0:00:05 > 0:00:10Weather, one of the most astonishing forces on Earth.
0:00:10 > 0:00:14Capable of both devastating power
0:00:14 > 0:00:16and spectacular beauty.
0:00:17 > 0:00:23Wherever you live on the planet, weather 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.
0:00:36 > 0:00:41So, 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 > 0:00:51ambitious and sometimes just plain unlikely experiments...
0:00:52 > 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.
0:01:10 > 0:01:14Water lies at the heart of our weather,
0:01:14 > 0:01:16but not just as rain.
0:01:18 > 0:01:21Because water can transform itself,
0:01:21 > 0:01:26redefining its powers in the process,
0:01:26 > 0:01:30creating the fastest, the slowest, the softest
0:01:30 > 0:01:34and the hardest weather on Earth.
0:01:35 > 0:01:39Often changing from one to another with alarming speed
0:01:39 > 0:01:41and striking consequences.
0:01:44 > 0:01:49In this programme, I'll reveal water in all its shapes.
0:01:52 > 0:01:54I'll capture a cloud...
0:01:54 > 0:01:57OK, little cloud, let's see what you've got!
0:01:57 > 0:01:59..to see just how much it weighs.
0:02:00 > 0:02:04Discover why hailstones are able to do so much damage...
0:02:04 > 0:02:06Ooh, look at that!
0:02:06 > 0:02:12..find out what would happen if rain fell in one big lump...
0:02:12 > 0:02:14It's amazing, isn't it?
0:02:14 > 0:02:18..and I'll experience water in its most ferociously powerful form...
0:02:18 > 0:02:20as an avalanche.
0:02:23 > 0:02:27I'm speechless, genuinely speechless!
0:02:33 > 0:02:38All our everyday weather appears to come from the clouds.
0:02:40 > 0:02:43They're the best clues most of us have as to what the weather
0:02:43 > 0:02:45is likely to do next.
0:02:47 > 0:02:50They dictate if it's sunny or dull...
0:02:52 > 0:02:56..and they're where all our watery weather seems to come from.
0:02:58 > 0:03:02But how? What exactly is a cloud?
0:03:04 > 0:03:06Come on, you've done it!
0:03:06 > 0:03:08If not, you should.
0:03:10 > 0:03:13Gazing at clouds, dreaming up shapes.
0:03:15 > 0:03:18And the next time you do, two things you should
0:03:18 > 0:03:21know about clouds that might just change the way you think.
0:03:21 > 0:03:24Number one, clouds are really heavy.
0:03:24 > 0:03:27Even that fluffy little cumulus
0:03:27 > 0:03:30could weigh as much as two elephants.
0:03:30 > 0:03:33And secondly, because of that weight,
0:03:33 > 0:03:38all clouds are falling slowly, steadily down to earth.
0:03:40 > 0:03:44I know, both those things sound pretty unlikely.
0:03:46 > 0:03:49Which is why I'm going to put them to the test.
0:03:54 > 0:03:58And I'm going to start by trying to discover just how much a
0:03:58 > 0:04:01"small cloud" really does weigh.
0:04:03 > 0:04:06Now, I know I'm not the only one who,
0:04:06 > 0:04:10when presented with a sign on a bench saying "wet paint",
0:04:10 > 0:04:14has to touch the bench just to check it really is.
0:04:14 > 0:04:18So, when I heard that a cloud can weigh as much as two elephants,
0:04:18 > 0:04:20I had to check it out.
0:04:20 > 0:04:25The only thing is, it turns out that weighing a cloud is a bit
0:04:25 > 0:04:30more of a faff than checking to see if paint is wet.
0:04:30 > 0:04:34Obviously, you can't hang a cloud off a spring balance
0:04:34 > 0:04:39or pop it on a set of scales, but you could measure the moisture in it
0:04:39 > 0:04:40and work it out from that.
0:04:40 > 0:04:44So, I thought, what if we could fly a giant ball of cotton wool
0:04:44 > 0:04:47into the cloud to gather the moisture?
0:04:47 > 0:04:50As an idea, it needs a bit of finessing, yes.
0:04:50 > 0:04:53So, I got an engineering mate of mine to iron out
0:04:53 > 0:04:56some of the wrinkles, and he came up with this!
0:04:56 > 0:04:59OK, so it's not actually cotton wool.
0:04:59 > 0:05:04It's an industrial version - ceramic wool.
0:05:04 > 0:05:06And it's not one solid ball either.
0:05:08 > 0:05:12My friend reckoned that by making the centre hollow, it would
0:05:12 > 0:05:15double the amount of wool that came into contact with the cloud.
0:05:17 > 0:05:20He calls it his "sky-sponge".
0:05:22 > 0:05:27And then we've got that to put it in the cloud.
0:05:27 > 0:05:29It's all fairly standard stuff.
0:05:31 > 0:05:36So, first off, let's check how much this sky-sponge weighs dry.
0:05:38 > 0:05:44That's 37 kilos, which for a sponge is already pretty heavy.
0:05:44 > 0:05:50But we need that weight to be able to fly it accurately.
0:05:50 > 0:05:53Especially when the pilot is someone
0:05:53 > 0:05:56not that used to carrying freight.
0:05:58 > 0:06:01I know, I know, it's not a good start.
0:06:04 > 0:06:07But as nobody has ever done anything like this before,
0:06:07 > 0:06:10I'm as good a choice as anybody.
0:06:18 > 0:06:22In the end, it was deemed not a job for an amateur, no matter how
0:06:22 > 0:06:26enthusiastic, so I took a co-pilot, Andrew, with me to keep an eye
0:06:26 > 0:06:29on things...mostly on me.
0:07:04 > 0:07:06So, helicopter, check.
0:07:06 > 0:07:09Basket full of highly absorbent ceramic wool, check.
0:07:09 > 0:07:13All I need now is a nice, little cloud to dip it into.
0:07:14 > 0:07:18And that's not as easy as you might think.
0:07:19 > 0:07:23Because when you get close to them, clouds are...
0:07:23 > 0:07:24Well, they're enormous!
0:07:26 > 0:07:28You look at them from the ground, they all look perfect
0:07:28 > 0:07:30and fluffy and small.
0:07:30 > 0:07:32Get up and they look entirely different.
