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One of the world's largest airships is taking | 0:00:08 | 0:00:10 | |
a team of scientists and adventurers on a unique expedition | 0:00:10 | 0:00:15 | |
a voyage deep into one of the most mysterious | 0:00:15 | 0:00:18 | |
and precious environments on earth - the atmosphere. | 0:00:18 | 0:00:22 | |
So we have this dynamic bubble of air constantly moving, | 0:00:24 | 0:00:28 | |
constantly changing, | 0:00:28 | 0:00:30 | |
and that's what we are here with Cloud Lab to explore. | 0:00:30 | 0:00:33 | |
This quest is taking the team coast to coast | 0:00:34 | 0:00:37 | |
across America to discover the many surprising ways | 0:00:37 | 0:00:41 | |
in which the atmosphere shapes our world. | 0:00:41 | 0:00:44 | |
It's difficult to imagine, | 0:01:04 | 0:01:06 | |
but the skies are home to a vast ocean of water. Yet it is beyond | 0:01:06 | 0:01:12 | |
our reach, suspended all around us as an invisible, vaporous gas. | 0:01:12 | 0:01:17 | |
Only once it is transformed into clouds does it become liquid water. | 0:01:20 | 0:01:24 | |
It's this deceptively simple transformation of water, | 0:01:26 | 0:01:30 | |
from gas to liquid, | 0:01:30 | 0:01:32 | |
that ultimately brings water from the sea to the Earth's land surfaces | 0:01:32 | 0:01:35 | |
by generating 1.4 trillion tonnes of rainfall every day. | 0:01:35 | 0:01:41 | |
Yet clouds are as mysterious as they are beautiful. | 0:01:45 | 0:01:49 | |
How can such delicate, ephemeral structures carry so much water? | 0:01:52 | 0:01:56 | |
To begin to understand exactly how much water clouds carry, | 0:02:00 | 0:02:05 | |
meteorologist Felicity Aston wants to try something | 0:02:05 | 0:02:08 | |
that's never been attempted before. | 0:02:08 | 0:02:10 | |
So what would be really great, | 0:02:13 | 0:02:15 | |
I don't know if it's going to be possible or not, | 0:02:15 | 0:02:17 | |
but what would be really great is if we could weigh a cloud - | 0:02:17 | 0:02:20 | |
see how heavy it is, and work out how much water is in one of those clouds. | 0:02:20 | 0:02:24 | |
But to do that, we've got to get up there. | 0:02:25 | 0:02:28 | |
So we've got to do a bit of cloud hunting. | 0:02:28 | 0:02:30 | |
Energy from the sun evaporates water from the sea | 0:02:34 | 0:02:37 | |
into the air above. | 0:02:37 | 0:02:38 | |
When this moist air is warm enough, | 0:02:39 | 0:02:41 | |
it starts to rise in a column of air known as a thermal. | 0:02:41 | 0:02:45 | |
As it rises, it gets colder, | 0:02:47 | 0:02:50 | |
and cold air can't hold as much water as warm air. | 0:02:50 | 0:02:54 | |
So you get to a certain level, when it's cold enough, | 0:02:55 | 0:02:58 | |
when all that water from the sea starts to re-materialise as tiny, | 0:02:58 | 0:03:02 | |
little droplets of water. | 0:03:02 | 0:03:04 | |
That is the birth of a cloud. | 0:03:04 | 0:03:06 | |
OK - that's the one I want. That one. | 0:03:11 | 0:03:13 | |
It'll be really great to go right through the middle | 0:03:14 | 0:03:16 | |
and right into the heart of it. | 0:03:16 | 0:03:18 | |
Atmospheric chemist Dr Jim McQuaid primes the instrumentation. | 0:03:20 | 0:03:24 | |
So we have a... There's a laser beam here. | 0:03:27 | 0:03:29 | |
So this is one instrument we've got, it is called a LiDAR. | 0:03:29 | 0:03:32 | |
The LiDAR, a kind of light radar, will measure the cloud's dimensions | 0:03:34 | 0:03:39 | |
by emitting a laser and analysing the light reflected back. | 0:03:39 | 0:03:43 | |
