0:00:08 > 0:00:10One of the world's largest airships is taking
0:00:10 > 0:00:15a team of scientists and adventurers on a unique expedition
0:00:15 > 0:00:18a voyage deep into one of the most mysterious
0:00:18 > 0:00:22and precious environments on earth - the atmosphere.
0:00:24 > 0:00:28So we have this dynamic bubble of air constantly moving,
0:00:28 > 0:00:30constantly changing,
0:00:30 > 0:00:33and that's what we are here with Cloud Lab to explore.
0:00:34 > 0:00:37This quest is taking the team coast to coast
0:00:37 > 0:00:41across America to discover the many surprising ways
0:00:41 > 0:00:44in which the atmosphere shapes our world.
0:01:04 > 0:01:06It's difficult to imagine,
0:01:06 > 0:01:12but the skies are home to a vast ocean of water. Yet it is beyond
0:01:12 > 0:01:17our reach, suspended all around us as an invisible, vaporous gas.
0:01:20 > 0:01:24Only once it is transformed into clouds does it become liquid water.
0:01:26 > 0:01:30It's this deceptively simple transformation of water,
0:01:30 > 0:01:32from gas to liquid,
0:01:32 > 0:01:35that ultimately brings water from the sea to the Earth's land surfaces
0:01:35 > 0:01:41by generating 1.4 trillion tonnes of rainfall every day.
0:01:45 > 0:01:49Yet clouds are as mysterious as they are beautiful.
0:01:52 > 0:01:56How can such delicate, ephemeral structures carry so much water?
0:02:00 > 0:02:05To begin to understand exactly how much water clouds carry,
0:02:05 > 0:02:08meteorologist Felicity Aston wants to try something
0:02:08 > 0:02:10that's never been attempted before.
0:02:13 > 0:02:15So what would be really great,
0:02:15 > 0:02:17I don't know if it's going to be possible or not,
0:02:17 > 0:02:20but what would be really great is if we could weigh a cloud -
0:02:20 > 0:02:24see how heavy it is, and work out how much water is in one of those clouds.
0:02:25 > 0:02:28But to do that, we've got to get up there.
0:02:28 > 0:02:30So we've got to do a bit of cloud hunting.
0:02:34 > 0:02:37Energy from the sun evaporates water from the sea
0:02:37 > 0:02:38into the air above.
0:02:39 > 0:02:41When this moist air is warm enough,
0:02:41 > 0:02:45it starts to rise in a column of air known as a thermal.
0:02:47 > 0:02:50As it rises, it gets colder,
0:02:50 > 0:02:54and cold air can't hold as much water as warm air.
0:02:55 > 0:02:58So you get to a certain level, when it's cold enough,
0:02:58 > 0:03:02when all that water from the sea starts to re-materialise as tiny,
0:03:02 > 0:03:04little droplets of water.
0:03:04 > 0:03:06That is the birth of a cloud.
0:03:11 > 0:03:13OK - that's the one I want. That one.
0:03:14 > 0:03:16It'll be really great to go right through the middle
0:03:16 > 0:03:18and right into the heart of it.
0:03:20 > 0:03:24Atmospheric chemist Dr Jim McQuaid primes the instrumentation.
0:03:27 > 0:03:29So we have a... There's a laser beam here.
0:03:29 > 0:03:32So this is one instrument we've got, it is called a LiDAR.
0:03:34 > 0:03:39The LiDAR, a kind of light radar, will measure the cloud's dimensions
0:03:39 > 0:03:43by emitting a laser and analysing the light reflected back.
0:03:45 > 0:03:49A second probe will measure the exact size
0:03:49 > 0:03:51and density of the individual droplets of liquid
0:03:51 > 0:03:54as the airship passes through the cloud.
0:03:55 > 0:03:56OK, Jim, are you ready?
0:03:56 > 0:03:58OK.
0:04:04 > 0:04:06So I'm picking up cloud droplets now.
0:04:06 > 0:04:08The humidity has gone up to 100%.
0:04:18 > 0:04:21Wow, so that cloud was nearly a kilometre long.