0:07:32 > 0:07:35I've got to find a nice, individual one,
0:07:35 > 0:07:37drop it in and see
0:07:37 > 0:07:39how much water it pulls out again.
0:07:39 > 0:07:41I'm assuming it won't soak up the entire cloud
0:07:41 > 0:07:45and we'll be left with underslung the weight of two elephants.
0:07:45 > 0:07:46That would be bad.
0:07:48 > 0:07:50A full-grown African elephant weighs,
0:07:50 > 0:07:53on average, four and a half tonnes.
0:07:53 > 0:07:57A quarter of that would pull my helicopter straight out the air.
0:08:03 > 0:08:09But if I just weigh a fraction of a cloud, then multiply my results,
0:08:09 > 0:08:15it should give us some idea how much a whole cloud actually weighs.
0:08:15 > 0:08:17Pick a victim.
0:08:17 > 0:08:19What about the one in front, up here?
0:08:19 > 0:08:20Yeah, that's a nice one.
0:08:20 > 0:08:23Right, cloud has been sourced.
0:08:23 > 0:08:26It is quite important that the helicopter itself
0:08:26 > 0:08:28doesn't go in the cloud.
0:08:28 > 0:08:32We have to remain visual with, well, pretty much everything.
0:08:33 > 0:08:39I need to fly low enough to dip the sky-sponge into the cloud
0:08:39 > 0:08:42but high enough to keep the chopper above it...
0:08:45 > 0:08:50..which is trickier than it sounds.
0:08:50 > 0:08:51'Well, for me.'
0:08:52 > 0:08:57Oh, great, well, that's all round bad.
0:08:57 > 0:09:00First time round, I miss the cloud altogether.
0:09:00 > 0:09:03This is a fairly unusual exercise, cloud collecting.
0:09:06 > 0:09:09Yeah, that's my excuse, anyway.
0:09:09 > 0:09:11This one will do a treat.
0:09:13 > 0:09:16OK, little cloud, let's see what you've got.
0:09:30 > 0:09:33Close up, the cloud seems so wispy,
0:09:33 > 0:09:37it's hard to imagine we're going to soak any water up at all.
0:09:45 > 0:09:50OK, we dipped it, let's get this thing down and see what we've got.
0:10:03 > 0:10:06Well, it's wet, that's a start...
0:10:06 > 0:10:08but how wet?
0:10:08 > 0:10:12Have we managed to collect enough moisture to make
0:10:12 > 0:10:14a difference on the scales?
0:10:19 > 0:10:23We have ten whole kilograms of difference.
0:10:25 > 0:10:28I know that doesn't sound like much,
0:10:28 > 0:10:31but look at the size of the cloud.
0:10:31 > 0:10:35Then look how much of it the sky-sponge flew through.
0:10:36 > 0:10:40Just that small section had ten kilos of water in it.
0:10:40 > 0:10:46If every section that size weighs the same, then that little cloud
0:10:46 > 0:10:49must be getting on for, well,
0:10:49 > 0:10:53not quite nine tonnes...but a lot.
0:10:55 > 0:10:58THUNDER BOOMS
0:11:00 > 0:11:04And a good-sized thunder cloud might be ten kilometres tall
0:11:04 > 0:11:07and ten kilometres wide...
0:11:10 > 0:11:15..which would make its total weight more like a million elephants.
0:11:15 > 0:11:19Or, if you prefer, about 60,000 jumbo jets.
0:11:21 > 0:11:24So, how on earth does all that weight stay up there?
0:11:26 > 0:11:31To find that out, we're going to have to build a cloud of our own.
0:11:46 > 0:11:51Right, what I've asked to achieve here is an indoor cloud.
0:11:51 > 0:11:56What I've got is a cattle trough full of water...
0:11:56 > 0:11:59and I don't know what these things are.
0:11:59 > 0:12:03Fortunately, what I've also got is Jim, who is an atmospheric scientist
0:12:03 > 0:12:04and can hopefully help.
0:12:04 > 0:12:06What is this? How's it going to work?
0:12:06 > 0:12:09So, this is how we're going to make something akin to clouds.
0:12:09 > 0:12:11- Right.- Obviously, it's not a cloud
0:12:11 > 0:12:14- but it's the closest we've got to a cloud-making machine.- Right.
0:12:14 > 0:12:18So, what we've got in here are some ultrasonic humidifiers.
0:12:18 > 0:12:21So, you quite often see these things
0:12:21 > 0:12:23in garden centres and things like that.
0:12:23 > 0:12:26- They just produce very, very fine mist.- Garden centres?
0:12:26 > 0:12:29- Garden centres.- It's just sounding less hi tech now, I'll be honest.
0:12:29 > 0:12:31They're masquerading as nice, ornamental devices
0:12:31 > 0:12:34but secretly they're cloud-making devices.
0:12:34 > 0:12:36Well, there we go. Well, come on then, make it work.
0:12:36 > 0:12:38So, all we need to do is turn this on.
0:12:41 > 0:12:43- Oh, hello!- There you go.
0:12:43 > 0:12:47Suddenly, miniature clouds appear.
0:12:47 > 0:12:51And that's just breaking the water down into smaller bits?
0:12:51 > 0:12:56We're breaking the liquid water into very, very tiny droplets of water.
0:12:57 > 0:13:03These garden pond devices turn the water into tiny droplets,
0:13:03 > 0:13:07and that is exactly how a cloud works.
0:13:08 > 0:13:13Clouds float because the water drops inside them are so small
0:13:13 > 0:13:15and so light.
0:13:15 > 0:13:17What's the difference in size?
0:13:17 > 0:13:19How big is a droplet of this compared to a droplet of water?
0:13:19 > 0:13:21So, a droplet of that is five microns,
0:13:21 > 0:13:24- but that means absolutely nothing to you.- Small?!
0:13:24 > 0:13:26OK, but a rain droplet, you can get your head round
0:13:26 > 0:13:28the size of a rain droplet.
0:13:28 > 0:13:30A rain droplet is about two millimetres.
0:13:30 > 0:13:33So the difference in size between these and the rain droplets
0:13:33 > 0:13:37is the same as if you got a sugar cube and a caravan.
0:13:37 > 0:13:39Hang on, which is the caravan?
0:13:39 > 0:13:41So, the caravan is the rain droplet.
0:13:41 > 0:13:44- Right.- And the sugar cube is these tiny little droplets.