A second probe will measure the exact size | 0:03:45 | 0:03:49 | |
and density of the individual droplets of liquid | 0:03:49 | 0:03:51 | |
as the airship passes through the cloud. | 0:03:51 | 0:03:54 | |
OK, Jim, are you ready? | 0:03:55 | 0:03:56 | |
OK. | 0:03:56 | 0:03:58 | |
So I'm picking up cloud droplets now. | 0:04:04 | 0:04:06 | |
The humidity has gone up to 100%. | 0:04:06 | 0:04:08 | |
Wow, so that cloud was nearly a kilometre long. | 0:04:18 | 0:04:21 | |
So, Jim, have you got an idea of how wide the cloud was? | 0:04:22 | 0:04:26 | |
200 metres across. | 0:04:27 | 0:04:29 | |
So we are going to assume it was as tall as it was wide, | 0:04:29 | 0:04:32 | |
because it looked like a fairly solid elliptical shape, | 0:04:32 | 0:04:35 | |
so we just use a simple formula to work out the volume of the cloud. | 0:04:35 | 0:04:39 | |
How wide was it - 200 metres? | 0:04:39 | 0:04:42 | |
20 million... | 0:04:42 | 0:04:43 | |
-20 million? -Cubic metres. | 0:04:43 | 0:04:45 | |
In that small, compact cloud. 20 million cubic metres. | 0:04:45 | 0:04:50 | |
To calculate the cloud's weight, | 0:04:53 | 0:04:55 | |
they factor in the size and density of the water droplets within it. | 0:04:55 | 0:04:59 | |
The weight per cubic metre is about...say the average is 0.2. | 0:05:06 | 0:05:10 | |
-0.2g per cubic metre. -Per cubic metre. | 0:05:10 | 0:05:13 | |
OK. So we times 0.2 by 20 million. | 0:05:15 | 0:05:18 | |
Yes... | 0:05:18 | 0:05:20 | |
So that small cloud weighed four tonnes. That's incredible. | 0:05:23 | 0:05:27 | |
It is. | 0:05:27 | 0:05:29 | |
The experiment has revealed that even a small | 0:05:32 | 0:05:34 | |
cumulus cloud converts large amounts of vapour to liquid water. | 0:05:34 | 0:05:38 | |
The average cumulus is 50 times larger than the one the team | 0:05:41 | 0:05:44 | |
have measured, so it carries around 200 tonnes of water. | 0:05:44 | 0:05:49 | |
But the greatest water bearers are cumulonimbus clouds. | 0:05:52 | 0:05:56 | |
Up to ten times more dense than cumulus cloud, and measuring, | 0:05:57 | 0:06:02 | |
on average, 1,000 times larger, these can weigh one million tonnes. | 0:06:02 | 0:06:06 | |
At any one point in time, the world's clouds hold | 0:06:08 | 0:06:12 | |
an astonishing 129 billion tonnes of water in the sky. | 0:06:12 | 0:06:16 | |
The team want to investigate how the rain cycle works. | 0:06:33 | 0:06:36 | |
It all comes down to the little understood process | 0:06:37 | 0:06:40 | |
that causes raindrops to form. | 0:06:40 | 0:06:43 | |
Rain doesn't form easily, which people in the UK | 0:06:43 | 0:06:47 | |
and, frankly, people in Florida, | 0:06:47 | 0:06:48 | |
are going to think is a bit odd, because it rains a lot, | 0:06:48 | 0:06:51 | |
but you need a little catalyst - a nucleus - to help raindrops form. | 0:06:51 | 0:06:56 | |
It's a bit like a grain of sand at the heart of a pearl. | 0:06:56 | 0:07:00 | |
Normally, tiny particles like dust or sea-salt | 0:07:02 | 0:07:05 | |
suspended in clouds do the job. | 0:07:05 | 0:07:07 | |
But a new idea has emerged suggesting that rain drops could be | 0:07:09 | 0:07:13 | |
seeded by another kind of particle. | 0:07:13 | 0:07:15 | |
Life, in the form of bacteria. | 0:07:16 | 0:07:19 | |
Micro-biologist Dr Chris Van Tulleken | 0:07:19 | 0:07:22 | |
wants to know whether that's the case. | 0:07:22 | 0:07:24 | |
Often, the first thing to form around particles are ice crystals, | 0:07:26 | 0:07:30 | |
