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One of the world's largest airships is taking
a team of scientists and adventurers on a unique expedition
a voyage deep into one of the most mysterious
and precious environments on earth - the atmosphere.
So we have this dynamic bubble of air constantly moving,
and that's what we are here with Cloud Lab to explore.
This quest is taking the team coast to coast
across America to discover the many surprising ways
in which the atmosphere shapes our world.
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 clouds carry,
meteorologist Felicity Aston 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.
Energy from the sun evaporates water from the sea
into the air above.
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,
when all that water from the sea starts to re-materialise as tiny,
little droplets of water.
That is the birth of a cloud.
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.
Atmospheric chemist 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 is 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.
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?
So I'm picking up cloud droplets now.
The humidity has gone up to 100%.
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 are going to assume it was as tall as it was 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?
In that 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.
-Per cubic metre.
OK. So we times 0.2 by 20 million.
So that small cloud weighed four tonnes. That's incredible.
The experiment has revealed that even a small
cumulus cloud converts large amounts of vapour to liquid water.
The average cumulus is 50 times larger than the one the team
have measured, so it carries around 200 tonnes of water.
But the greatest water bearers are cumulonimbus clouds.
Up to ten times more dense than 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.
The team want to investigate how the rain cycle works.
It all comes down to the little understood process
that causes raindrops to form.
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.
Normally, tiny particles like dust or sea-salt
suspended in clouds do the job.
But a new idea has emerged suggesting that rain drops could be
seeded by another kind of particle.
Life, in the form of bacteria.
Micro-biologist Dr Chris Van Tulleken
wants to know whether that's the case.
Often, the first thing to form around particles are ice crystals,
because high up inside clouds
temperatures can be 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.
So Chris is mounting an experiment to find out which is
the best at producing ice.
Is it dust or bacteria?
So we've got three rows of drops here.
We've got the first row, near me,
that's pure water, and then the second row has mineral dust in it
and the third row has a 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.
So we are below... We are below freezing.
It's funny, isn't it? So 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.
It's minus eight, almost minus eight and a half.
Nothing's frozen yet.
There you go. There, there.
But that was only the bacterial ones.
None of the mineral ones have frozen.
Only when it is two degrees colder does the mineral dust
finally start to freeze.
Ah, those... At 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 mineral,
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.
The experiment raises the intriguing possibility that
bacteria will make clouds rain more readily.
So, knowing whether a cloud is a home to bacteria or not could help
forecasters predict if it's going to rain.
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
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.
Cloud Lab's Andy Torbet 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'll check them every...?
The question they're trying to answer is do the birds time
their departures to take advantage
of favourable atmospheric conditions?
Now is an ideal time to test the idea.
A cold front, a mass of cooler air, has just swept through the region,
bringing with it torrential rain.
But now conditions have improved.
I really wanted to see a humming bird.
-They just seem so delicate.
-Yeah, they are very delicate.
That's why we put them in the bags instead of the boxes,
just for purely 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 head winds.
So he's making a spot there to attach the transmitter.
The birds, in this case, a hummingbird,
are fitted with radio transmitters to track their departure.
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.
So 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 - then hold the wings?
And if you just let your hands go
he can just fly off or maybe with a little encouragement.
-Good luck, little one.
There he goes. Wow! Pretty impressive.
That was brilliant.
Now all they can do is wait and see if the birds use the better weather
to make the crossing that evening.
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.
-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 tail-wind that the birds seem to have exploited.
And the data Felicity has gathered suggests
they are 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 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 Cloud Lab team want to explore a surprising
consequence of human impact upon 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.
Hurricanes have battered these shores
since long before there were human settlements.
It's a consequence of the particular geography in this area.
As water is evaporated into the sky to form clouds,
it brings with it vast amounts of heat energy.
In the warm, shallow waters of the Gulf, that process takes place
with such intensity that it can help to generate a hurricane.
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.
There's a newly emerging idea that the temperature
of the Gulf may be influenced by pollutants in the atmosphere.
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 are 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.
Well, not dirty, but something that's either over
-this shipping channel or over the oil refineries.
Jim detects methane and carbon dioxide -
important markers for other pollutants.