0:13:44 > 0:13:47Right, well, that is working!
0:13:47 > 0:13:50The humidifiers have split all our caravans up
0:13:50 > 0:13:54into billions of sugar cubes.
0:13:54 > 0:13:56- OK, lid goes on.- OK.
0:13:56 > 0:13:59'But to really complete the effect, we want to see
0:13:59 > 0:14:04'if we can get those tiny moisture droplets to float in the air.'
0:14:04 > 0:14:09We'll turn the fan on now and we'll see our clouds emerge.
0:14:10 > 0:14:12And there it is!
0:14:12 > 0:14:15Weirdly, it feels dry.
0:14:15 > 0:14:19Hard to believe our sky-sponge managed to soak this stuff up.
0:14:19 > 0:14:22So, this doesn't just look like a cloud,
0:14:22 > 0:14:24this is pretty close to a cloud.
0:14:24 > 0:14:26These are just droplets of water, very, very small droplets of water,
0:14:26 > 0:14:28and that's what a cloud is.
0:14:28 > 0:14:31Jim, not being critical of your cloud,
0:14:31 > 0:14:33but it looks a lot more frantic.
0:14:33 > 0:14:37I think of clouds as just solid state, really, just drifting.
0:14:37 > 0:14:39What you're seeing here is what's happening around the edge
0:14:39 > 0:14:41of the cloud, it's constantly changing.
0:14:41 > 0:14:45- So you get up close to a cloud and it's really quite busy?- Yes.
0:14:45 > 0:14:49So, whilst I'm very impressed with your home-made cloud here,
0:14:49 > 0:14:52it's kind of not...up enough!
0:15:00 > 0:15:05Now, this might look like overkill, but actually our cattle trough
0:15:05 > 0:15:10is surprisingly heavy, just like the water in a real cloud.
0:15:13 > 0:15:18And I do need to get all that water off the ground to check
0:15:18 > 0:15:19that second fact.
0:15:21 > 0:15:24Are all clouds really falling back to earth?
0:15:27 > 0:15:30Jim and I wait with baited breath.
0:15:31 > 0:15:33We might have made the water droplets
0:15:33 > 0:15:36small enough to float but...
0:15:37 > 0:15:39..it's true,
0:15:39 > 0:15:43once they're up in the air, they drift back towards the ground.
0:15:43 > 0:15:46So this effect where I can see it rolling over the top
0:15:46 > 0:15:49and then sort of falling, that's accurate?
0:15:49 > 0:15:52Yes, our cloud is dropping out. So, if you look at clouds with
0:15:52 > 0:15:56binoculars or something like that, you'll see bits of streams of cloud.
0:15:56 > 0:15:59So, because this is small, it all looks faster,
0:15:59 > 0:16:03but if this were as big as a real cloud, this effect,
0:16:03 > 0:16:05this exact effect, is what's going on all of the time.
0:16:05 > 0:16:07Yes, just continuously all the time.
0:16:07 > 0:16:10Round the edges of clouds, round the periphery of clouds,
0:16:10 > 0:16:13you've got this going on all the time.
0:16:14 > 0:16:17So, there you have it - clouds are heavy
0:16:17 > 0:16:20and they are all falling slowly down to earth.
0:16:25 > 0:16:29It's just that most evaporate before they ever get there.
0:16:36 > 0:16:40In fact, the typical life span of a small cumulus cloud is only
0:16:40 > 0:16:43ten to 15 minutes.
0:16:44 > 0:16:46But while they're up there,
0:16:46 > 0:16:49they act as a sort of a public transport system for water,
0:16:49 > 0:16:52carrying it from one place to another...
0:16:56 > 0:16:59..until either the service goes off duty
0:16:59 > 0:17:03or they dump all their passengers out as rain.
0:17:06 > 0:17:10There are about 13 trillion tonnes of water being
0:17:10 > 0:17:12moved around in the atmosphere.
0:17:17 > 0:17:22And every day, about a tenth of that comes crashing back down to earth.
0:17:22 > 0:17:24WIND HOWLS
0:17:24 > 0:17:29Sometimes, these storms are incredibly intense.
0:17:29 > 0:17:32The quickest on record dumped 12 centimetres of water
0:17:32 > 0:17:34in just eight minutes.
0:17:35 > 0:17:39The heaviest managed nearly a metre and a half of rain
0:17:39 > 0:17:41in under ten hours.
0:17:45 > 0:17:49And so to my home territory, where, on average, it rains
0:17:49 > 0:17:52one day out of every three.
0:17:56 > 0:18:01This is my favourite place in the entire world.
0:18:01 > 0:18:03It's in the Lake District, Honister Pass,
0:18:03 > 0:18:06running down to Lake Buttermere.
0:18:06 > 0:18:10I've been coming here for 27 years.
0:18:10 > 0:18:14It has one of the best views in the world.
0:18:14 > 0:18:16I've seen it once.
0:18:16 > 0:18:23That's because this specific place is the wettest in England.
0:18:23 > 0:18:27On average, four metres of rain falls here every year.
0:18:29 > 0:18:33And yet, on the one day when I am here specifically to talk to you
0:18:33 > 0:18:37about rain, it's not actually raining!
0:18:37 > 0:18:39We've had to resort to this...
0:18:40 > 0:18:45Yes, sprinklers, in the wettest place in England.
0:18:45 > 0:18:49However, this will suffice perfectly to allow me
0:18:49 > 0:18:51to show you what I want to show you.
0:18:51 > 0:18:53Puddles.
0:18:53 > 0:18:59Puddles hold the key to seeing how those tiny cloud droplets
0:18:59 > 0:19:01turn into raindrops.
0:19:01 > 0:19:07We can't look into a cloud to see how raindrops form
0:19:07 > 0:19:10but we can get an idea of what's going on by looking in a puddle.
0:19:14 > 0:19:20As the raindrop hits, part of it is attracted to the water.
0:19:20 > 0:19:25What bounces back up is a smaller droplet about half the size.
0:19:26 > 0:19:31When that droplet hits, the same thing happens again,
0:19:31 > 0:19:33around half of it stays in the puddle.
0:19:43 > 0:19:48Now, imagine that in reverse and upside down.
0:19:50 > 0:19:52The puddle is the cloud.
0:19:54 > 0:19:59A water droplet doubles in size by attracting other water droplets.
