0:00:04 > 0:00:06Weather.
0:00:06 > 0:00:11One of the most astonishing forces on earth.
0:00:11 > 0:00:14Capable of both devastating power
0:00:14 > 0:00:15and spectacular beauty.
0:00:17 > 0:00:20Wherever you live on the planet,
0:00:20 > 0:00:23weather shapes your world.
0:00:23 > 0:00:26Yet for most of us, how it works is a mystery.
0:00:29 > 0:00:33To really understand weather, you have to get inside it.
0:00:37 > 0:00:39So, I'm going to strip weather back to basics.
0:00:41 > 0:00:42All in the name of science.
0:00:42 > 0:00:46'Uncovering its secrets in a series of brave,
0:00:46 > 0:00:53'ambitious, and sometimes just plain unlikely experiments...
0:00:53 > 0:00:56Well, it certainly feels like a dust storm from here.
0:00:56 > 0:01:00'..to show you weather like you've never seen it before.'
0:01:08 > 0:01:13All weather, no matter how rare or how unusual
0:01:13 > 0:01:18can be broken down to three simple ingredients.
0:01:18 > 0:01:20Wind...
0:01:20 > 0:01:22water...
0:01:22 > 0:01:23and temperature.
0:01:24 > 0:01:27With just those three things
0:01:27 > 0:01:31you can create pretty much any weather you want.
0:01:33 > 0:01:37But the most important of the three is temperature.
0:01:39 > 0:01:44'In this programme, I'll discover how without it,
0:01:44 > 0:01:47'we wouldn't have any weather at all.'
0:01:47 > 0:01:48Oh, yeah!
0:01:52 > 0:01:54Oh, that's a lot of dust now.
0:01:54 > 0:01:59How a dust storm in Africa can make rain right here.
0:02:01 > 0:02:06And how heat can produce snow as hard as concrete.
0:02:24 > 0:02:29Almost all our planet's heat is provided by the sun.
0:02:31 > 0:02:36And we tend to think of the sun as the source of all our best weather.
0:02:41 > 0:02:44But if you're looking to unlock the secrets of the weather,
0:02:44 > 0:02:46the heat coming from up there
0:02:46 > 0:02:51is not as important as when it's coming from down there,
0:02:51 > 0:02:52from the ground.
0:02:55 > 0:02:58I know it sounds unlikely but it's all to do with
0:02:58 > 0:03:02the fact that the sun heats the earth unevenly.
0:03:05 > 0:03:07Sand gets hotter than water.
0:03:07 > 0:03:10Tarmac gets hotter than sand.
0:03:12 > 0:03:14Concrete gets hotter than grass.
0:03:16 > 0:03:20And these differences produce pockets of warm rising air
0:03:20 > 0:03:22called thermals
0:03:22 > 0:03:25which drive winds and create clouds.
0:03:27 > 0:03:31But how can you see that effect for yourself?
0:03:32 > 0:03:35Well, with a quarry,
0:03:35 > 0:03:38five vehicles worth of kit,
0:03:38 > 0:03:41and two specially built metal tables.
0:03:43 > 0:03:46These tables are going to be our hot ground.
0:03:48 > 0:03:50Because they're dark in colour,
0:03:50 > 0:03:53they should soak up lots of heat from the hot sun.
0:03:55 > 0:03:58And to make sure they get hot enough
0:03:58 > 0:04:00we're going to give the sun a little help.
0:04:02 > 0:04:03With 14 gas canisters.
0:04:05 > 0:04:08All connected up to 18 high-power burners.
0:04:11 > 0:04:15We reckon with these we can get our table up to 200C.
0:04:17 > 0:04:18Maybe even higher.
0:04:20 > 0:04:23And I'm hoping that's enough to show you
0:04:23 > 0:04:25what hot land does to our weather.
0:04:26 > 0:04:28To be perfectly clear,
0:04:28 > 0:04:31my ambition here is not to actually make weather with this.
0:04:31 > 0:04:34I'm not hoping for a little square cloud overhead.
0:04:34 > 0:04:38The theory is right, it's just the scale is a bit small.
0:04:38 > 0:04:41What I will be doing is creating that rising column of air,
0:04:41 > 0:04:44that thermal which is part of the weather and it is something
0:04:44 > 0:04:47I will be able to show you, once I've got it established.
0:04:53 > 0:04:56So, let's do just that.
0:04:56 > 0:04:59Turn on the gas,
0:04:59 > 0:05:00fire up the burners,
0:05:00 > 0:05:05and get those metal tables as hot as we feasibly can,
0:05:05 > 0:05:09enabling me to fly some paper helicopters.
0:05:12 > 0:05:14'Yeah, I know. But bear with me.
0:05:14 > 0:05:16'There is method in this.'
0:05:16 > 0:05:17Can I have my box of...
0:05:19 > 0:05:20Thank you.
0:05:23 > 0:05:25Right, going up.
0:05:30 > 0:05:33Time for my hi tech thermal indicators.
0:05:36 > 0:05:41Now, normally a paper helicopter would just spin slowly to the floor.
0:05:45 > 0:05:48But it doesn't.
0:05:48 > 0:05:49It hovers.
0:06:15 > 0:06:18You can see how we've created a thermal down there
0:06:18 > 0:06:20and the helicopters that catch it
0:06:20 > 0:06:22are flying in that column of rising air.
0:06:44 > 0:06:48The updraft is enough to hold the helicopters in place.
0:06:50 > 0:06:55Just as they do naturally with clouds, rain drops and hail stones.
0:07:04 > 0:07:06Oh, yeah!
0:07:06 > 0:07:09But as the heat coming off the metal table increases
0:07:09 > 0:07:11the helicopters begin to climb.
0:07:16 > 0:07:18Until they're disappearing out of sight.
0:07:21 > 0:07:23Which is how it should be.
0:07:26 > 0:07:30A real thermal can reach 1,500 metres.
0:07:32 > 0:07:35And they have an important role to play in our weather.
0:07:37 > 0:07:40We've all seen how puddles dry up on a hot day.
0:07:41 > 0:07:43But where does that water go?
0:07:45 > 0:07:48Well, those thermals take it up into the air
0:07:48 > 0:07:51until it gets high enough, and cold enough,
0:07:51 > 0:07:55that it condenses back into drops and forms a cloud.
0:07:58 > 0:08:01But there's another result of this uneven heating of the earth.
0:08:03 > 0:08:04It produces deserts.
0:08:06 > 0:08:11And deserts play a very important role in what happens next
0:08:11 > 0:08:12to those clouds.
0:08:20 > 0:08:22It's hard to imagine, I know,
0:08:22 > 0:08:25but right now, I am surrounded by desert.
0:08:29 > 0:08:31And not just any desert either,
0:08:31 > 0:08:34probably the most famous desert of them all.
0:08:34 > 0:08:35The Sahara.