because high up inside clouds | 0:07:30 | 0:07:32 | |
temperatures can be well below freezing. | 0:07:32 | 0:07:34 | |
Those crystals of ice act like a magnet, attracting water vapour | 0:07:35 | 0:07:40 | |
and growing rapidly. | 0:07:40 | 0:07:42 | |
When they are big enough and heavy enough they fall - | 0:07:44 | 0:07:47 | |
and as they fall, they melt to become rain. | 0:07:47 | 0:07:50 | |
So Chris is mounting an experiment to find out which is | 0:07:54 | 0:07:58 | |
the best at producing ice. | 0:07:58 | 0:08:00 | |
Is it dust or bacteria? | 0:08:00 | 0:08:04 | |
So we've got three rows of drops here. | 0:08:07 | 0:08:10 | |
We've got the first row, near me, | 0:08:10 | 0:08:12 | |
that's pure water, and then the second row has mineral dust in it | 0:08:12 | 0:08:17 | |
and the third row has a bacteria that we know does live in clouds. | 0:08:17 | 0:08:22 | |
And we are just going to drop the temperature on this plate | 0:08:22 | 0:08:25 | |
and see which freezes more easily. | 0:08:25 | 0:08:27 | |
So we are below... We are below freezing. | 0:08:29 | 0:08:31 | |
It's funny, isn't it? So we talk about freezing as zero, | 0:08:31 | 0:08:35 | |
but it's actually really hard to get water to freeze. | 0:08:35 | 0:08:39 | |
In fact, pure water doesn't freeze until well below zero. | 0:08:41 | 0:08:45 | |
There needs to be impurities in the water for it to | 0:08:46 | 0:08:49 | |
freeze at higher temperatures. | 0:08:49 | 0:08:51 | |
It's minus eight, almost minus eight and a half. | 0:08:51 | 0:08:56 | |
Nothing's frozen yet. | 0:08:58 | 0:09:00 | |
There you go. There, there. | 0:09:02 | 0:09:04 | |
But that was only the bacterial ones. | 0:09:05 | 0:09:07 | |
None of the mineral ones have frozen. | 0:09:07 | 0:09:10 | |
Only when it is two degrees colder does the mineral dust | 0:09:10 | 0:09:13 | |
finally start to freeze. | 0:09:13 | 0:09:15 | |
Ah, those... At almost minus 11, | 0:09:16 | 0:09:20 | |
some of the mineral ones are going. | 0:09:20 | 0:09:22 | |
Not only has the experiment demonstrated that ice | 0:09:23 | 0:09:26 | |
forms around bacteria, | 0:09:26 | 0:09:28 | |
but that it does so at a higher temperature than around dust. | 0:09:28 | 0:09:32 | |
So, the bacterial protein is more efficient than the mineral, | 0:09:41 | 0:09:45 | |
the main mineral that we think causes rain, | 0:09:45 | 0:09:49 | |
and, to me, the key thing is here - bacteria have evolved a protein - | 0:09:49 | 0:09:53 | |
they've made something that helps water freeze, that helps ice form. | 0:09:53 | 0:09:58 | |
The experiment raises the intriguing possibility that | 0:10:00 | 0:10:03 | |
bacteria will make clouds rain more readily. | 0:10:03 | 0:10:06 | |
So, knowing whether a cloud is a home to bacteria or not could help | 0:10:07 | 0:10:11 | |
forecasters predict if it's going to rain. | 0:10:11 | 0:10:14 | |
This is Gulf Shores, Alabama. | 0:10:37 | 0:10:40 | |
It's an important staging post for a number of different migratory | 0:10:40 | 0:10:44 | |
bird species, all of which are trying to escape the approaching | 0:10:44 | 0:10:48 | |
American winter. | 0:10:48 | 0:10:49 | |
They are resting up here before the most perilous | 0:10:53 | 0:10:55 | |
part of their journey to South and Central America. | 0:10:55 | 0:10:58 | |
The 600-mile flight across the Gulf of Mexico. | 0:10:58 | 0:11:02 | |
Cloud Lab's Andy Torbet is joining a group of scientists | 0:11:08 | 0:11:12 | |