0:20:03 > 0:20:08These stick on in a process scientists call "coalescence".
0:20:14 > 0:20:19It increases again and again until it's so heavy...
0:20:19 > 0:20:21it falls away.
0:20:23 > 0:20:27And that is, roughly, how rain is formed.
0:20:37 > 0:20:41It feels right like this - this is how it feels here.
0:20:41 > 0:20:45Which is just as well, because I've got one more thing
0:20:45 > 0:20:48I want to tell you before I get them to turn these sprinklers off.
0:20:50 > 0:20:56And it's about the official difference between rain and drizzle.
0:20:57 > 0:21:00Look closely at a puddle's surface.
0:21:00 > 0:21:05If the drops are splashing, like here, then it's rain.
0:21:08 > 0:21:10But if there are no splashes,
0:21:10 > 0:21:15then it only qualifies as drizzle, officially.
0:21:17 > 0:21:19Clever, isn't it?
0:21:19 > 0:21:21Splashes, rain.
0:21:21 > 0:21:24No splashes, drizzle.
0:21:27 > 0:21:31But what they've both got in common is that they are just too
0:21:31 > 0:21:33heavy to be held aloft.
0:21:35 > 0:21:40We talk about heavy rain but water is heavy, very heavy!
0:21:40 > 0:21:44To give us an idea of just how heavy, we are about to see what
0:21:44 > 0:21:48would happen if all of Borrowdale's four metres of water
0:21:48 > 0:21:51fell in one go.
0:21:52 > 0:21:57Obviously, we can't get a digger the size of the Lake District.
0:21:57 > 0:22:01So, we're just going to recreate what it's like
0:22:01 > 0:22:06when four metres of water hits one small area.
0:22:06 > 0:22:10So, we have four cubic metres of water in the bucket, which amounts
0:22:10 > 0:22:14to four tonnes, at height.
0:22:14 > 0:22:17Then, beneath it, you'll see we've found a car...
0:22:17 > 0:22:19for scientific purposes.
0:22:20 > 0:22:26Let's see just how much damage that amount of water can do.
0:22:30 > 0:22:33Hmm...looks like rain.
0:22:46 > 0:22:51Yeah, pretty brutal, but I shouldn't be surprised...
0:22:53 > 0:22:55..because the water actually weighed
0:22:55 > 0:22:58four times more than the car underneath it.
0:23:08 > 0:23:12Every minute of every day, 900 million tonnes of rain
0:23:12 > 0:23:14land on our planet.
0:23:18 > 0:23:22That's about the same amount of water as in all 16 lakes
0:23:22 > 0:23:24of the Lake District.
0:23:31 > 0:23:34Oh, they're going to notice!
0:23:34 > 0:23:39But it does prove the point - water is really heavy.
0:23:39 > 0:23:43That is just the annual rainfall for Borrowdale,
0:23:43 > 0:23:45where I've been going on holiday all of my life.
0:23:45 > 0:23:48Explains something about it.
0:23:48 > 0:23:51It is amazing, isn't it?
0:23:51 > 0:23:54Luckily, this could never happen with real rain.
0:23:57 > 0:24:02Not even in a tropical storm where sometimes it feels that the
0:24:02 > 0:24:04heavens have literally opened.
0:24:07 > 0:24:10Partly because, as we saw earlier,
0:24:10 > 0:24:13raindrops fall the moment they get heavy enough.
0:24:15 > 0:24:19And partly because of what happens to rain as it falls.
0:24:26 > 0:24:31To show you what I mean, I'm hard at work building a sand castle...
0:24:35 > 0:24:38..and Professor Jane Rickson from Cranfield University is
0:24:38 > 0:24:43filling a plastic bucket from a pond.
0:24:51 > 0:24:54There were always kids like you on the beach, weren't there?
0:24:56 > 0:24:57OK, so what's all this about?
0:24:57 > 0:25:02Well, pour water on a sand castle
0:25:02 > 0:25:03and you completely flatten it.
0:25:05 > 0:25:08No surprises there.
0:25:09 > 0:25:13But rain doesn't fall from waist height.
0:25:13 > 0:25:18It falls from clouds that are at least 300 metres above the ground.
0:25:18 > 0:25:21And that makes all the difference.
0:25:23 > 0:25:25Let me show you,
0:25:25 > 0:25:28by building another sand castle
0:25:28 > 0:25:32and throwing the water off something just a little bit higher.
0:25:41 > 0:25:46Now, obviously, this isn't as high as a real cloud.
0:25:46 > 0:25:51They start at around 300 metres. This tower is 30,
0:25:51 > 0:25:54but it's tall enough for what we want to do.
0:26:03 > 0:26:05OK, Richard, let it go!
0:26:14 > 0:26:16Idiot!
0:26:16 > 0:26:18Yeah, wrong side.
0:26:18 > 0:26:20How was I to know?
0:26:22 > 0:26:24Let's try it again.
0:26:28 > 0:26:31OK, Richard, let it fall!
0:26:31 > 0:26:34And so another bucketful leaves the tower...
0:26:34 > 0:26:38but what arrives below is rain.
0:26:42 > 0:26:44And there it is, it's still standing.
0:26:44 > 0:26:47So why is it if I throw the water from up there...?
0:26:47 > 0:26:49You'd think it would smash it
0:26:49 > 0:26:52to bits even more, but it's still standing. What's the difference?
0:26:52 > 0:26:55Well, what happens, as you were throwing that water down,
0:26:55 > 0:26:58air resistance, the turbulence in the air is overcoming the surface
0:26:58 > 0:27:03tension of that lump of water, breaking it into smaller drops.
0:27:03 > 0:27:06Do you want to go and see that? Shall we do it again?
0:27:06 > 0:27:09Yes...I'll get the water.
0:27:12 > 0:27:16As the water falls, it meets air resistance,
0:27:16 > 0:27:21and the larger the lump of water, the more resistance it experiences.
0:27:24 > 0:27:27That friction breaks the water up into smaller pieces,
0:27:27 > 0:27:30sometimes inflating the drops like parachutes,
0:27:30 > 0:27:32before blowing them apart.
0:27:34 > 0:27:38The further they fall, the smaller those drops become,
0:27:38 > 0:27:42until finally they're so small that air has little effect on them
0:27:42 > 0:27:45and they land as rain.