0:08:40 > 0:08:46Because Saharan sand regularly makes it all the way to the UK.
0:08:48 > 0:08:55Where it leaves a fine dusty layer on cars, benches, windows.
0:08:55 > 0:08:58Pretty much everything, in fact.
0:08:58 > 0:09:02Which is more than a little surprising because it comes
0:09:02 > 0:09:06all the way from Africa, more than 2,000 miles away.
0:09:07 > 0:09:09So how on Earth did it get here?
0:09:12 > 0:09:16Well, first you need a particularly sun-parched part of the planet.
0:09:18 > 0:09:21And then you need a dust storm.
0:09:25 > 0:09:29Dust storms are the way nature gets dust off the ground
0:09:29 > 0:09:31and into the air.
0:09:33 > 0:09:39A big one can easily be a mile high and 100 miles wide.
0:09:41 > 0:09:43A vast moving wall of dirt.
0:09:49 > 0:09:54But even the big ones only travel between 25 and 50 miles
0:09:54 > 0:09:55before they die out.
0:09:56 > 0:10:00So how does the dust end up 2,000 miles away
0:10:00 > 0:10:03on the bonnet of a car in Bristol?
0:10:03 > 0:10:06Well, believe it or not, it bounces there.
0:10:07 > 0:10:10Let me try and show you what's going on.
0:10:10 > 0:10:13Imagine this tennis ball were a grain of sand.
0:10:13 > 0:10:16Drop it from waist height and it bounces up about two feet.
0:10:16 > 0:10:21Now imagine this ping pong ball were a smaller particle of dust.
0:10:21 > 0:10:26Drop it from the same height and it bounces up about the same distance.
0:10:26 > 0:10:29But if I drop them both together, watch what happens then.
0:10:30 > 0:10:34Yeah, the ping pong ball flies off.
0:10:34 > 0:10:36What's happening, actually, is the ping pong ball is smaller
0:10:36 > 0:10:39and lighter and all the kinetic energy, the bounce in this ball
0:10:39 > 0:10:42is being transferred into it and away it goes.
0:10:42 > 0:10:48Obviously, real dust comes in many more than just two different sizes
0:10:48 > 0:10:50which is why this is maybe a better analogy.
0:10:50 > 0:10:53Four different sizes of ball this time,
0:10:53 > 0:10:57stacked loosely on this plastic spike in the centre.
0:10:57 > 0:11:01Obviously, real dust doesn't have a plastic spike connecting it
0:11:01 > 0:11:04but your alternative is that you watch for ten hours whilst
0:11:04 > 0:11:07I try and drop all four in a line.
0:11:07 > 0:11:09Let's see what happens this time.
0:11:25 > 0:11:28It's gone, the small ball. I mean it's just...
0:11:28 > 0:11:30I'd show you again
0:11:30 > 0:11:34but I'd have to wait for it to re-enter the atmosphere, I think.
0:11:35 > 0:11:36Seriously, it's gone.
0:11:40 > 0:11:42So, that's the principle.
0:11:43 > 0:11:46But can actual dust really do the same thing?
0:11:48 > 0:11:51Even with the power of a huge dust storm behind it?
0:11:56 > 0:12:01To find out, I'm going to the source of most of the world's dust.
0:12:02 > 0:12:05Not the Sahara but South Australia...
0:12:08 > 0:12:10..where Dr Craig Strong has offered
0:12:10 > 0:12:13to help me start a dust storm of my own.
0:12:14 > 0:12:16What are you actually looking for?
0:12:16 > 0:12:18Well, I'm looking, Richard,
0:12:18 > 0:12:22for the landscape that's going to produce dust.
0:12:22 > 0:12:25And I think this stony plain is probably really good.
0:12:25 > 0:12:26Because you can see these rocks?
0:12:26 > 0:12:29They're acting as a trap for dust.
0:12:29 > 0:12:32So, I think if we dig down, we'll find that there's plenty of dust,
0:12:32 > 0:12:34it's just it means it hasn't blown away yet
0:12:34 > 0:12:36because the rocks are locking all that dust in.
0:12:36 > 0:12:39So, when I see an area of rocks like this out here,
0:12:39 > 0:12:42I assume all the dust has gone.
0:12:42 > 0:12:44You think it's trapped underneath?
0:12:44 > 0:12:45Absolutely.
0:12:49 > 0:12:51So, if we have a look down here, Richard,
0:12:51 > 0:12:56once we get under there, it's just dust gold.
0:12:56 > 0:12:58- I mean, look at this! - It's incredibly fine!
0:12:58 > 0:13:00You can see that just blowing away,
0:13:00 > 0:13:02so there's lots and lots of fine material.
0:13:02 > 0:13:04This is exactly what we want.
0:13:04 > 0:13:06That's what dust storms are really made of.
0:13:06 > 0:13:09The problem here is that the dust is trapped under these stones.
0:13:09 > 0:13:12That's why you know it's here, but it is trapped.
0:13:12 > 0:13:13Absolutely.
0:13:13 > 0:13:15How do we get it out?
0:13:15 > 0:13:17The rocks are doing the job of protecting the soil,
0:13:17 > 0:13:19so I reckon we probably should pick up the rocks
0:13:19 > 0:13:21and move them out of the way.
0:13:21 > 0:13:22That's the first step.
0:13:22 > 0:13:23Churn it up a bit?
0:13:23 > 0:13:24Churn it up a bit, that's right.
0:13:24 > 0:13:28That's easy. I can do that for you. I'm going to do it now.
0:13:31 > 0:13:37Well, I say me, but actually I mean this chap, Trevor,
0:13:37 > 0:13:40who just happens to have the very tool
0:13:40 > 0:13:42for pushing aside all those stones.
0:13:51 > 0:13:54It's not long before he's cleared an oblong area
0:13:54 > 0:13:59the size of a couple of football pitches.
0:14:00 > 0:14:03And it has an immediate effect.
0:14:05 > 0:14:07Look at that! Dust devil! That's amazing!
0:14:10 > 0:14:13Probably those swirling winds come through here all the time,
0:14:13 > 0:14:16but because we've taken the stones away and uncovered the dust
0:14:16 > 0:14:20for it to be picked up we can suddenly see them. It's beautiful.
0:14:21 > 0:14:24But it's not what we're looking for.
0:14:25 > 0:14:28We want to make something just that little bit bigger.
0:14:34 > 0:14:36One dust storm coming right up.
0:14:39 > 0:14:41Right, where do you want it?
0:14:41 > 0:14:43Here, I would say. This will do nicely.
0:14:46 > 0:14:50Doing this in what is effectively the home of half the world's dust,
0:14:50 > 0:14:54I hope I don't trigger an international incident.
0:14:54 > 0:14:56If you get up tomorrow and your sideboard is covered in dust
0:14:56 > 0:14:58because I started this...
0:14:59 > 0:15:01That's a lot of dust!
0:15:05 > 0:15:07I don't know who that bloke is.