0:04:22 > 0:04:26So, Jim, have you got an idea of how wide the cloud was?
0:04:27 > 0:04:29200 metres across.
0:04:29 > 0:04:32So we are going to assume it was as tall as it was wide,
0:04:32 > 0:04:35because it looked like a fairly solid elliptical shape,
0:04:35 > 0:04:39so we just use a simple formula to work out the volume of the cloud.
0:04:39 > 0:04:42How wide was it - 200 metres?
0:04:42 > 0:04:4320 million...
0:04:43 > 0:04:45- 20 million?- Cubic metres.
0:04:45 > 0:04:50In that small, compact cloud. 20 million cubic metres.
0:04:53 > 0:04:55To calculate the cloud's weight,
0:04:55 > 0:04:59they factor in the size and density of the water droplets within it.
0:05:06 > 0:05:10The weight per cubic metre is about...say the average is 0.2.
0:05:10 > 0:05:13- 0.2g per cubic metre. - Per cubic metre.
0:05:15 > 0:05:18OK. So we times 0.2 by 20 million.
0:05:18 > 0:05:20Yes...
0:05:23 > 0:05:27So that small cloud weighed four tonnes. That's incredible.
0:05:27 > 0:05:29It is.
0:05:32 > 0:05:34The experiment has revealed that even a small
0:05:34 > 0:05:38cumulus cloud converts large amounts of vapour to liquid water.
0:05:41 > 0:05:44The average cumulus is 50 times larger than the one the team
0:05:44 > 0:05:49have measured, so it carries around 200 tonnes of water.
0:05:52 > 0:05:56But the greatest water bearers are cumulonimbus clouds.
0:05:57 > 0:06:02Up to ten times more dense than cumulus cloud, and measuring,
0:06:02 > 0:06:06on average, 1,000 times larger, these can weigh one million tonnes.
0:06:08 > 0:06:12At any one point in time, the world's clouds hold
0:06:12 > 0:06:16an astonishing 129 billion tonnes of water in the sky.
0:06:33 > 0:06:36The team want to investigate how the rain cycle works.
0:06:37 > 0:06:40It all comes down to the little understood process
0:06:40 > 0:06:43that causes raindrops to form.
0:06:43 > 0:06:47Rain doesn't form easily, which people in the UK
0:06:47 > 0:06:48and, frankly, people in Florida,
0:06:48 > 0:06:51are going to think is a bit odd, because it rains a lot,
0:06:51 > 0:06:56but you need a little catalyst - a nucleus - to help raindrops form.
0:06:56 > 0:07:00It's a bit like a grain of sand at the heart of a pearl.
0:07:02 > 0:07:05Normally, tiny particles like dust or sea-salt
0:07:05 > 0:07:07suspended in clouds do the job.
0:07:09 > 0:07:13But a new idea has emerged suggesting that rain drops could be
0:07:13 > 0:07:15seeded by another kind of particle.
0:07:16 > 0:07:19Life, in the form of bacteria.
0:07:19 > 0:07:22Micro-biologist Dr Chris Van Tulleken
0:07:22 > 0:07:24wants to know whether that's the case.
0:07:26 > 0:07:30Often, the first thing to form around particles are ice crystals,
0:07:30 > 0:07:32because high up inside clouds
0:07:32 > 0:07:34temperatures can be well below freezing.
0:07:35 > 0:07:40Those crystals of ice act like a magnet, attracting water vapour
0:07:40 > 0:07:42and growing rapidly.
0:07:44 > 0:07:47When they are big enough and heavy enough they fall -
0:07:47 > 0:07:50and as they fall, they melt to become rain.
0:07:54 > 0:07:58So Chris is mounting an experiment to find out which is
0:07:58 > 0:08:00the best at producing ice.
0:08:00 > 0:08:04Is it dust or bacteria?
0:08:07 > 0:08:10So we've got three rows of drops here.
0:08:10 > 0:08:12We've got the first row, near me,
0:08:12 > 0:08:17that's pure water, and then the second row has mineral dust in it
0:08:17 > 0:08:22and the third row has a bacteria that we know does live in clouds.