0:27:46 > 0:27:49So, that's why the water landed in drops
0:27:49 > 0:27:52and didn't smash it, rather than a big bucket-shaped lump?
0:27:52 > 0:27:55That's right, and in fact you can actually see the point
0:27:55 > 0:27:58at which that lump starts to break up into those smaller drops.
0:27:58 > 0:28:03Well, you can if I climb the tower again.
0:28:10 > 0:28:13It actually happens surprisingly quickly.
0:28:16 > 0:28:19Within ten metres, there is enough air
0:28:19 > 0:28:23blowing on our bucketful of water to break it down into drops.
0:28:24 > 0:28:28If our digger had been just a few metres higher,
0:28:28 > 0:28:32then the car might well have survived.
0:28:32 > 0:28:37So, even if it was possible for water to fall out of the sky in one
0:28:37 > 0:28:43big lump, by the time it got to the ground, it would still be rain.
0:28:45 > 0:28:48Because they break down like this,
0:28:48 > 0:28:52the average raindrop ends up about two millimetres across.
0:28:55 > 0:28:59But there is a way that water can fall out of the air in bigger,
0:28:59 > 0:29:01more dangerous pieces.
0:29:04 > 0:29:08By shape-shifting...into ice.
0:29:09 > 0:29:11Now, most of us think that
0:29:11 > 0:29:16when we see ice falling out of the sky, it's hail.
0:29:16 > 0:29:20So, what if I told you this wasn't hail at all.
0:29:20 > 0:29:24Sure, it looks like hail but it can't be hail.
0:29:24 > 0:29:28You can't get hail in winter, it only happens in summer.
0:29:28 > 0:29:31I know, you think you've seen hail in winter,
0:29:31 > 0:29:32but trust me, you haven't.
0:29:32 > 0:29:37What you've seen is this...
0:29:37 > 0:29:39an ice pellet.
0:29:39 > 0:29:42Ice pellets are formed when a snowflake partially melts
0:29:42 > 0:29:45on the way down, losing all its pretty branches.
0:29:45 > 0:29:49It then refreezes, forming a small ball before it hits the ground.
0:29:49 > 0:29:52Just to make things even more confusing,
0:29:52 > 0:29:55in North America, they call this sleet,
0:29:55 > 0:30:00which over here means a sort of slushy mix of rain and snow.
0:30:00 > 0:30:03Either way, this is not hail.
0:30:03 > 0:30:06Hail is something entirely different.
0:30:12 > 0:30:16Charles Knight has been studying hailstones
0:30:16 > 0:30:18for the last 50 years.
0:30:18 > 0:30:22And in his refrigerated laboratory, in Boulder, Colorado,
0:30:22 > 0:30:27he offers to show me exactly how hailstones are different...
0:30:27 > 0:30:29by sawing one in half.
0:30:29 > 0:30:32It's very simple, we just use a hobby band saw like this.
0:30:32 > 0:30:35- You're just going to slice it in half?- Yes.
0:30:35 > 0:30:37How long have you had that hailstone?
0:30:37 > 0:30:39Oh, about ten years, actually.
0:30:39 > 0:30:41What?!
0:30:46 > 0:30:49But it's worth it.
0:30:49 > 0:30:53As soon as Charles opens it up, the difference is revealed.
0:30:53 > 0:30:56Hail is made of layers.
0:30:56 > 0:30:58There, you can see one layer there, anyway.
0:30:58 > 0:31:01So where there's that little circle?
0:31:01 > 0:31:04Yes. On the bigger hailstones, there's much more obvious layering.
0:31:04 > 0:31:08This is an example of really what you would call a giant hailstone.
0:31:08 > 0:31:10- It's enormous!- It's enormous, yes.
0:31:12 > 0:31:15But that's obviously not going to stop him cutting it in half,
0:31:15 > 0:31:18even though this one is 15 years old.
0:31:20 > 0:31:22Oh, wow!
0:31:24 > 0:31:28This time, the layers are crystal clear.
0:31:32 > 0:31:37If you make a thin section, then you can really see the layering.
0:31:37 > 0:31:41It's a slice right through it, that's absolutely beautiful!
0:31:41 > 0:31:44That's really telling its own story, isn't it?
0:31:44 > 0:31:49Just like the rings of a tree, these layers chart the story of how
0:31:49 > 0:31:51this hailstone grew.
0:31:56 > 0:31:57It's a story that starts...
0:31:57 > 0:31:59THUNDER CRACKS
0:31:59 > 0:32:02..with a thunderstorm.
0:32:02 > 0:32:05And thunderstorms only tend to happen in summer.
0:32:07 > 0:32:11Because of the height of thunder clouds, some of the water droplets
0:32:11 > 0:32:13inside them freeze.
0:32:13 > 0:32:17But the powerful updraughts created by the warm weather
0:32:17 > 0:32:21keep the droplets supported in the cloud...
0:32:21 > 0:32:24where they collect more water,
0:32:24 > 0:32:29with new layers freezing on in a separate shell.
0:32:29 > 0:32:34Until, finally, there are so many layers that they're too heavy to
0:32:34 > 0:32:36be supported and they fall to the ground.
0:32:40 > 0:32:42Which got me thinking.
0:32:42 > 0:32:46Because it's made in layers, does that mean hail is stronger than a
0:32:46 > 0:32:48single, solid ball of ice?
0:32:48 > 0:32:52You make wood stronger by laminating it. You make glass stronger
0:32:52 > 0:32:54by laminating it.
0:32:54 > 0:32:57So, does laminating ice make it stronger?
0:32:59 > 0:33:02Certainly, hail is powerful.
0:33:02 > 0:33:07It causes over £1 billion worth of damage a year.
0:33:08 > 0:33:12But is it any harder than conventional ice?
0:33:17 > 0:33:21To find out, we're going to have to go into uncharted territory,
0:33:21 > 0:33:26with an experiment that hasn't been done before, using that.
0:33:33 > 0:33:37Yeah, I know, it looks like a lump of plastic pipe on some tables,
0:33:37 > 0:33:40in a field, and to some extent, well, it is.
0:33:40 > 0:33:43But you should see what it's about to do to that table tennis bat.
0:33:47 > 0:33:52Its inventors, Purdue University's Jim Stratton and Craig Zehrung,
0:33:52 > 0:33:54wanted to see just how fast they
0:33:54 > 0:33:57could get an ordinary ping pong ball to fly.