tracking the migration patterns of the birds that depart from here. | 0:11:12 | 0:11:16 | |
They're on a dawn raid to catch and then tag some. | 0:11:19 | 0:11:24 | |
-And you'll check them every...? -30 minutes. -OK. | 0:11:29 | 0:11:31 | |
The question they're trying to answer is do the birds time | 0:11:34 | 0:11:37 | |
their departures to take advantage | 0:11:37 | 0:11:39 | |
of favourable atmospheric conditions? | 0:11:39 | 0:11:41 | |
Now is an ideal time to test the idea. | 0:11:43 | 0:11:46 | |
A cold front, a mass of cooler air, has just swept through the region, | 0:11:49 | 0:11:54 | |
bringing with it torrential rain. | 0:11:54 | 0:11:55 | |
But now conditions have improved. | 0:11:59 | 0:12:01 | |
I really wanted to see a humming bird. | 0:12:02 | 0:12:04 | |
-They just seem so delicate. -Yeah, they are very delicate. | 0:12:04 | 0:12:06 | |
That's why we put them in the bags instead of the boxes, | 0:12:06 | 0:12:09 | |
just for purely that reason. | 0:12:09 | 0:12:11 | |
The birds are caught between two conflicting pressures. | 0:12:11 | 0:12:14 | |
On the one hand, winter is coming, | 0:12:14 | 0:12:16 | |
and they have to move before food becomes scarce. | 0:12:16 | 0:12:19 | |
On the other, if they get their timing wrong, | 0:12:21 | 0:12:24 | |
they may find themselves fighting head winds. | 0:12:24 | 0:12:27 | |
So he's making a spot there to attach the transmitter. | 0:12:30 | 0:12:34 | |
The birds, in this case, a hummingbird, | 0:12:34 | 0:12:36 | |
are fitted with radio transmitters to track their departure. | 0:12:36 | 0:12:39 | |
How much does that weigh? | 0:12:41 | 0:12:43 | |
It weighs about 4% of the bird's body mass. | 0:12:43 | 0:12:46 | |
It may look invasive, | 0:12:47 | 0:12:49 | |
but the procedures have been honed over many years. | 0:12:49 | 0:12:51 | |
So where will you be picking up the data from that transmitter? | 0:12:51 | 0:12:55 | |
From the towers that we have here on the peninsula that | 0:12:55 | 0:12:58 | |
will pick up the signal when the bird departs across the Gulf of Mexico. | 0:12:58 | 0:13:02 | |
-Do you want to let him go? -Oh, yes, please. | 0:13:04 | 0:13:06 | |
-OK, how do I hold him? -Open your hand. | 0:13:06 | 0:13:09 | |
OK - then hold the wings? | 0:13:11 | 0:13:14 | |
And if you just let your hands go | 0:13:15 | 0:13:17 | |
he can just fly off or maybe with a little encouragement. | 0:13:17 | 0:13:20 | |
-OK. -Good luck, little one. | 0:13:22 | 0:13:23 | |
There he goes. Wow! Pretty impressive. | 0:13:23 | 0:13:25 | |
That was brilliant. | 0:13:25 | 0:13:27 | |
Now all they can do is wait and see if the birds use the better weather | 0:13:31 | 0:13:35 | |
to make the crossing that evening. | 0:13:35 | 0:13:36 | |
We put radio tags on some birds to see | 0:13:45 | 0:13:47 | |
whether they actually made it across the gulf, | 0:13:47 | 0:13:50 | |
and three of the birds that we tagged made the journey. | 0:13:50 | 0:13:53 | |
And it took them between 16 and 24 hours. | 0:13:53 | 0:13:57 | |
-Wow! -But it just showed that they were able to make that journey. | 0:13:57 | 0:14:01 | |
The tagged hummingbirds and thrushes departed that same evening | 0:14:04 | 0:14:09 | |
and reached their destination. | 0:14:09 | 0:14:11 | |
The passage of the cold front led to an improvement in the weather, | 0:14:16 | 0:14:19 | |
and delivered a tail-wind that the birds seem to have exploited. | 0:14:19 | 0:14:23 | |
And the data Felicity has gathered suggests | 0:14:26 | 0:14:28 | |