0:08:22 > 0:08:25And we are just going to drop the temperature on this plate
0:08:25 > 0:08:27and see which freezes more easily.
0:08:29 > 0:08:31So we are below... We are below freezing.
0:08:31 > 0:08:35It's funny, isn't it? So we talk about freezing as zero,
0:08:35 > 0:08:39but it's actually really hard to get water to freeze.
0:08:41 > 0:08:45In fact, pure water doesn't freeze until well below zero.
0:08:46 > 0:08:49There needs to be impurities in the water for it to
0:08:49 > 0:08:51freeze at higher temperatures.
0:08:51 > 0:08:56It's minus eight, almost minus eight and a half.
0:08:58 > 0:09:00Nothing's frozen yet.
0:09:02 > 0:09:04There you go. There, there.
0:09:05 > 0:09:07But that was only the bacterial ones.
0:09:07 > 0:09:10None of the mineral ones have frozen.
0:09:10 > 0:09:13Only when it is two degrees colder does the mineral dust
0:09:13 > 0:09:15finally start to freeze.
0:09:16 > 0:09:20Ah, those... At almost minus 11,
0:09:20 > 0:09:22some of the mineral ones are going.
0:09:23 > 0:09:26Not only has the experiment demonstrated that ice
0:09:26 > 0:09:28forms around bacteria,
0:09:28 > 0:09:32but that it does so at a higher temperature than around dust.
0:09:41 > 0:09:45So, the bacterial protein is more efficient than the mineral,
0:09:45 > 0:09:49the main mineral that we think causes rain,
0:09:49 > 0:09:53and, to me, the key thing is here - bacteria have evolved a protein -
0:09:53 > 0:09:58they've made something that helps water freeze, that helps ice form.
0:10:00 > 0:10:03The experiment raises the intriguing possibility that
0:10:03 > 0:10:06bacteria will make clouds rain more readily.
0:10:07 > 0:10:11So, knowing whether a cloud is a home to bacteria or not could help
0:10:11 > 0:10:14forecasters predict if it's going to rain.
0:10:37 > 0:10:40This is Gulf Shores, Alabama.
0:10:40 > 0:10:44It's an important staging post for a number of different migratory
0:10:44 > 0:10:48bird species, all of which are trying to escape the approaching
0:10:48 > 0:10:49American winter.
0:10:53 > 0:10:55They are resting up here before the most perilous
0:10:55 > 0:10:58part of their journey to South and Central America.
0:10:58 > 0:11:02The 600-mile flight across the Gulf of Mexico.
0:11:08 > 0:11:12Cloud Lab's Andy Torbet is joining a group of scientists
0:11:12 > 0:11:16tracking the migration patterns of the birds that depart from here.
0:11:19 > 0:11:24They're on a dawn raid to catch and then tag some.
0:11:29 > 0:11:31- And you'll check them every...? - 30 minutes.- OK.
0:11:34 > 0:11:37The question they're trying to answer is do the birds time
0:11:37 > 0:11:39their departures to take advantage
0:11:39 > 0:11:41of favourable atmospheric conditions?
0:11:43 > 0:11:46Now is an ideal time to test the idea.
0:11:49 > 0:11:54A cold front, a mass of cooler air, has just swept through the region,
0:11:54 > 0:11:55bringing with it torrential rain.
0:11:59 > 0:12:01But now conditions have improved.
0:12:02 > 0:12:04I really wanted to see a humming bird.
0:12:04 > 0:12:06- They just seem so delicate. - Yeah, they are very delicate.
0:12:06 > 0:12:09That's why we put them in the bags instead of the boxes,
0:12:09 > 0:12:11just for purely that reason.
0:12:11 > 0:12:14The birds are caught between two conflicting pressures.
0:12:14 > 0:12:16On the one hand, winter is coming,
0:12:16 > 0:12:19and they have to move before food becomes scarce.
0:12:21 > 0:12:24On the other, if they get their timing wrong,
0:12:24 > 0:12:27they may find themselves fighting head winds.