0:34:03 > 0:34:07And the answer, using this contraption,
0:34:07 > 0:34:09turns out to be very fast indeed.
0:34:20 > 0:34:21That is astonishing!
0:34:21 > 0:34:24This projectile is moving when it comes out of there.
0:34:24 > 0:34:26- Oh, yeah.- About 919 miles an hour.
0:34:26 > 0:34:28- That's brisk, isn't it?- Yeah.
0:34:28 > 0:34:31So, you brought along your device which, if you think about it, is a
0:34:31 > 0:34:34sort of nightmarish serving machine, and you've agreed to help us?
0:34:34 > 0:34:36Mm-hm, yeah!
0:34:36 > 0:34:37OK, right, so here's the plan.
0:34:37 > 0:34:41We're going to see which is harder, ice or hail.
0:34:41 > 0:34:44But first of all, we've got to make some hailstones.
0:34:46 > 0:34:50We've already seen how much of a faff that is,
0:34:50 > 0:34:51even for Mother Nature...
0:34:53 > 0:34:57..but luckily, Jim and Craig have a plan.
0:34:58 > 0:35:02A plan that starts with dry ice.
0:35:02 > 0:35:05It's like an '80s pop video!
0:35:05 > 0:35:10A pop video starring...a bead on a bit of string.
0:35:12 > 0:35:16The dry ice makes the bead really, really cold...
0:35:16 > 0:35:18- Two roles?- Yup.
0:35:18 > 0:35:21..before it's dropped into cold water.
0:35:21 > 0:35:23You'll notice every time he puts it in there, you can hear
0:35:23 > 0:35:26a little bit of a crack, you can hear a little bit of a fizz.
0:35:26 > 0:35:29That's the water instantaneously freezing to the outside.
0:35:29 > 0:35:32So that's one layer of ice round that little seed?
0:35:32 > 0:35:33Very small layer.
0:35:33 > 0:35:35How long does this take?
0:35:35 > 0:35:37Erm, about ten minutes.
0:35:37 > 0:35:39- Oh, God!- How many of these do we need?- Quite a few.
0:35:39 > 0:35:43'And they need to be the size of ping pong balls to fire them
0:35:43 > 0:35:45'from Jim and Craig's gun.'
0:35:45 > 0:35:47- Can I have a go?- Yeah!
0:35:47 > 0:35:48Right, dip it in here...
0:35:51 > 0:35:53..fairly quickly into there.
0:35:54 > 0:35:59That's it! Look at that! It's already the size of a...pea.
0:35:59 > 0:36:02I'm just suggesting,
0:36:02 > 0:36:05we probably need to find a way of mass-producing these.
0:36:05 > 0:36:07I mean, this is the land of Henry Ford.
0:36:07 > 0:36:10Right! One is good, we could try three.
0:36:10 > 0:36:14- Erm...- And now you've tripled your efficiency.
0:36:14 > 0:36:16Haven't I, haven't I?
0:36:18 > 0:36:22Sometimes on TV, we don't do things in actual time.
0:36:22 > 0:36:24This is one of those occasions.
0:36:38 > 0:36:40You going to do anything?
0:36:40 > 0:36:42I'm reading this.
0:36:42 > 0:36:44There's no words, you're looking at the pictures!
0:36:44 > 0:36:46- It's my turn again?- Yes!
0:36:46 > 0:36:48Whoo-hoo, you've been busy!
0:36:59 > 0:37:02Hail's ready!
0:37:02 > 0:37:04- They're done!- They are done!
0:37:04 > 0:37:07Magnificent they are as well. Look at that.
0:37:07 > 0:37:11OK, they might need a little bit of rounding off to get them
0:37:11 > 0:37:16down the barrel of the gun, but the size is good.
0:37:16 > 0:37:18Say goodbye.
0:37:18 > 0:37:20- One...- Excellent!
0:37:20 > 0:37:23- ..two.- 'Three of 30.'
0:37:23 > 0:37:25We're going to have to do some more, aren't we?
0:37:25 > 0:37:27We are, yeah. A whole bunch, yeah.
0:37:27 > 0:37:30So we have something to compare them with,
0:37:30 > 0:37:33we've also frozen some water into ordinary ice,
0:37:33 > 0:37:37using a few of Craig and Jim's spare ping pong balls as moulds.
0:37:39 > 0:37:42So, we've got solid ice
0:37:42 > 0:37:45and we've got hail, which is ice in layers.
0:37:45 > 0:37:49Time to put them up against each other to see
0:37:49 > 0:37:51if there really is a difference.
0:37:51 > 0:37:56And we can't resist starting with one of our home-made hailstones.
0:37:56 > 0:37:58I'll give you the honours. All you have to do is puncture it.
0:37:58 > 0:38:01Scoot back a little bit so we can look at the...
0:38:01 > 0:38:03Why is everybody else standing back?
0:38:03 > 0:38:05- Well, we're getting somewhere we can see.- Right.
0:38:05 > 0:38:07- Ahhh... - LAUGHTER
0:38:07 > 0:38:10What?! I've not done this before, have I?
0:38:10 > 0:38:12How wrong can it go? Are we ready?
0:38:12 > 0:38:14Yup, we're ready.
0:38:14 > 0:38:15Punching a hole in there now.
0:38:18 > 0:38:21Oh, it's quite dramatic, as it turns out! Yeah!
0:38:21 > 0:38:24Let's have a look at the footage.
0:38:24 > 0:38:28Believe it or not, we're breaking new scientific ground here.
0:38:28 > 0:38:33So, to make sure we capture any differences between the ice
0:38:33 > 0:38:37and the hail, we're recording everything at ultra high speed.
0:38:37 > 0:38:41And sure enough, our cameras capture every detail,
0:38:41 > 0:38:44from the plastic seal popping off the tube,
0:38:44 > 0:38:47to our hurtling hailstone punching through the target.
0:38:49 > 0:38:53- Look at that! - That is awesome.- Beautiful.
0:38:54 > 0:38:57Is it worth experimenting now with just seeing how much more
0:38:57 > 0:38:59resilient one is than the other?
0:38:59 > 0:39:01Yeah, we've brought plenty of materials we can shoot at.