0:15:07 > 0:15:10Just a helpful local who decided to lend a hand. That's good of him.
0:15:20 > 0:15:22The blokes live round here.
0:15:22 > 0:15:25Happy to come out all in the name of science.
0:15:30 > 0:15:34Yeah, now we're talking, this is a dust storm.
0:15:41 > 0:15:44Oh, that's a lot of dust now!
0:15:45 > 0:15:48And it seems to be working.
0:15:48 > 0:15:53In amongst all the cars and chaos the dust is starting to bounce.
0:15:57 > 0:16:00Individual grains are colliding against each other,
0:16:00 > 0:16:02just like the rubber balls did.
0:16:02 > 0:16:05And notice that they're not just bouncing
0:16:05 > 0:16:07in the direction of the wind.
0:16:07 > 0:16:09They're being propelled upwards.
0:16:25 > 0:16:28Well, there it is. We've got the dust bouncing,
0:16:28 > 0:16:31just like it does in a real dust storm.
0:16:31 > 0:16:36But in a real dust storm, it bounces much higher than the storm itself.
0:16:36 > 0:16:38Can we do that here today?
0:16:38 > 0:16:40There is only one way to find out.
0:16:43 > 0:16:46Craig has bought in another dust expert to help,
0:16:46 > 0:16:49Professor Nigel Tapper,
0:16:49 > 0:16:52who specialises in measuring airborne dust.
0:16:54 > 0:16:58With his assistance we should be able to see just how high
0:16:58 > 0:17:01we can get our dust to bounce.
0:17:01 > 0:17:03OK. Beautiful!
0:17:04 > 0:17:06Nigel, to be honest, it looks like this is something
0:17:06 > 0:17:07you're about to fire at your balloon.
0:17:07 > 0:17:09What is it?
0:17:09 > 0:17:10We've got a pump arrangement here
0:17:10 > 0:17:13that pumps at about 2.5 litres per minute
0:17:13 > 0:17:16through this cyclone sampler that we can put underneath
0:17:16 > 0:17:20the balloon to sample the dust at various levels.
0:17:20 > 0:17:21That was a fly.
0:17:21 > 0:17:23They're quite tasty.
0:17:23 > 0:17:25Particularly tasty here.
0:17:25 > 0:17:26- A bit dry.- Yeah, yeah.
0:17:26 > 0:17:29So, tie it on to the balloon string, then?
0:17:29 > 0:17:31Yep. You've just got to run it through here.
0:17:31 > 0:17:34This is the tricky bit cos you've got to remember which way to roll.
0:17:34 > 0:17:36Oh, yeah. I'll do this one.
0:17:36 > 0:17:38And we'll do it on the bottom too.
0:17:38 > 0:17:40Watch this go well. I've got it.
0:17:40 > 0:17:42No, watch, it's that way...
0:17:42 > 0:17:44- Perfect.- You see?
0:17:44 > 0:17:45You've done that before.
0:17:45 > 0:17:49Well, no! Why would I have done this before, ever?
0:17:49 > 0:17:51OK. Winching up.
0:17:51 > 0:17:53We're winching, not whingeing.
0:17:53 > 0:17:54That's a pom joke.
0:17:54 > 0:17:57- Yeah, I know. - Well, I just chucked it in.
0:18:00 > 0:18:05Nigel plans to put three dust samplers under the balloon.
0:18:05 > 0:18:08One at three metres, which was about the height of the cloud
0:18:08 > 0:18:09we created with the cars.
0:18:11 > 0:18:15One at eight metres, more than double that height.
0:18:16 > 0:18:20And one at 20 metres, because,
0:18:20 > 0:18:22well, that's how much string we've got.
0:18:23 > 0:18:27That extra vane at the top is carrying a miniature camera.
0:18:28 > 0:18:32So we can keep a close eye on what's going on.
0:18:32 > 0:18:36And to make sure we're doing our very best impression
0:18:36 > 0:18:40of a real dust storm, we've wheeled in a couple of enormous fans
0:18:40 > 0:18:42to supply some extra wind.
0:18:43 > 0:18:46So, my job now is to try and keep the balloon,
0:18:46 > 0:18:50which is suspended from the winch over there through this hook,
0:18:50 > 0:18:52at the right height and in the best place
0:18:52 > 0:18:55to catch the most dust kicked up by our dust storm.
0:18:55 > 0:18:58One important thing to bear in mind, this all seems very big.
0:18:58 > 0:18:59I mean, it's a very big balloon.
0:18:59 > 0:19:01We're using big tools to make the dust
0:19:01 > 0:19:04but we're imitating the weather.
0:19:04 > 0:19:07This whole experiment, in fact, is tiny,
0:19:07 > 0:19:09but the principles are just the same.
0:19:09 > 0:19:11Hopefully the fine particles will end up on top,
0:19:11 > 0:19:15or me, if this balloon goes much higher.
0:19:15 > 0:19:17I did point out that I'm the smallest bloke here.
0:19:17 > 0:19:21Why I've got this job, I don't know. We'll be all right. Right. we ready?
0:19:30 > 0:19:32So, the vehicle is churning the surface,
0:19:32 > 0:19:34the fans are doing the job of the wind.
0:19:47 > 0:19:51Maybe I should have put my goggles on. That would have been better.
0:19:55 > 0:19:58Well, it certainly feels like a dust storm from here.
0:20:05 > 0:20:11So now I just need to keep the balloon in the densest part of it.
0:20:11 > 0:20:13I'm going to go over here a little bit.
0:20:21 > 0:20:23Basically, if I can't see...
0:20:25 > 0:20:26..or breathe,
0:20:26 > 0:20:29then I'm probably in the right spot.
0:20:30 > 0:20:34The different pumps at different levels are sampling the air
0:20:34 > 0:20:38and the dust carried by it at different heights.
0:20:38 > 0:20:43There's no way that these fans could actually blow the dust directly
0:20:43 > 0:20:48up to ten or 12 metres, but they do inject the energy into the system.
0:20:48 > 0:20:50It's then exchanged kinetically,
0:20:50 > 0:20:52the particles bouncing off one another...
0:20:52 > 0:20:54hopefully ending up at the top.
0:20:57 > 0:21:00But there's only one way to know for sure -
0:21:00 > 0:21:02check what's in those pumps.
0:21:07 > 0:21:12'Luckily, Nigel has a makeshift laboratory right on-site.'
0:21:12 > 0:21:16It'll be really interesting to crack these open and see what we've got.
0:21:16 > 0:21:20We've got to be a little bit... a little bit clean here.
0:21:20 > 0:21:22What are you trying to say?
0:21:22 > 0:21:26All right, now we've got the insults out of the way, time to see
0:21:26 > 0:21:29'what we collected in the lowest pump.
0:21:29 > 0:21:33'And if it's not dust, we've got a problem,
0:21:33 > 0:21:36'because that was slap-bang in the middle of our home-made dust storm.'