0:12:30 > 0:12:34So he's making a spot there to attach the transmitter.
0:12:34 > 0:12:36The birds, in this case, a hummingbird,
0:12:36 > 0:12:39are fitted with radio transmitters to track their departure.
0:12:41 > 0:12:43How much does that weigh?
0:12:43 > 0:12:46It weighs about 4% of the bird's body mass.
0:12:47 > 0:12:49It may look invasive,
0:12:49 > 0:12:51but the procedures have been honed over many years.
0:12:51 > 0:12:55So where will you be picking up the data from that transmitter?
0:12:55 > 0:12:58From the towers that we have here on the peninsula that
0:12:58 > 0:13:02will pick up the signal when the bird departs across the Gulf of Mexico.
0:13:04 > 0:13:06- Do you want to let him go? - Oh, yes, please.
0:13:06 > 0:13:09- OK, how do I hold him? - Open your hand.
0:13:11 > 0:13:14OK - then hold the wings?
0:13:15 > 0:13:17And if you just let your hands go
0:13:17 > 0:13:20he can just fly off or maybe with a little encouragement.
0:13:22 > 0:13:23- OK.- Good luck, little one.
0:13:23 > 0:13:25There he goes. Wow! Pretty impressive.
0:13:25 > 0:13:27That was brilliant.
0:13:31 > 0:13:35Now all they can do is wait and see if the birds use the better weather
0:13:35 > 0:13:36to make the crossing that evening.
0:13:45 > 0:13:47We put radio tags on some birds to see
0:13:47 > 0:13:50whether they actually made it across the gulf,
0:13:50 > 0:13:53and three of the birds that we tagged made the journey.
0:13:53 > 0:13:57And it took them between 16 and 24 hours.
0:13:57 > 0:14:01- Wow!- But it just showed that they were able to make that journey.
0:14:04 > 0:14:09The tagged hummingbirds and thrushes departed that same evening
0:14:09 > 0:14:11and reached their destination.
0:14:16 > 0:14:19The passage of the cold front led to an improvement in the weather,
0:14:19 > 0:14:23and delivered a tail-wind that the birds seem to have exploited.
0:14:26 > 0:14:28And the data Felicity has gathered suggests
0:14:28 > 0:14:30they are not the only birds
0:14:30 > 0:14:32that take advantage of a change in the wind.
0:14:34 > 0:14:39This is national radar data, so any of the green,
0:14:39 > 0:14:43red and yellow signals you can see - that's bad weather that was
0:14:43 > 0:14:46sitting right on top of you and pinning all those birds down.
0:14:46 > 0:14:49But then as that front moves across,
0:14:49 > 0:14:54there's a sudden explosion of these sort of rosette blue colours.
0:14:54 > 0:14:56And nobody knew what they were at first,
0:14:56 > 0:15:00but now they know that it's biological matter showing up
0:15:00 > 0:15:04on the radar - so that is the birds leaving - it shows up on the radar.
0:15:07 > 0:15:09And if I just let this play, you can
0:15:09 > 0:15:13see that over the whole country, as fronts move across,
0:15:13 > 0:15:17behind the fronts you'll see this sudden explosion of birds leaving.
0:15:22 > 0:15:25After the passage of a front,
0:15:25 > 0:15:28many millions of birds take to the skies in an attempt to reduce
0:15:28 > 0:15:31the energy required to make their migration.
0:15:34 > 0:15:37It really just shows how important these weather fronts
0:15:37 > 0:15:39are for the birds.
0:15:39 > 0:15:43They have to fly in air that's following these cold fronts along.
0:15:45 > 0:15:50And just seeing it on this level shows that these weather fronts
0:15:50 > 0:15:53you know, they are vital for movement.
0:15:53 > 0:15:56Not just on a small scale but on a global scale.
0:16:16 > 0:16:19The Cloud Lab team want to explore a surprising
0:16:19 > 0:16:21consequence of human impact upon the atmosphere.
0:16:23 > 0:16:25The apparent increase in the frequency
0:16:25 > 0:16:27and intensity of hurricanes.