0:39:01 > 0:39:04We can actually shoot two at the same thing and see what one will and
0:39:04 > 0:39:07won't go through and the type of force that we have in them.
0:39:07 > 0:39:09That's exactly what I was meaning... Do that.
0:39:09 > 0:39:11- All right.- Let's do it!
0:39:11 > 0:39:15So, here's the set-up. We've got lots of different sorts of wood
0:39:15 > 0:39:18and we're going to take two shots at each piece.
0:39:20 > 0:39:23First, with plain ice, then with our home-made hail.
0:39:25 > 0:39:28First up, chipboard.
0:39:28 > 0:39:31Right, three, two, one!
0:39:32 > 0:39:36Ice, straight through.
0:39:36 > 0:39:38Hail, straight through.
0:39:40 > 0:39:43OK, slightly thicker piece of chipboard.
0:39:45 > 0:39:46Same result.
0:39:46 > 0:39:48Plywood.
0:39:53 > 0:39:55The ice barely dents it.
0:39:55 > 0:39:56Come on, hail!
0:39:56 > 0:39:59Three, two, one!
0:40:01 > 0:40:03Well, there is a difference.
0:40:03 > 0:40:06The hail splintered the back of the plywood.
0:40:08 > 0:40:10Let's try a slightly thinner piece.
0:40:12 > 0:40:15This time, the ice barely makes it through.
0:40:15 > 0:40:19The hole it makes is far smaller than the projectile itself.
0:40:21 > 0:40:23Right, fingers crossed.
0:40:27 > 0:40:30- Ooh, nice!- Awesome!
0:40:30 > 0:40:32- Did it work? What happened?- It did!
0:40:32 > 0:40:34It smashed and there's your impact.
0:40:34 > 0:40:36- That's right the way through.- Yup!
0:40:36 > 0:40:38In fact, that's completely different.
0:40:40 > 0:40:45Same piece of wood, same shooting speed, different results.
0:40:45 > 0:40:49In slow-mo, you can clearly see how much of the ice ball never
0:40:49 > 0:40:52makes it through the board.
0:40:52 > 0:40:56Well, it might be crude, but that is what I'd hope we'd see.
0:40:56 > 0:40:58This mark here,
0:40:58 > 0:41:01that's from the straight ice, barely getting through.
0:41:01 > 0:41:06That is our home-made hail with its laminated layers around it.
0:41:06 > 0:41:10Clearly a more fearsome projectile.
0:41:13 > 0:41:16Both balls are made of frozen water,
0:41:16 > 0:41:20so you wouldn't expect any difference in how hard they are.
0:41:21 > 0:41:25But the layers in hail do appear to make it stronger.
0:41:28 > 0:41:34So summer hail does seem to be harder than winter ice.
0:41:41 > 0:41:46But water can shape-shift into something even more dangerous...
0:41:49 > 0:41:54..naturally quicker than hail, with a mightier punch than hail.
0:41:55 > 0:41:58And what it is might well surprise you.
0:42:08 > 0:42:12This is how most of us are used to seeing snow move.
0:42:14 > 0:42:17Delicate flakes floating gently down to earth.
0:42:21 > 0:42:24Floating so gently that a snowflake can take nearly
0:42:24 > 0:42:27an hour before it finally reaches the ground.
0:42:29 > 0:42:32Travelling at just four miles an hour,
0:42:32 > 0:42:35little more than walking speed.
0:42:38 > 0:42:44And yet snow can be the fastest form of water that there is.
0:42:45 > 0:42:48RUMBLING
0:42:52 > 0:42:56Because when it's in an avalanche, it can hit 80 miles an hour
0:42:56 > 0:42:59in six seconds flat.
0:42:59 > 0:43:03And then, well, it just keeps on accelerating.
0:43:05 > 0:43:09The fastest one ever recorded, on Mount St Helens in America,
0:43:09 > 0:43:13clocked a staggering 250 miles an hour.
0:43:16 > 0:43:22So how can snow move down a mountain faster than water can?
0:43:25 > 0:43:30Walter Steinkogler, of The Institute for Snow and Avalanche Research,
0:43:30 > 0:43:34is trying to find out how that incredible speed is possible...
0:43:35 > 0:43:40..by starting an avalanche of his own.
0:43:40 > 0:43:41- Walter.- Yes.
0:43:41 > 0:43:43Is this where it's going to happen?
0:43:43 > 0:43:45Yes, absolutely. You can see it quite nicely now.
0:43:45 > 0:43:47That's the whole slope.
0:43:47 > 0:43:49You see two spontaneous avalanches already
0:43:49 > 0:43:52and we're going to try to release the avalanches from the very top.
0:43:52 > 0:43:54Don't those two avalanches mean it's already happened?
0:43:54 > 0:43:55No, no, no, not at all,
0:43:55 > 0:43:57you see there's plenty of snow still on the slope
0:43:57 > 0:44:00and actually this is a really good indicator that there
0:44:00 > 0:44:02is the potential to produce nice avalanche.
0:44:02 > 0:44:06- When that's going on, you're going to be conducting experiments and learning.- Yes.
0:44:06 > 0:44:08This is part of an ongoing piece of work for you, isn't it?
0:44:08 > 0:44:10It is, it's actually my part of my PhD thesis,
0:44:10 > 0:44:13- and this data is really essential for my work, yes.- Right.
0:44:16 > 0:44:19There are several different types of avalanche,
0:44:19 > 0:44:24but the fastest by far is what's known as a "dry powder avalanche".
0:44:26 > 0:44:29And that's the type we're hoping to get.
0:44:31 > 0:44:35If he can trigger a dry powder avalanche, Walter can find out
0:44:35 > 0:44:40more about how they move so fast, and we've offered to help...
0:44:40 > 0:44:44by putting a barrage of slow motion cameras on the slope.
0:44:44 > 0:44:46We're not going to mess with your PhD?
0:44:46 > 0:44:49I will tell you afterwards, but I would appreciate it if you don't.
0:44:49 > 0:44:52I won't. If I do, send him the bill.
0:44:52 > 0:44:54- I send to this guy?- Graham.- Perfect.
0:44:54 > 0:44:56He's in charge, I'm not.
0:44:56 > 0:44:58Let's hope it doesn't come to that.
0:45:00 > 0:45:03But I would like to add an extra element into his experiment.