0:21:36 > 0:21:39- OK, beautiful.- Oh, there it is!
0:21:39 > 0:21:41We were only sampling for a short time
0:21:41 > 0:21:43- and there's a lot of it, as you can see.- Yeah.
0:21:43 > 0:21:46So let's move eight metres up into our home-made dust storm...
0:21:46 > 0:21:47That's right.
0:21:47 > 0:21:51Let's have a look at the filter paper. Turn it over.
0:21:51 > 0:21:53- Whoops, you've got...right there... - Oh, there it is!
0:21:53 > 0:21:55- ..a little bit of smudging.- Yeah.
0:21:55 > 0:21:58So we've actually got a bit of fine material at eight metres.
0:21:58 > 0:22:01- So, now, at 20 metres... - At 20 metres...
0:22:01 > 0:22:04- Which was well out of the cloud of dust.- Absolutely.
0:22:04 > 0:22:07If there is any here, this is the finer particles that have
0:22:07 > 0:22:11managed to bounce themselves up well beyond the top of our plume of dust.
0:22:12 > 0:22:14Come on.
0:22:15 > 0:22:18Turn it up the right way.
0:22:18 > 0:22:20- There we go. Well, look... - There is smudging on it.
0:22:20 > 0:22:22It is... There is a smudge.
0:22:22 > 0:22:25- So we did get a bit of material up that far.- Definitely some there.
0:22:25 > 0:22:28'So I think we can count that as a success.
0:22:28 > 0:22:32'We've got dust at least five times higher than our cloud.'
0:22:32 > 0:22:35So, we must bear in mind, this experiment, though to us it
0:22:35 > 0:22:38was quite big, it's actually tiny in terms of the weather, isn't it?
0:22:38 > 0:22:41Minute. So, this upper-level, for us, 20 metres,
0:22:41 > 0:22:46that was outside our plume of dust, if that were scaled up to be
0:22:46 > 0:22:50weather, to be a dust storm, that could be thousands of feet high up.
0:22:50 > 0:22:53If we were looking at a real dust storm, making it up to 2,000,
0:22:53 > 0:22:563,000 metres, it's an amazing process.
0:22:58 > 0:23:03A large dust storm can move 15 million tonnes of sand
0:23:03 > 0:23:05in a single go.
0:23:07 > 0:23:11Many are so big that they can be seen from space.
0:23:12 > 0:23:15And when that dust has bounced high enough,
0:23:15 > 0:23:18it gets caught in global wind patterns...
0:23:19 > 0:23:22..which move it around the planet.
0:23:24 > 0:23:29Once in the clouds, dust plays a crucial role in our weather.
0:23:32 > 0:23:37Because dust is central to the story of rain.
0:23:39 > 0:23:42Water vapour needs something to stick to
0:23:42 > 0:23:45if it's going to turn into raindrops.
0:23:46 > 0:23:49And dust is perfect.
0:23:50 > 0:23:55So, down the dust comes, carried by the water drops...
0:23:55 > 0:23:58out of the sky...
0:23:58 > 0:24:00and onto your car.
0:24:01 > 0:24:05So, without the sun beating down,
0:24:05 > 0:24:08creating deserts and dust,
0:24:08 > 0:24:09you might not get rain.
0:24:10 > 0:24:12Kind of ironic, isn't it?
0:24:14 > 0:24:20But there is one rare type of rain that doesn't need dust.
0:24:20 > 0:24:22What it does need is cold.
0:24:23 > 0:24:27It's a weather phenomenon unlike any other.
0:24:30 > 0:24:33One that can take any of these objects...
0:24:36 > 0:24:40..and trap them like flies in amber.
0:24:43 > 0:24:47Encasing them in a hard, plastic-looking shell.
0:24:50 > 0:24:53It's called freezing rain.
0:24:54 > 0:24:59And, as its name suggests, it's completely dependent on temperature.
0:25:00 > 0:25:05But it's not just the weather that needs to be below freezing...
0:25:05 > 0:25:07the rainwater does.
0:25:07 > 0:25:11And I'm going to try and recreate it for you right here, right now.
0:25:11 > 0:25:14Actually, I'm probably in the best possible place to do that -
0:25:14 > 0:25:16Montreal, Quebec, Canada,
0:25:16 > 0:25:17because it happens more here than
0:25:17 > 0:25:20just about anywhere else in the world.
0:25:20 > 0:25:22Right...
0:25:22 > 0:25:24Oh, yeah, that's...perfect.
0:25:24 > 0:25:27Really...very cold, which is what I want.
0:25:27 > 0:25:30The air temperature here right now is about -10 degrees C,
0:25:30 > 0:25:33but as long as it's somewhere near freezing, the air temperature
0:25:33 > 0:25:37doesn't matter - it's the temperature of these objects.
0:25:37 > 0:25:41I need them to be really cold, and they definitely are.
0:25:42 > 0:25:46So, I've got water that's below freezing but still in liquid form,
0:25:46 > 0:25:50and I've got...a hose.
0:25:50 > 0:25:52Let's see what happens when super-cold water
0:25:52 > 0:25:54hits super-cold objects.
0:25:56 > 0:25:58It's not complicated. I've begun.
0:26:00 > 0:26:04Well, it might not be complicated, but it is effective.
0:26:07 > 0:26:13The moment the spray hits the hydrant, it turns instantly to ice.
0:26:15 > 0:26:18Fully formed blobs of ice that appear
0:26:18 > 0:26:20right in front of your eyes.
0:26:22 > 0:26:27Instead of dripping into icicles, it solidifies immediately.
0:26:30 > 0:26:32So, how does it work?
0:26:34 > 0:26:40We've already seen how raindrops need an impurity like dust to form,
0:26:40 > 0:26:43but freezing rain is formed when a snowflake falls
0:26:43 > 0:26:47through a freak layer of warm air on its way down.
0:26:48 > 0:26:52Now it's rain, but rain without any dust inside it.
0:26:54 > 0:26:57The temperature of the drop can go below freezing
0:26:57 > 0:26:59without turning to ice.
0:27:01 > 0:27:03Until it touches something cold.
0:27:06 > 0:27:10To try and recreate that, I'm spraying distilled water.
0:27:11 > 0:27:12It's starting.
0:27:15 > 0:27:19This, I feel, is good, but it's going to take a while.
0:27:22 > 0:27:25I could be patient and wait...
0:27:25 > 0:27:29or just tweak my approach a bit.
0:27:45 > 0:27:48This is bigger, this is better.
0:27:51 > 0:27:54The water in the truck has been outside for days,
0:27:54 > 0:27:57so, normally, it would be frozen too,
0:27:57 > 0:27:59but fire trucks in Canada
0:27:59 > 0:28:02have a constantly revolving drum inside them
0:28:02 > 0:28:07that keeps the water moving, a bit like a giant slushy drink dispenser.