they are not the only birds | 0:14:28 | 0:14:30 | |
that take advantage of a change in the wind. | 0:14:30 | 0:14:32 | |
This is national radar data, so any of the green, | 0:14:34 | 0:14:39 | |
red and yellow signals you can see - that's bad weather that was | 0:14:39 | 0:14:43 | |
sitting right on top of you and pinning all those birds down. | 0:14:43 | 0:14:46 | |
But then as that front moves across, | 0:14:46 | 0:14:49 | |
there's a sudden explosion of these sort of rosette blue colours. | 0:14:49 | 0:14:54 | |
And nobody knew what they were at first, | 0:14:54 | 0:14:56 | |
but now they know that it's biological matter showing up | 0:14:56 | 0:15:00 | |
on the radar - so that is the birds leaving - it shows up on the radar. | 0:15:00 | 0:15:04 | |
And if I just let this play, you can | 0:15:07 | 0:15:09 | |
see that over the whole country, as fronts move across, | 0:15:09 | 0:15:13 | |
behind the fronts you'll see this sudden explosion of birds leaving. | 0:15:13 | 0:15:17 | |
After the passage of a front, | 0:15:22 | 0:15:25 | |
many millions of birds take to the skies in an attempt to reduce | 0:15:25 | 0:15:28 | |
the energy required to make their migration. | 0:15:28 | 0:15:31 | |
It really just shows how important these weather fronts | 0:15:34 | 0:15:37 | |
are for the birds. | 0:15:37 | 0:15:39 | |
They have to fly in air that's following these cold fronts along. | 0:15:39 | 0:15:43 | |
And just seeing it on this level shows that these weather fronts | 0:15:45 | 0:15:50 | |
you know, they are vital for movement. | 0:15:50 | 0:15:53 | |
Not just on a small scale but on a global scale. | 0:15:53 | 0:15:56 | |
The Cloud Lab team want to explore a surprising | 0:16:16 | 0:16:19 | |
consequence of human impact upon the atmosphere. | 0:16:19 | 0:16:21 | |
The apparent increase in the frequency | 0:16:23 | 0:16:25 | |
and intensity of hurricanes. | 0:16:25 | 0:16:27 | |
They've arrived at New Orleans. | 0:16:31 | 0:16:33 | |
In 2005, this was the scene of the deadliest hurricane to hit | 0:16:33 | 0:16:37 | |
the United States in more than half a century. | 0:16:37 | 0:16:40 | |
Hurricanes have battered these shores | 0:16:41 | 0:16:43 | |
since long before there were human settlements. | 0:16:43 | 0:16:46 | |
It's a consequence of the particular geography in this area. | 0:16:46 | 0:16:50 | |
THUNDER ROLLS | 0:16:51 | 0:16:53 | |
As water is evaporated into the sky to form clouds, | 0:16:53 | 0:16:57 | |
it brings with it vast amounts of heat energy. | 0:16:57 | 0:17:00 | |
In the warm, shallow waters of the Gulf, that process takes place | 0:17:02 | 0:17:06 | |
with such intensity that it can help to generate a hurricane. | 0:17:06 | 0:17:10 | |
We already know that the sea surface temperatures, | 0:17:20 | 0:17:23 | |
drive the hurricanes, they're the hurricane fuel. | 0:17:23 | 0:17:27 | |
And so if we look at a graph of sea surface temperatures, | 0:17:27 | 0:17:30 | |
we can see that there's a very obvious, upward trend, | 0:17:30 | 0:17:34 | |
so temperatures are getting warmer and warmer, decade after decade. | 0:17:34 | 0:17:40 | |
And that's what's driving not only more hurricanes but worse hurricanes. | 0:17:40 | 0:17:44 | |
So what I'd like to know now is what's driving that upward | 0:17:44 | 0:17:49 | |
trend in temperature. | 0:17:49 | 0:17:50 | |
There's a newly emerging idea that the temperature | 0:17:52 | 0:17:54 | |
of the Gulf may be influenced by pollutants in the atmosphere. | 0:17:54 | 0:17:58 | |