The high levels of pollution
mean 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 are seeing more small droplets
-than you would in a clean cloud.
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 clouds here are dirty clouds and because they are
thicker and denser, they are blocking out more sunlight than clean clouds.
So they are having a net cooling effect
on the climate underneath them.
-So dirty clouds are cooling down temperatures.
It seems that polluted clouds cool the world's oceans.
And yet sea surface temperatures are on the rise.
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
as when there were less hurricanes.
-So it's possible that pollution is suppressing hurricanes.
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
So there are fewer of these dense, polluted clouds.
As a result, the seas have slowly warmed up again.
So, what we are saying is that by cleaning up
our atmosphere...we have allowed there to be more hurricanes.
So we are not seeing an upward trend in hurricanes.
What we have seen in past decades, when the air was dirty,
was a suppression in hurricanes. What we are seeing
at the moment is a return to the natural state of things, a return
to the normal number of hurricanes you would expect to find in a season.
The airship is approaching the desert city of Phoenix, Arizona,
where the team want to answer a question about human impact
upon the atmosphere.
Can cities make their own weather?
So I've been looking at historical data
and you can see that Phoenix in the last 100 years has gone
from being really a small, agricultural settlement
into a large, urban city.
In the same period of time, there has been a distinct change
in the amount of rainfall in the city.
There are areas of Phoenix that have had up to
a 12% increase in the amount of rainfall,
which is really significant,
and it looks like there might be a correlation between the two.
So we want to see
if we can unravel how the city might be creating its own weather.
The rain that falls here has followed the same
cycle for millennia.
Every summer, warm, moist air is swept up from the oceans to
As this air meets the hot desert, variations in the landscape
drive pockets of air upwards as thermals.
Where the moisture cools, condenses,
and ultimately falls in sudden downpours of rain.
This process should make rainfall across the region fairly random.
But something appears to be concentrating it upon the city.
To see why, Felicity is going to start by surveying
temperatures in Phoenix and the surrounding desert.
I took several readings of the surface temperature
and I was getting between 37 and 38 degrees centigrade.
So, it's pretty hot down there, it's soaking up all the heat from the sun.
For the city to be concentrating rainfall,
it needs to be hotter than the desert,
driving extra thermal activity.
I'm getting a real variety in surface temperatures.
So if I take a reading from the road or a car park, it's pretty
much the same surface temperature as in the desert,
but if I point the camera at a garden or a swimming pool or a roof top,
then it's a lot less.
So, on average, the surface temperature here will overall
be a lot less than the desert.
The city is a little cooler than the surrounding desert.
So there's no evidence for the increased
thermal activity that can explain the rainfall.
As the day wears on, that picture soon changes.
See, look, look, look, look, see the city.
-It's hotter than the desert.
OK, yeah, you can see definitely the boundary.
So that's the desert cooling down and that's the hot city.
That's a really nice example of it.
Whilst the natural landscape has quickly cooled,
the camera reveals the city to have remained warm.
They've identified an effect called the urban heat island.
Earlier today, we measured the ground temperature of the suburbs
to be 24, 25 degrees,
and see, I'm measuring, 23, 22 -
I mean it's still as hot as when we measured it in the middle of the day.
The city surfaces are continuing to radiate
the energy of the sun they absorbed earlier in the day.
The question is whether the urban heat island is generating thermals.
If it is, they should be able to detect
an increase in temperature at altitude from the airship.
So, I've just had a look at the temperatures
and this is the temperature going down and that's going down simply
because the sun's going down - you know, we're turning the heater off.
So this is the temperature over the desert
and this is the temperature over the city.
Oh, wow, so this is where we hit the city?
-So we've got this big parcel of warm air sitting
over the city.
It makes a lot of logical sense that that air is going to start rising and
that's going to start convection and the consequence of that is weather.
So the increased rainfall in Phoenix could be caused by
the urban heat island effect.
It generates thermals over the city,
that force air upward, where it begins to cool.
That in turn can cause the vapour to condense and form rain,
concentrated here upon Phoenix.
So we've found the connection we were looking for,
between cities, and the increased rainfall that Phoenix has been
experiencing in the last 100 years.
And the really exciting thing about that is that we've
hard evidence that human beings are creating their own weather.