0:28:07 > 0:28:10This is strangely addictive. I mean, I've done a little bit there
0:28:10 > 0:28:13and now I just want to do everything...more!
0:28:15 > 0:28:16Let's have a go at this.
0:28:21 > 0:28:23Certainly, it gets the job done quick.
0:28:27 > 0:28:29So let's see what we've got.
0:28:51 > 0:28:53Look at that!
0:28:53 > 0:28:57Completely encased in crystal clear ice, and that's exactly what I want.
0:28:59 > 0:29:04It's the clarity of the ice that makes freezing rain so unusual.
0:29:05 > 0:29:10That and the fact that it completely surrounds any object it touches.
0:29:12 > 0:29:16It's not just icy where the objects faced the hose,
0:29:16 > 0:29:19it's icy everywhere,
0:29:19 > 0:29:22in a perfect, even coating.
0:29:23 > 0:29:29Leaving the objects rigid but unharmed.
0:29:29 > 0:29:30Erm, "ish".
0:29:30 > 0:29:32ELECTRICAL CRACKLING
0:29:34 > 0:29:36There's going to be shouting about that.
0:29:38 > 0:29:41Luckily, freezing rain is fairly rare,
0:29:41 > 0:29:46but it does hold the secret of how we get frost.
0:29:46 > 0:29:49Just like our fire hydrant and phone box,
0:29:49 > 0:29:52these leaves have cooled below freezing.
0:29:54 > 0:29:59The difference is that frost grows without falling as a liquid first.
0:30:03 > 0:30:09The ice crystals just magically appear, literally out of thin air.
0:30:12 > 0:30:18But when ice crystals grow in the air instead, then something
0:30:18 > 0:30:20even more magical happens.
0:30:20 > 0:30:22They become snow.
0:30:32 > 0:30:37All snowflakes start off as an ice crystal -
0:30:37 > 0:30:40a six-sided shape a bit like this.
0:30:47 > 0:30:50But then temperature begins to play its part.
0:30:53 > 0:30:57Just a little extra moisture in the air
0:30:57 > 0:31:00and arms start to form at the corners.
0:31:04 > 0:31:10A degree rise in temperature, and a plate forms on one of those arms.
0:31:13 > 0:31:17A two-degree drop, and tiny needles form around them.
0:31:21 > 0:31:27Each of these minute changes stamp their identity on the ice.
0:31:30 > 0:31:35And they are so subtle that scientists aren't sure
0:31:35 > 0:31:38exactly why it happens.
0:31:38 > 0:31:40What they do know is you end up with
0:31:40 > 0:31:43something like this...
0:31:44 > 0:31:46..a snowflake.
0:31:46 > 0:31:50Water in its most beautiful and complicated form.
0:31:50 > 0:31:53Except I made this one, it's all my own work.
0:31:53 > 0:31:55Nothing to do at all with professional snow artist,
0:31:55 > 0:31:58Simon Kemp, over there, who's just out for a picnic.
0:32:00 > 0:32:01Well, I did the fiddly bits.
0:32:01 > 0:32:04I did that bit, that's mine.
0:32:04 > 0:32:05All my own work.
0:32:08 > 0:32:11The really cool thing about all of this is that every one
0:32:11 > 0:32:13of these shapes is different.
0:32:13 > 0:32:16I know it's a bit of a cliche, "no two snowflakes are the same",
0:32:16 > 0:32:18but they're not.
0:32:18 > 0:32:22I guess because of this, well, infinite number of variations
0:32:22 > 0:32:27in temperature and humidity, every snowflake really is unique, not just
0:32:27 > 0:32:32in a handful of snow or in all the snow in this giant snowflake, but
0:32:32 > 0:32:37in all the snow that's ever fallen in the world, or ever will fall.
0:32:37 > 0:32:42That's because the conditions that create each snowflake are
0:32:42 > 0:32:47so unique that an individual shape can never be repeated.
0:32:50 > 0:32:55But what's really amazing is that a snowflake never stops changing.
0:32:55 > 0:32:57Even after it's landed,
0:32:57 > 0:33:03temperature continues to transform it in the most surprising ways.
0:33:03 > 0:33:05HORN BLASTS
0:33:10 > 0:33:13I'm interested to see exactly how.
0:33:13 > 0:33:16So I thought I'd kill two birds with one stone -
0:33:16 > 0:33:18provide a civic service for the good people of Davos
0:33:18 > 0:33:20by clearing this car park...
0:33:22 > 0:33:23Sorry!
0:33:23 > 0:33:27..and conduct a little experiment.
0:33:27 > 0:33:29Well, it's more of an illustration, really.
0:33:38 > 0:33:40Yeah, we're ploughing!
0:33:42 > 0:33:47First off, I need to compress the snow as hard as I can.
0:33:48 > 0:33:50Manly work taking place.
0:33:59 > 0:34:00One last load.
0:34:02 > 0:34:05Right, I'm going to give it a real press this time. Come on!
0:34:06 > 0:34:09Yeah, I think that's about as solid as I can get that.
0:34:15 > 0:34:19Inevitably, the hardness of snow is something scientists have
0:34:19 > 0:34:23considered, and they've developed a scale associated with it.
0:34:23 > 0:34:26Five stages, and it goes like this -
0:34:26 > 0:34:28can you push a push a fist into it?
0:34:28 > 0:34:30No. Next, four fingers.
0:34:30 > 0:34:32This isn't as rough and ready as it sounds, actually.
0:34:32 > 0:34:37The fist equates to ten grams per square centimetre of pressure.
0:34:37 > 0:34:39After the four fingers, it's a single finger -
0:34:39 > 0:34:41that's 100 grams per square centimetre.
0:34:41 > 0:34:44No. If all that fails, they move on.
0:34:44 > 0:34:46It gets serious - a pen!
0:34:48 > 0:34:49Yeah! So it would end there.
0:34:49 > 0:34:52But if that doesn't work, the next scale is a knife.
0:34:52 > 0:34:54I haven't got a knife.
0:34:54 > 0:34:58I have, actually, just for the purposes of this.
0:34:58 > 0:35:00Yeah, I know.
0:35:00 > 0:35:04And then they see if the knife goes in and...oh, look, it does.
0:35:04 > 0:35:07I haven't actually done that just because I happen to have
0:35:07 > 0:35:10a sword in my snowplough truck - there is a reason.
0:35:10 > 0:35:13This is all about the changing nature of snow,
0:35:13 > 0:35:15the fact that it never seems to stop altering.
0:35:15 > 0:35:17I've made it in to this big mound and made it as firm as I can,
0:35:17 > 0:35:19but it's not finished there.
0:35:19 > 0:35:21Unless there is a sudden heat wave,
0:35:21 > 0:35:23by tomorrow morning another significant change
0:35:23 > 0:35:26will have happened, and I may have set the locals an interesting
0:35:26 > 0:35:29and unexpected challenge with my sword in the snow.