So we've come to an area that has a lot of heavy industry | 0:17:59 | 0:18:03 | |
and also one of the busiest shipping lanes in the US, | 0:18:03 | 0:18:06 | |
because here we are likely to see what impact that's | 0:18:06 | 0:18:10 | |
having on the clouds that are forming in this area. | 0:18:10 | 0:18:13 | |
Clouds have an important effect on sea temperatures | 0:18:15 | 0:18:18 | |
because of the way they block out the sun's heat. | 0:18:18 | 0:18:22 | |
But the extent to which they block the sun depends upon what | 0:18:22 | 0:18:25 | |
they're made from, because polluted clouds have different | 0:18:25 | 0:18:29 | |
properties compared to clean clouds. | 0:18:29 | 0:18:31 | |
What we'd now like to do is to try | 0:18:32 | 0:18:34 | |
and get into some of these clouds over here. | 0:18:34 | 0:18:37 | |
-We're looking for a dirty cloud. -Dirty, yep. | 0:18:37 | 0:18:39 | |
Well, not dirty, but something that's either over | 0:18:39 | 0:18:41 | |
-this shipping channel or over the oil refineries. -OK. | 0:18:41 | 0:18:45 | |
Jim detects methane and carbon dioxide - | 0:18:50 | 0:18:53 | |
important markers for other pollutants. | 0:18:53 | 0:18:55 | |
The high levels of pollution | 0:19:01 | 0:19:02 | |
mean there are more particles on which the cloud droplets can form. | 0:19:02 | 0:19:06 | |
And that has an important knock-on effect. | 0:19:09 | 0:19:11 | |
This is the size distribution and the average | 0:19:19 | 0:19:21 | |
is about six microns, and that's quite small, | 0:19:21 | 0:19:25 | |
whereas in the cleaner clouds | 0:19:25 | 0:19:27 | |
which we've flown through in Florida, the average size is more like ten. | 0:19:27 | 0:19:31 | |
Right. So we are seeing more small droplets | 0:19:31 | 0:19:35 | |
-than you would in a clean cloud. -Yes. | 0:19:35 | 0:19:39 | |
In dirty clouds you have more and smaller particles, | 0:19:39 | 0:19:44 | |
so they are going to be denser clouds, there's more droplets. | 0:19:44 | 0:19:48 | |
The consequences of this are far-reaching. | 0:19:51 | 0:19:54 | |
The clouds here are dirty clouds and because they are | 0:19:54 | 0:19:59 | |
thicker and denser, they are blocking out more sunlight than clean clouds. | 0:19:59 | 0:20:04 | |
So they are having a net cooling effect | 0:20:04 | 0:20:07 | |
on the climate underneath them. | 0:20:07 | 0:20:09 | |
-So dirty clouds are cooling down temperatures. -Yes. -Right. | 0:20:11 | 0:20:18 | |
It seems that polluted clouds cool the world's oceans. | 0:20:22 | 0:20:25 | |
And yet sea surface temperatures are on the rise. | 0:20:29 | 0:20:32 | |
Fuelling hurricanes. | 0:20:37 | 0:20:38 | |
Felicity calls upon the one piece of data that can make | 0:20:44 | 0:20:47 | |
sense of this confusing picture. | 0:20:47 | 0:20:50 | |
The way in which pollution levels | 0:20:50 | 0:20:52 | |
have changed over the past few decades. | 0:20:52 | 0:20:55 | |
What I'm thinking is that the period when the atmosphere | 0:20:55 | 0:20:58 | |
was at its dirtiest. | 0:20:58 | 0:21:00 | |
And if you look at these hurricane seasons... | 0:21:03 | 0:21:05 | |
..it's pretty much the same period of time | 0:21:07 | 0:21:10 | |
as when there were less hurricanes. | 0:21:10 | 0:21:13 | |
-So it's possible that pollution is suppressing hurricanes. -Yeah. | 0:21:15 | 0:21:21 | |
It's an extraordinary idea, | 0:21:25 | 0:21:27 | |
that higher levels of pollution in the past might have been suppressing | 0:21:27 | 0:21:32 | |
