Original subtitles

WWW.MY-SUBS.CO

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.