
scatters and reflects.

So less heat reaches
the earth and the sea.

The clouds here are dirty clouds, and
because they're thicker and denser,

they're blocking out more sunlight
than clean clouds.

So they're having
a net cooling effect

on the climate underneath them.

So dirty clouds are cooling down
temperatures.

Right.

It seems that polluted clouds
cool the world's oceans.

And yet sea surface temperatures
are on the rise...

..fuelling hurricanes.

Felicity calls upon the one piece
of data that can make sense

of this confusing picture -

the way in which pollution levels
have changed

over the past few decades.

What I'm thinking

is that the period when the
atmosphere was at its dirtiest...

And if you look at these
hurricane seasons...

..it's pretty much the same period of
time when there were less hurricanes.

So it's possible that pollution
is suppressing hurricanes. Yeah.

It's an extraordinary idea,
that higher levels of pollution

in the past might have been
suppressing hurricanes,

because polluted clouds were
cooling the world's oceans.

But environmental legislation

has improved air quality
across America.

So there are fewer of these
dense, polluted clouds.

As a result, the seas
have slowly warmed up again.

So, what we're saying is that
by cleaning up our atmosphere

we have allowed there to be
more hurricanes.

So, we're not seeing an upward trend
in hurricanes,

what we HAVE seen in past decades
when the air was dirty,

was a suppression in hurricanes.
So what we're seeing at the moment

is a return to the
natural state of things,

a return to the normal number
of hurricanes

that you would expect
to find in a season.

And that's really...

a fantastic story.

Felicity has picked her way through
the intricate evidence

that might explain the rise in
hurricane frequency and intensity.

And the answer is as complex
as it is surprising.

The team have now completed
the first half

of their epic voyage across America.

Next time, the team journey across
the harsh desert of the west,

and on to the Pacific Ocean.

We've made it!

Andy will take to
the skies once again,

searching for life
at the edge of existence.

Felicity and Jim will investigate
our role in making rain.

And Sarah experiences
life on the wing.