hurricanes because polluted clouds were cooling the world's oceans. | 0:21:32 | 0:21:36 | |
But environmental legislation has improved air quality | 0:21:39 | 0:21:42 | |
across America. | 0:21:42 | 0:21:44 | |
So there are fewer of these dense, polluted clouds. | 0:21:48 | 0:21:53 | |
As a result, the seas have slowly warmed up again. | 0:21:55 | 0:21:59 | |
So, what we are saying is that by cleaning up | 0:22:03 | 0:22:07 | |
our atmosphere...we have allowed there to be more hurricanes. | 0:22:07 | 0:22:12 | |
So we are not seeing an upward trend in hurricanes. | 0:22:13 | 0:22:16 | |
What we have seen in past decades, when the air was dirty, | 0:22:16 | 0:22:19 | |
was a suppression in hurricanes. What we are seeing | 0:22:19 | 0:22:22 | |
at the moment is a return to the natural state of things, a return | 0:22:22 | 0:22:26 | |
to the normal number of hurricanes you would expect to find in a season. | 0:22:26 | 0:22:31 | |
The airship is approaching the desert city of Phoenix, Arizona, | 0:22:51 | 0:22:54 | |
where the team want to answer a question about human impact | 0:22:54 | 0:22:58 | |
upon the atmosphere. | 0:22:58 | 0:23:00 | |
Can cities make their own weather? | 0:23:00 | 0:23:02 | |
So I've been looking at historical data | 0:23:07 | 0:23:11 | |
and you can see that Phoenix in the last 100 years has gone | 0:23:11 | 0:23:14 | |
from being really a small, agricultural settlement | 0:23:14 | 0:23:18 | |
into a large, urban city. | 0:23:18 | 0:23:20 | |
In the same period of time, there has been a distinct change | 0:23:20 | 0:23:24 | |
in the amount of rainfall in the city. | 0:23:24 | 0:23:26 | |
There are areas of Phoenix that have had up to | 0:23:28 | 0:23:31 | |
a 12% increase in the amount of rainfall, | 0:23:31 | 0:23:33 | |
which is really significant, | 0:23:33 | 0:23:36 | |
and it looks like there might be a correlation between the two. | 0:23:36 | 0:23:39 | |
So we want to see | 0:23:39 | 0:23:41 | |
if we can unravel how the city might be creating its own weather. | 0:23:41 | 0:23:45 | |
The rain that falls here has followed the same | 0:23:47 | 0:23:50 | |
cycle for millennia. | 0:23:50 | 0:23:51 | |
Every summer, warm, moist air is swept up from the oceans to | 0:23:54 | 0:23:57 | |
the south. | 0:23:57 | 0:23:58 | |
As this air meets the hot desert, variations in the landscape | 0:24:00 | 0:24:05 | |
drive pockets of air upwards as thermals. | 0:24:05 | 0:24:07 | |
Where the moisture cools, condenses, | 0:24:10 | 0:24:15 | |
and ultimately falls in sudden downpours of rain. | 0:24:15 | 0:24:19 | |
This process should make rainfall across the region fairly random. | 0:24:25 | 0:24:29 | |
But something appears to be concentrating it upon the city. | 0:24:30 | 0:24:34 | |
To see why, Felicity is going to start by surveying | 0:24:40 | 0:24:43 | |
temperatures in Phoenix and the surrounding desert. | 0:24:43 | 0:24:46 | |
I took several readings of the surface temperature | 0:24:52 | 0:24:55 | |
and I was getting between 37 and 38 degrees centigrade. | 0:24:55 | 0:25:01 | |
So, it's pretty hot down there, it's soaking up all the heat from the sun. | 0:25:01 | 0:25:04 | |
For the city to be concentrating rainfall, | 0:25:09 | 0:25:12 | |
it needs to be hotter than the desert, | 0:25:12 | 0:25:14 | |
driving extra thermal activity. | 0:25:14 | 0:25:16 | |
I'm getting a real variety in surface temperatures. | 0:25:19 | 0:25:22 | |
So if I take a reading from the road or a car park, it's pretty | 0:25:22 | 0:25:27 | |