0:35:29 > 0:35:32Mustn't forget my pen.
0:35:35 > 0:35:39Overnight, the snowflakes undergo a remarkable change.
0:35:41 > 0:35:46All the arms and branches broke off as my snowplough crushed them up
0:35:46 > 0:35:47next to each other.
0:35:49 > 0:35:52And now they begin to fuse together
0:35:52 > 0:35:55in a process scientists call sintering.
0:35:57 > 0:35:59Joining on to each other
0:35:59 > 0:36:02and the blade of the sword in one rigid structure.
0:36:04 > 0:36:09This isn't freezing - in fact, the whole process works better
0:36:09 > 0:36:11when conditions are slightly warmer.
0:36:11 > 0:36:13It's restructuring.
0:36:15 > 0:36:21The next morning, we hid our cameras in a nearby, um, shed thing,
0:36:21 > 0:36:25and waited for the first curious locals to come past.
0:36:44 > 0:36:48The sword is fixed as if it's set in concrete.
0:36:57 > 0:37:02This isn't ice, remember - it's still snow.
0:37:05 > 0:37:07Snow that never stops changing.
0:37:08 > 0:37:11All because of temperature.
0:37:13 > 0:37:14THEY LAUGH
0:37:16 > 0:37:20When we think of temperature, we tend to think of sunshine.
0:37:20 > 0:37:22Or lack of it.
0:37:24 > 0:37:27But, in fact, the biggest influence that temperature has
0:37:27 > 0:37:32on weather is controlling the water vapour in our air.
0:37:32 > 0:37:36Evaporating it from the ground and the oceans...
0:37:36 > 0:37:40freezing it into frost and snow...
0:37:40 > 0:37:43or condensing it into fog...
0:37:51 > 0:37:54I've come to one of the most predictably foggy places
0:37:54 > 0:37:59on the planet - the Appalachian Mountains,
0:37:59 > 0:38:02near Blacksburg, Virginia.
0:38:04 > 0:38:07Almost every morning, fog rolls up
0:38:07 > 0:38:11the Bluestone River and floods the valley.
0:38:17 > 0:38:19This must be one of the off days,
0:38:19 > 0:38:24which is why it's just as well I'm on this particular road,
0:38:24 > 0:38:28because here, they can make their own fog at the flick of a switch.
0:38:34 > 0:38:37This is the Virginia Smart Road...
0:38:39 > 0:38:41..a two-mile highway designed
0:38:41 > 0:38:45to test vehicle and traffic systems in different sorts of weather.
0:38:49 > 0:38:53However, we're going to use it to take a short diversion
0:38:53 > 0:38:56and answer a question I've often wondered about.
0:38:56 > 0:39:02If fog is made of water, then why isn't it clear?
0:39:02 > 0:39:05Why is fog white?
0:39:09 > 0:39:13- LOUD HISSING - Do you know, I never noticed how loud fog is.
0:39:13 > 0:39:16It's loud! Isn't it? Oh!
0:39:16 > 0:39:21London in Victorian times must have been deafening!
0:39:21 > 0:39:25Luckily we're not planning on doing anything with sound.
0:39:26 > 0:39:29We're doing it with light.
0:39:29 > 0:39:33Light is made up of lots of different wavelengths,
0:39:33 > 0:39:35each a different colour.
0:39:35 > 0:39:39And we see those colours when objects absorb
0:39:39 > 0:39:43one wavelength and reflect another.
0:39:43 > 0:39:46The light from this laser is
0:39:46 > 0:39:50scattering off the tiny particles of fog, making each one visible.
0:39:50 > 0:39:54But we are only projecting one colour here - green -
0:39:54 > 0:39:56and they're reflecting it.
0:39:58 > 0:40:01The air around them hasn't reflected any wavelengths,
0:40:01 > 0:40:02so it looks black.
0:40:04 > 0:40:08Project red, and the droplets change colour.
0:40:10 > 0:40:13Same thing with violet.
0:40:13 > 0:40:15In fact, they reflect EVERY colour.
0:40:20 > 0:40:24Add all the different ones together, and they become white.
0:40:33 > 0:40:38Fog is just a cloud that's in contact with the ground.
0:40:39 > 0:40:44So the reason fog looks white is the same reason clouds look white.
0:40:46 > 0:40:49Because they're scattering every colour of light.
0:40:52 > 0:40:54Now, you might be thinking,
0:40:54 > 0:40:58"Hold on, clouds aren't ALWAYS white - sometimes they're black."
0:40:58 > 0:41:00Well, yes, sometimes they appear to be black.
0:41:00 > 0:41:03But that's mainly an optical illusion.
0:41:03 > 0:41:05It's your brain exaggerating any differences there are,
0:41:05 > 0:41:08to give you what it thinks is a more useful picture.
0:41:08 > 0:41:12And I can demonstrate. I've cut two holes in this piece of cardboard.
0:41:12 > 0:41:16And if I put one hole over a white bit of cloud
0:41:16 > 0:41:19and the other what looks like a black bit...
0:41:21 > 0:41:25In fact, there is barely any difference.
0:41:27 > 0:41:30And what tiny difference there is is caused
0:41:30 > 0:41:35by those minute water droplets fusing together to form raindrops.
0:41:37 > 0:41:41The bigger drops of water make the cloud more dense,
0:41:41 > 0:41:45which makes it harder for sunlight to pass through.
0:41:45 > 0:41:50So we see dark patches that our brain exaggerates.
0:41:58 > 0:42:02But sometimes there's no doubt that a cloud is black.
0:42:03 > 0:42:07The sort of brooding storm cloud that serves as warning
0:42:07 > 0:42:08for one final type of weather.
0:42:10 > 0:42:13The type that no show on weather should be without.
0:42:14 > 0:42:16And it's a fitting conclusion.
0:42:16 > 0:42:20Because it requires all three of the key ingredients
0:42:20 > 0:42:22that we've looked at in this series.
0:42:24 > 0:42:26Temperature...
0:42:26 > 0:42:28water...
0:42:28 > 0:42:31and wind, in equal measure.
0:42:32 > 0:42:35When heat makes the ground intensely warm...
0:42:35 > 0:42:39and the air is heavy with water vapour...
0:42:39 > 0:42:43and strong winds mould the clouds...
0:42:43 > 0:42:46you create...
0:42:50 > 0:42:51..a lightning storm.
0:42:52 > 0:42:57This is one of the planet's most lightning-prone regions -
0:42:57 > 0:43:00Florida, USA.
0:43:04 > 0:43:08For most of us, the most dramatic weather we're likely to encounter
0:43:08 > 0:43:10is thunder and lightning.
0:43:10 > 0:43:13We've all heard thunder and seen lightning.
0:43:13 > 0:43:16But... here's the interesting thing.
0:43:16 > 0:43:20It is actually possible to do the exact opposite -
0:43:20 > 0:43:23to SEE thunder and HEAR lightning.