0:16:31 > 0:16:33They've arrived at New Orleans.
0:16:33 > 0:16:37In 2005, this was the scene of the deadliest hurricane to hit
0:16:37 > 0:16:40the United States in more than half a century.
0:16:41 > 0:16:43Hurricanes have battered these shores
0:16:43 > 0:16:46since long before there were human settlements.
0:16:46 > 0:16:50It's a consequence of the particular geography in this area.
0:16:51 > 0:16:53THUNDER ROLLS
0:16:53 > 0:16:57As water is evaporated into the sky to form clouds,
0:16:57 > 0:17:00it brings with it vast amounts of heat energy.
0:17:02 > 0:17:06In the warm, shallow waters of the Gulf, that process takes place
0:17:06 > 0:17:10with such intensity that it can help to generate a hurricane.
0:17:20 > 0:17:23We already know that the sea surface temperatures,
0:17:23 > 0:17:27drive the hurricanes, they're the hurricane fuel.
0:17:27 > 0:17:30And so if we look at a graph of sea surface temperatures,
0:17:30 > 0:17:34we can see that there's a very obvious, upward trend,
0:17:34 > 0:17:40so temperatures are getting warmer and warmer, decade after decade.
0:17:40 > 0:17:44And that's what's driving not only more hurricanes but worse hurricanes.
0:17:44 > 0:17:49So what I'd like to know now is what's driving that upward
0:17:49 > 0:17:50trend in temperature.
0:17:52 > 0:17:54There's a newly emerging idea that the temperature
0:17:54 > 0:17:58of the Gulf may be influenced by pollutants in the atmosphere.
0:17:59 > 0:18:03So we've come to an area that has a lot of heavy industry
0:18:03 > 0:18:06and also one of the busiest shipping lanes in the US,
0:18:06 > 0:18:10because here we are likely to see what impact that's
0:18:10 > 0:18:13having on the clouds that are forming in this area.
0:18:15 > 0:18:18Clouds have an important effect on sea temperatures
0:18:18 > 0:18:22because of the way they block out the sun's heat.
0:18:22 > 0:18:25But the extent to which they block the sun depends upon what
0:18:25 > 0:18:29they're made from, because polluted clouds have different
0:18:29 > 0:18:31properties compared to clean clouds.
0:18:32 > 0:18:34What we'd now like to do is to try
0:18:34 > 0:18:37and get into some of these clouds over here.
0:18:37 > 0:18:39- We're looking for a dirty cloud. - Dirty, yep.
0:18:39 > 0:18:41Well, not dirty, but something that's either over
0:18:41 > 0:18:45- this shipping channel or over the oil refineries.- OK.
0:18:50 > 0:18:53Jim detects methane and carbon dioxide -
0:18:53 > 0:18:55important markers for other pollutants.
0:19:01 > 0:19:02The high levels of pollution
0:19:02 > 0:19:06mean there are more particles on which the cloud droplets can form.
0:19:09 > 0:19:11And that has an important knock-on effect.
0:19:19 > 0:19:21This is the size distribution and the average
0:19:21 > 0:19:25is about six microns, and that's quite small,
0:19:25 > 0:19:27whereas in the cleaner clouds
0:19:27 > 0:19:31which we've flown through in Florida, the average size is more like ten.
0:19:31 > 0:19:35Right. So we are seeing more small droplets
0:19:35 > 0:19:39- than you would in a clean cloud.- Yes.
0:19:39 > 0:19:44In dirty clouds you have more and smaller particles,
0:19:44 > 0:19:48so they are going to be denser clouds, there's more droplets.
0:19:51 > 0:19:54The consequences of this are far-reaching.
0:19:54 > 0:19:59The clouds here are dirty clouds and because they are
0:19:59 > 0:20:04thicker and denser, they are blocking out more sunlight than clean clouds.
0:20:04 > 0:20:07So they are having a net cooling effect
0:20:07 > 0:20:09on the climate underneath them.
0:20:11 > 0:20:18- So dirty clouds are cooling down temperatures.- Yes.- Right.