much the same surface temperature as in the desert, | 0:25:27 | 0:25:30 | |
but if I point the camera at a garden or a swimming pool or a roof top, | 0:25:30 | 0:25:37 | |
then it's a lot less. | 0:25:37 | 0:25:39 | |
So, on average, the surface temperature here will overall | 0:25:39 | 0:25:44 | |
be a lot less than the desert. | 0:25:44 | 0:25:46 | |
The city is a little cooler than the surrounding desert. | 0:25:48 | 0:25:51 | |
So there's no evidence for the increased | 0:25:51 | 0:25:53 | |
thermal activity that can explain the rainfall. | 0:25:53 | 0:25:56 | |
As the day wears on, that picture soon changes. | 0:26:05 | 0:26:09 | |
See, look, look, look, look, see the city. | 0:26:16 | 0:26:19 | |
-Yeah. -It's hotter than the desert. | 0:26:19 | 0:26:21 | |
OK, yeah, you can see definitely the boundary. | 0:26:21 | 0:26:24 | |
So that's the desert cooling down and that's the hot city. | 0:26:25 | 0:26:29 | |
That's a really nice example of it. | 0:26:29 | 0:26:31 | |
Whilst the natural landscape has quickly cooled, | 0:26:36 | 0:26:39 | |
the camera reveals the city to have remained warm. | 0:26:39 | 0:26:42 | |
They've identified an effect called the urban heat island. | 0:26:45 | 0:26:48 | |
Earlier today, we measured the ground temperature of the suburbs | 0:26:51 | 0:26:55 | |
to be 24, 25 degrees, | 0:26:55 | 0:26:56 | |
and see, I'm measuring, 23, 22 - | 0:26:58 | 0:27:02 | |
I mean it's still as hot as when we measured it in the middle of the day. | 0:27:02 | 0:27:05 | |
The city surfaces are continuing to radiate | 0:27:08 | 0:27:11 | |
the energy of the sun they absorbed earlier in the day. | 0:27:11 | 0:27:14 | |
The question is whether the urban heat island is generating thermals. | 0:27:16 | 0:27:20 | |
If it is, they should be able to detect | 0:27:22 | 0:27:25 | |
an increase in temperature at altitude from the airship. | 0:27:25 | 0:27:28 | |
So, I've just had a look at the temperatures | 0:27:31 | 0:27:33 | |
and this is the temperature going down and that's going down simply | 0:27:33 | 0:27:36 | |
because the sun's going down - you know, we're turning the heater off. | 0:27:36 | 0:27:39 | |
So this is the temperature over the desert | 0:27:39 | 0:27:42 | |
and this is the temperature over the city. | 0:27:42 | 0:27:45 | |
Oh, wow, so this is where we hit the city? | 0:27:45 | 0:27:47 | |
-Yeah. -So we've got this big parcel of warm air sitting | 0:27:47 | 0:27:50 | |
over the city. | 0:27:50 | 0:27:53 | |
It makes a lot of logical sense that that air is going to start rising and | 0:27:54 | 0:27:58 | |
that's going to start convection and the consequence of that is weather. | 0:27:58 | 0:28:02 | |
So the increased rainfall in Phoenix could be caused by | 0:28:06 | 0:28:10 | |
the urban heat island effect. | 0:28:10 | 0:28:12 | |
It generates thermals over the city, | 0:28:13 | 0:28:16 | |
that force air upward, where it begins to cool. | 0:28:16 | 0:28:19 | |
That in turn can cause the vapour to condense and form rain, | 0:28:20 | 0:28:24 | |
concentrated here upon Phoenix. | 0:28:24 | 0:28:27 | |
So we've found the connection we were looking for, | 0:28:30 | 0:28:32 | |
between cities, and the increased rainfall that Phoenix has been | 0:28:32 | 0:28:37 | |
experiencing in the last 100 years. | 0:28:37 | 0:28:39 | |
And the really exciting thing about that is that we've | 0:28:39 | 0:28:42 | |
hard evidence that human beings are creating their own weather. | 0:28:42 | 0:28:47 |