0:43:25 > 0:43:30And I'm going to show you one way to hear lightning right now,
0:43:30 > 0:43:33without even leaving my car.
0:43:34 > 0:43:38Right, first of all, turn the radio on.
0:43:38 > 0:43:43Then tune it to the AM frequency because this works best on that.
0:43:44 > 0:43:47And then, look for a point where you haven't got a radio station.
0:43:50 > 0:43:53Yeah, it's tricky, there's a lot of radio stations in the States.
0:43:53 > 0:43:55But here we go.
0:43:55 > 0:43:56There.
0:43:56 > 0:43:59Right, between radio stations,
0:43:59 > 0:44:01what we've got, well, it's static -
0:44:01 > 0:44:03not surprisingly, I know.
0:44:03 > 0:44:07But some of that static is quite important to us.
0:44:07 > 0:44:10Listen for those quite distinct pops.
0:44:10 > 0:44:12CRACKLING AND POPPING
0:44:12 > 0:44:14Those are lightning strikes.
0:44:14 > 0:44:16They might be happening some distance away,
0:44:16 > 0:44:20but the huge electrical discharge is interfering with the radio signal.
0:44:20 > 0:44:23So we're listening to lightning happen.
0:44:23 > 0:44:27And this system is so reliable that storm chasers actually use it
0:44:27 > 0:44:29to track down storms.
0:44:33 > 0:44:36The louder the pops are,
0:44:36 > 0:44:39the nearer the storm is.
0:44:41 > 0:44:46Let me just prove to you that this is electricity making this happen.
0:45:00 > 0:45:02Never could do this.
0:45:20 > 0:45:23The static electricity I've built up on the balloon
0:45:23 > 0:45:25affects the radio too.
0:45:25 > 0:45:26CRACKLING AND POPPING
0:45:29 > 0:45:31You see? Static. It is interesting,
0:45:31 > 0:45:35but no matter how effective, or sometimes useful,
0:45:35 > 0:45:40that method is, it isn't the real and actual sound of lightning.
0:45:40 > 0:45:44To get closer to hearing that, I need to set something up.
0:45:47 > 0:45:50Because static isn't the true sound of lightning.
0:45:51 > 0:45:54Just as it isn't the real noise of a balloon.
0:45:59 > 0:46:03This is a very low frequency detector.
0:46:03 > 0:46:07And it can pick up lightning strikes from thousands of miles away.
0:46:08 > 0:46:12Too far away for any static to be an issue.
0:46:14 > 0:46:18As the planet has more than 100 lightning strikes a second,
0:46:18 > 0:46:22I should have a fairly good chance of hearing a few between here...
0:46:22 > 0:46:24and the other side of the globe.
0:46:26 > 0:46:28CRACKLING
0:46:32 > 0:46:34WHISTLING
0:46:35 > 0:46:38What you're listening for particularly is whistles.
0:46:42 > 0:46:46And that is the actual sound of a lightning bolt
0:46:46 > 0:46:48somewhere on the planet.
0:46:48 > 0:46:50WHISTLING
0:46:56 > 0:46:59But if this is the REAL sound of lightning,
0:46:59 > 0:47:01then what is thunder?
0:47:05 > 0:47:09I've got another balloon here - don't ask me why. Just have.
0:47:15 > 0:47:19Now, what happens next is no big surprise.
0:47:19 > 0:47:22There's a big bang.
0:47:22 > 0:47:25But what IS surprising is where the noise comes from,
0:47:25 > 0:47:28because it's not the material of the balloon.
0:47:28 > 0:47:32If I stretch the rubber like so, and pop it again...
0:47:33 > 0:47:36..there's no noise. The noise is coming from the air.
0:47:36 > 0:47:42As the balloon bursts, the air inside it explodes out.
0:47:42 > 0:47:46And that is a clue to how thunder works.
0:47:52 > 0:47:56But to find out more, I'll have to visit one of the few
0:47:56 > 0:48:00places in the world capable of creating full-blown thunder.
0:48:00 > 0:48:05They do it by firing 200,000 amps of electrical current
0:48:05 > 0:48:08down this narrow copper wire.
0:48:09 > 0:48:13Exactly the same amount as in real lightning.
0:48:17 > 0:48:19So this is it - this is where it's all controlled?
0:48:19 > 0:48:21Yep. So you'll need a pair of these.
0:48:21 > 0:48:24- Will I?- This is going to be quite loud.- Is it?
0:48:24 > 0:48:26- Because we're going to be producing thunder.- OK.
0:48:26 > 0:48:29Also, don't look directly at the arc cos it's very bright.
0:48:29 > 0:48:31Right, so I've come quite a long way
0:48:31 > 0:48:33to see something that I can't look at or listen to.
0:48:33 > 0:48:36- Pretty much.- Good. OK.
0:48:36 > 0:48:40Well, I suppose it is quite a lot of electricity we're playing with here.
0:48:40 > 0:48:42If your kettle goes off, he's nicked your electricity
0:48:42 > 0:48:45- to put in their capacitors. OK, these on?- Yes.- Go!
0:48:47 > 0:48:49Have we started?
0:48:49 > 0:48:50- Yes.- Right.
0:48:50 > 0:48:53So now we can see the voltage on the capacitors.
0:48:53 > 0:48:56Oh, yeah. Essentially this is going to build up a colossal charge
0:48:56 > 0:48:58- and then discharge it.- Yep.
0:48:58 > 0:49:01- Dan?- Yeah? - I can hear what you're saying.
0:49:01 > 0:49:04- HE LAUGHS - Oh, yeah. When the shock goes through,
0:49:04 > 0:49:06you might want to put your hands over these as well.
0:49:06 > 0:49:09- Shall I just cower under the table? - Or you could cower under the table.
0:49:09 > 0:49:11Right, it's 25.
0:49:11 > 0:49:12Yes, nearly there.
0:49:18 > 0:49:21- OK, so we're ready.- Well... - So we can fire?
0:49:21 > 0:49:23OK, but...I can't look? Or listen.
0:49:23 > 0:49:24SIREN BLARES
0:49:24 > 0:49:26- LOUD BANG - Whoa!
0:49:29 > 0:49:32That, in fact, was quite staggeringly loud!
0:49:32 > 0:49:36- I mean, really, amazingly loud!- Yep.
0:49:36 > 0:49:40So was that thunder, or was it just sort of the discharge
0:49:40 > 0:49:43of the electricity leaving and arriving?
0:49:43 > 0:49:45No, that was thunder.
0:49:45 > 0:49:47But it didn't sound anything like thunder, it was just like that.
0:49:47 > 0:49:52Yeah, that's because the lightning arc is only 20cm long here.
0:49:53 > 0:49:57In reality you've got a kilometre of an arc that's all producing sound
0:49:57 > 0:50:00on every little bit, and it all arrives at you,
0:50:00 > 0:50:03a couple of kilometres away, at different times.