0:20:22 > 0:20:25It seems that polluted clouds cool the world's oceans.
0:20:29 > 0:20:32And yet sea surface temperatures are on the rise.
0:20:37 > 0:20:38Fuelling hurricanes.
0:20:44 > 0:20:47Felicity calls upon the one piece of data that can make
0:20:47 > 0:20:50sense of this confusing picture.
0:20:50 > 0:20:52The way in which pollution levels
0:20:52 > 0:20:55have changed over the past few decades.
0:20:55 > 0:20:58What I'm thinking is that the period when the atmosphere
0:20:58 > 0:21:00was at its dirtiest.
0:21:03 > 0:21:05And if you look at these hurricane seasons...
0:21:07 > 0:21:10..it's pretty much the same period of time
0:21:10 > 0:21:13as when there were less hurricanes.
0:21:15 > 0:21:21- So it's possible that pollution is suppressing hurricanes.- Yeah.
0:21:25 > 0:21:27It's an extraordinary idea,
0:21:27 > 0:21:32that higher levels of pollution in the past might have been suppressing
0:21:32 > 0:21:36hurricanes because polluted clouds were cooling the world's oceans.
0:21:39 > 0:21:42But environmental legislation has improved air quality
0:21:42 > 0:21:44across America.
0:21:48 > 0:21:53So there are fewer of these dense, polluted clouds.
0:21:55 > 0:21:59As a result, the seas have slowly warmed up again.
0:22:03 > 0:22:07So, what we are saying is that by cleaning up
0:22:07 > 0:22:12our atmosphere...we have allowed there to be more hurricanes.
0:22:13 > 0:22:16So we are not seeing an upward trend in hurricanes.
0:22:16 > 0:22:19What we have seen in past decades, when the air was dirty,
0:22:19 > 0:22:22was a suppression in hurricanes. What we are seeing
0:22:22 > 0:22:26at the moment is a return to the natural state of things, a return
0:22:26 > 0:22:31to the normal number of hurricanes you would expect to find in a season.
0:22:51 > 0:22:54The airship is approaching the desert city of Phoenix, Arizona,
0:22:54 > 0:22:58where the team want to answer a question about human impact
0:22:58 > 0:23:00upon the atmosphere.
0:23:00 > 0:23:02Can cities make their own weather?
0:23:07 > 0:23:11So I've been looking at historical data
0:23:11 > 0:23:14and you can see that Phoenix in the last 100 years has gone
0:23:14 > 0:23:18from being really a small, agricultural settlement
0:23:18 > 0:23:20into a large, urban city.
0:23:20 > 0:23:24In the same period of time, there has been a distinct change
0:23:24 > 0:23:26in the amount of rainfall in the city.
0:23:28 > 0:23:31There are areas of Phoenix that have had up to
0:23:31 > 0:23:33a 12% increase in the amount of rainfall,
0:23:33 > 0:23:36which is really significant,
0:23:36 > 0:23:39and it looks like there might be a correlation between the two.
0:23:39 > 0:23:41So we want to see
0:23:41 > 0:23:45if we can unravel how the city might be creating its own weather.
0:23:47 > 0:23:50The rain that falls here has followed the same
0:23:50 > 0:23:51cycle for millennia.
0:23:54 > 0:23:57Every summer, warm, moist air is swept up from the oceans to
0:23:57 > 0:23:58the south.
0:24:00 > 0:24:05As this air meets the hot desert, variations in the landscape
0:24:05 > 0:24:07drive pockets of air upwards as thermals.
0:24:10 > 0:24:15Where the moisture cools, condenses,
0:24:15 > 0:24:19and ultimately falls in sudden downpours of rain.
0:24:25 > 0:24:29This process should make rainfall across the region fairly random.
0:24:30 > 0:24:34But something appears to be concentrating it upon the city.
0:24:40 > 0:24:43To see why, Felicity is going to start by surveying
0:24:43 > 0:24:46temperatures in Phoenix and the surrounding desert.
0:24:52 > 0:24:55I took several readings of the surface temperature
0:24:55 > 0:25:01and I was getting between 37 and 38 degrees centigrade.
0:25:01 > 0:25:04So, it's pretty hot down there, it's soaking up all the heat from the sun.
0:25:09 > 0:25:12For the city to be concentrating rainfall,
0:25:12 > 0:25:14it needs to be hotter than the desert,
0:25:14 > 0:25:16driving extra thermal activity.
0:25:19 > 0:25:22I'm getting a real variety in surface temperatures.
0:25:22 > 0:25:27So if I take a reading from the road or a car park, it's pretty
0:25:27 > 0:25:30much the same surface temperature as in the desert,
0:25:30 > 0:25:37but if I point the camera at a garden or a swimming pool or a roof top,
0:25:37 > 0:25:39then it's a lot less.
0:25:39 > 0:25:44So, on average, the surface temperature here will overall
0:25:44 > 0:25:46be a lot less than the desert.
0:25:48 > 0:25:51The city is a little cooler than the surrounding desert.
0:25:51 > 0:25:53So there's no evidence for the increased
0:25:53 > 0:25:56thermal activity that can explain the rainfall.
0:26:05 > 0:26:09As the day wears on, that picture soon changes.
0:26:16 > 0:26:19See, look, look, look, look, see the city.
0:26:19 > 0:26:21- Yeah.- It's hotter than the desert.
0:26:21 > 0:26:24OK, yeah, you can see definitely the boundary.
0:26:25 > 0:26:29So that's the desert cooling down and that's the hot city.
0:26:29 > 0:26:31That's a really nice example of it.
0:26:36 > 0:26:39Whilst the natural landscape has quickly cooled,
0:26:39 > 0:26:42the camera reveals the city to have remained warm.
0:26:45 > 0:26:48They've identified an effect called the urban heat island.
0:26:51 > 0:26:55Earlier today, we measured the ground temperature of the suburbs
0:26:55 > 0:26:56to be 24, 25 degrees,
0:26:58 > 0:27:02and see, I'm measuring, 23, 22 -
0:27:02 > 0:27:05I mean it's still as hot as when we measured it in the middle of the day.
0:27:08 > 0:27:11The city surfaces are continuing to radiate
0:27:11 > 0:27:14the energy of the sun they absorbed earlier in the day.
0:27:16 > 0:27:20The question is whether the urban heat island is generating thermals.
0:27:22 > 0:27:25If it is, they should be able to detect
0:27:25 > 0:27:28an increase in temperature at altitude from the airship.
0:27:31 > 0:27:33So, I've just had a look at the temperatures
0:27:33 > 0:27:36and this is the temperature going down and that's going down simply
0:27:36 > 0:27:39because the sun's going down - you know, we're turning the heater off.
0:27:39 > 0:27:42So this is the temperature over the desert
0:27:42 > 0:27:45and this is the temperature over the city.
0:27:45 > 0:27:47Oh, wow, so this is where we hit the city?
0:27:47 > 0:27:50- Yeah.- So we've got this big parcel of warm air sitting
0:27:50 > 0:27:53over the city.
0:27:54 > 0:27:58It makes a lot of logical sense that that air is going to start rising and
0:27:58 > 0:28:02that's going to start convection and the consequence of that is weather.
0:28:06 > 0:28:10So the increased rainfall in Phoenix could be caused by
0:28:10 > 0:28:12the urban heat island effect.
0:28:13 > 0:28:16It generates thermals over the city,
0:28:16 > 0:28:19that force air upward, where it begins to cool.
0:28:20 > 0:28:24That in turn can cause the vapour to condense and form rain,
0:28:24 > 0:28:27concentrated here upon Phoenix.
0:28:30 > 0:28:32So we've found the connection we were looking for,
0:28:32 > 0:28:37between cities, and the increased rainfall that Phoenix has been
0:28:37 > 0:28:39experiencing in the last 100 years.
0:28:39 > 0:28:42And the really exciting thing about that is that we've
0:28:42 > 0:28:47hard evidence that human beings are creating their own weather.