0:50:07 > 0:50:11Because a bolt of lightning is around a kilometre long,
0:50:11 > 0:50:15some of it is further away from us than other bits.
0:50:16 > 0:50:19So parts of the sound get to us quicker,
0:50:19 > 0:50:22meaning that what we hear is multiple rumbles
0:50:22 > 0:50:23of that short, sharp bang.
0:50:27 > 0:50:30But it still doesn't tell us what thunder actually is.
0:50:30 > 0:50:34Luckily Dan has a way to show us.
0:50:35 > 0:50:41Using slow-motion cameras and a line of lit candles.
0:50:41 > 0:50:43It's the director's birthday.
0:50:43 > 0:50:46I've just got another 56 candles to go.
0:50:46 > 0:50:48We're getting there!
0:50:48 > 0:50:51Once again we return to the control room.
0:50:51 > 0:50:54SIREN BLARES
0:50:57 > 0:50:58LOUD BANG
0:51:05 > 0:51:08The candles are all blown out.
0:51:12 > 0:51:18And if you watch carefully, you can see that they are blown out
0:51:18 > 0:51:19one by one.
0:51:20 > 0:51:22So what is going on?
0:51:24 > 0:51:27Well, of course, it's all to do with temperature.
0:51:30 > 0:51:32A typical bolt of lightning
0:51:32 > 0:51:35is somewhere between two and five centimetres wide.
0:51:35 > 0:51:38So, something close to that.
0:51:40 > 0:51:43These, by the way, are not for style reasons -
0:51:43 > 0:51:46they're for protection, because effectively I'm taking
0:51:46 > 0:51:49a shaft of sunlight as wide as this screen and focusing it down
0:51:49 > 0:51:51into something roughly the size of a bolt of lightning.
0:51:51 > 0:51:56It IS hot, but it's nothing compared with lightning.
0:51:56 > 0:51:59A typical bolt will reach 20,000 degrees Celsius.
0:51:59 > 0:52:01That's well over three times
0:52:01 > 0:52:05the temperature of the surface of the sun itself.
0:52:06 > 0:52:10Thankfully, it only lasts for about one ten-thousandth of a second.
0:52:10 > 0:52:14But that's still enough for something quite amazing to happen.
0:52:14 > 0:52:21Because lightning is so ferociously hot, it explodes the air around it.
0:52:23 > 0:52:27Causing it to rush outwards, like the air in my balloon.
0:52:29 > 0:52:33What we see with the candles is that air moving away
0:52:33 > 0:52:36from the lightning bolt in a shock wave.
0:52:38 > 0:52:42But just how powerful is this wave?
0:52:43 > 0:52:45Time for another experiment.
0:52:45 > 0:52:50Can thunder break these glass light bulbs?
0:52:50 > 0:52:52I agree, light bulbs are delicate.
0:52:52 > 0:52:55But are they delicate enough to be affected?
0:52:55 > 0:52:56We think so.
0:52:56 > 0:52:59We certainly think that the inner ring of light bulbs will go.
0:52:59 > 0:53:01We're not so quite so sure about the outer ring.
0:53:01 > 0:53:03- One way to find out.- Zap 'em!- Yes.
0:53:09 > 0:53:11LOUD BANG
0:53:17 > 0:53:21They've been destroyed by the sheer force of the hot air
0:53:21 > 0:53:23exploding outwards.
0:53:23 > 0:53:27Well, except that tough one at the back - it hardly flinched.
0:53:27 > 0:53:30But still, pretty impressive.
0:53:31 > 0:53:33I really want to see this one cos I still can't believe
0:53:33 > 0:53:36- it was strong enough.- OK, Let's have a look.- That's a heck of a wave.
0:53:40 > 0:53:42RUMBLING
0:53:48 > 0:53:52Notice it's not the lightning destroying the light bulbs -
0:53:52 > 0:53:57the arc never even touches them. It's the shock wave after the flash
0:53:57 > 0:54:00that does the damage.
0:54:03 > 0:54:08Notice also that the sound of thunder happens even later -
0:54:08 > 0:54:13after the flash, and after the bulbs have exploded.
0:54:17 > 0:54:19That is a lot of power, a lot of energy.
0:54:19 > 0:54:22Yeah. And that's just from the thunder.
0:54:22 > 0:54:24The arc hasn't attached to the light bulbs.
0:54:24 > 0:54:27So that's just the shock wave that's broken the light bulbs.
0:54:27 > 0:54:30Which tells us how strong that shock wave can be.
0:54:30 > 0:54:32But I want to see more.
0:54:33 > 0:54:35What we've been looking at,
0:54:35 > 0:54:39impressive though it is, is the effect of thunder.
0:54:39 > 0:54:42I want to look at the thunder itself.
0:54:44 > 0:54:46With very specialised cameras,
0:54:46 > 0:54:48we can actually attempt to capture
0:54:48 > 0:54:51that shock wave on screen.
0:54:52 > 0:54:56Not the effects, but the actual shock wave itself.
0:55:07 > 0:55:10That's fant...! That's absolutely brilliant!
0:55:11 > 0:55:16That is the air exploding away from the hot lightning bolt
0:55:16 > 0:55:18at over 700mph.
0:55:20 > 0:55:23I think we can count that one a definite success!
0:55:23 > 0:55:27So there you have it - you can hear lightning...
0:55:28 > 0:55:31..and you can see thunder.
0:55:32 > 0:55:36All because of the incredible temperature it gets to.
0:55:43 > 0:55:46We've seen how temperature drives weather.
0:55:49 > 0:55:50How heat gets water into the air...
0:55:54 > 0:55:57..and cold turns it into clouds.
0:56:00 > 0:56:06How warmth creates winds that can bounce dust into raindrops...
0:56:08 > 0:56:12..and tiny fluctuations in temperature shape snowflakes...
0:56:13 > 0:56:16..and frost.
0:56:19 > 0:56:24And it all goes to show how our weather is endlessly fascinating -
0:56:24 > 0:56:27a stunning display of magic
0:56:27 > 0:56:32and spectacle performed in front of us every single day.
0:56:36 > 0:56:39Even when conditions are wet and miserable,
0:56:39 > 0:56:43there are amazing events going on just behind the scenes.
0:56:47 > 0:56:50And though it may seem that only extreme weather
0:56:50 > 0:56:52is worthy of our attention...
0:56:54 > 0:57:01..the weather around us every day is equally full of wonder.
0:57:04 > 0:57:09This is not freak weather. It's OUR weather,
0:57:09 > 0:57:11and it's astonishing.
0:57:13 > 0:57:16You can find out more about wild weather
0:57:16 > 0:57:19with The Open University's free wall poster.
0:57:19 > 0:57:21Call -
0:57:23 > 0:57:26Or go to -
0:57:27 > 0:57:29And follow the links to the Open University.