0:45:03 > 0:45:08So, Walter, can I place these on the slope?
0:45:08 > 0:45:11If they're a known distance apart, I thought I could time
0:45:11 > 0:45:13when the front, the head...
0:45:13 > 0:45:15Yes, we call it the "front".
0:45:15 > 0:45:17..the front of the avalanche passes one of these,
0:45:17 > 0:45:20I can time it over that distance and I can work out how fast it's going.
0:45:20 > 0:45:23Sure, that's a nice approach. You can do that, yeah.
0:45:23 > 0:45:25Thank you very much. Right, we'll do it.
0:45:25 > 0:45:27Erm, I just need a helicopter.
0:45:36 > 0:45:40OK, well, that's that sorted, but now we need to work out how to
0:45:40 > 0:45:44fly our fences into precise positions without triggering
0:45:44 > 0:45:46an avalanche ourselves.
0:45:49 > 0:45:52Our safety team have been thinking long
0:45:52 > 0:45:54and hard about the best way to do it.
0:45:56 > 0:46:00And what they've come up with is dangling someone on a bit of rope.
0:46:03 > 0:46:05This someone, in fact,
0:46:05 > 0:46:09who apparently enjoys this kind of thing.
0:46:26 > 0:46:30That is the single coolest thing I have ever witnessed.
0:46:30 > 0:46:32That man is, without a doubt,
0:46:32 > 0:46:35the best helicopter pilot I've ever seen in action.
0:46:38 > 0:46:41I mean, that sky-sponge was difficult enough.
0:46:43 > 0:46:47Just to be flying that close to mountains
0:46:47 > 0:46:53and sheer rock faces in this gusty, windy, changeable weather.
0:46:53 > 0:46:56Just that, let alone with another bloke dangling from a piece of rope
0:46:56 > 0:47:01below you, and then below that, a huge, well, basically wooden sail.
0:47:01 > 0:47:04I'm speechless! Genuinely speechless!
0:47:07 > 0:47:12Walter has told us where he expects the avalanche to fall.
0:47:12 > 0:47:16So we position the first fence slap bang in its path.
0:47:27 > 0:47:31But the conditions up here are very changeable...
0:47:33 > 0:47:37..as we discover when we try to fly the second fence in.
0:47:40 > 0:47:45Suddenly, the winds quicken and start to gust alarmingly.
0:47:50 > 0:47:54At any moment, the whole fence could be dashed into the side of
0:47:54 > 0:47:58the mountain, taking that bloke with it...
0:48:00 > 0:48:02..not to mention the helicopter.
0:48:07 > 0:48:12And the fence needs to be exactly 100 metres from the first one.
0:48:15 > 0:48:17Never have the words "rather him than me"
0:48:17 > 0:48:20been more directly applicable.
0:48:23 > 0:48:24It's down.
0:48:29 > 0:48:31So, everything is now in place.
0:48:31 > 0:48:34My two boards, I know, are 100 metres apart.
0:48:34 > 0:48:37When the front of the avalanche passes the first one,
0:48:37 > 0:48:39I'll start the stopwatch on my phone, stop it
0:48:39 > 0:48:42when it passes the second and we'll get an idea of the speed.
0:48:42 > 0:48:44And I do know we're going to be surprised at how something that...
0:48:44 > 0:48:47a little snowflake that can take an hour to drift down out
0:48:47 > 0:48:52of the sky can suddenly be part of something so fast and so powerful.
0:48:58 > 0:49:03All we have to do now is wait for them to trigger it.
0:49:03 > 0:49:05THEY SPEAK GERMAN
0:49:19 > 0:49:21LOUD EXPLOSION
0:49:23 > 0:49:25LOUD EXPLOSION
0:49:29 > 0:49:31LOUD EXPLOSION
0:49:32 > 0:49:34LOUD EXPLOSION
0:49:35 > 0:49:37That obviously is the explosives.
0:49:45 > 0:49:48RUMBLING
0:49:48 > 0:49:51OK, we're off. Fence one.
0:49:51 > 0:49:52Fence two.
0:49:54 > 0:49:55Oh!
0:49:57 > 0:49:59Well, my boards have gone...
0:50:04 > 0:50:06I missed it.
0:50:08 > 0:50:11But I suppose it does prove, in a way,
0:50:11 > 0:50:14just how fast an avalanche can be.
0:50:14 > 0:50:18And luckily for me, our slow-motion cameras captured everything.
0:50:19 > 0:50:23So, let's take a look at that avalanche again.
0:50:23 > 0:50:28This is the moment the dynamite is dropped from the helicopter,
0:50:28 > 0:50:34causing this explosion at the top of the mountain.
0:50:34 > 0:50:38Immediately, it's surrounded by a powder cloud,
0:50:38 > 0:50:43made up of 1% snow and 99% air.
0:50:43 > 0:50:46This is a dry powder avalanche.
0:50:49 > 0:50:52The avalanche accelerates down the steep incline
0:50:52 > 0:50:55until it reaches our first fence.
0:50:55 > 0:50:59Though not exactly at the angle we expected.
0:51:00 > 0:51:03The leading edge passes the first one now.
0:51:05 > 0:51:08And that particular bit of snow
0:51:08 > 0:51:12reaches the second fence...now.
0:51:12 > 0:51:17Almost exactly the same time the first fence is destroyed.
0:51:17 > 0:51:20No wonder I had trouble timing it.
0:51:20 > 0:51:26Our avalanche was actually only travelling at 25 miles an hour,
0:51:26 > 0:51:31just a tenth of the speed of the fastest one ever measured.
0:51:31 > 0:51:37But still faster than if we'd just pushed that snow over a cliff.
0:51:38 > 0:51:41I want to know how that's possible.
0:51:43 > 0:51:45Let's imagine there's a chunk of snow at the top
0:51:45 > 0:51:46and then is starts to move.
0:51:46 > 0:51:51What's happening to that snow from the moment it starts to move down?
0:51:51 > 0:51:53Well, first, it will break into pieces
0:51:53 > 0:51:56and it gets rounded a bit and it also gets compressed.
0:51:56 > 0:51:59And these are the pieces which you can see up there,
0:51:59 > 0:52:01they look like snow balls?
0:52:01 > 0:52:04Technically, most of them they are snowballs, yes.
0:52:04 > 0:52:08These snowballs are the secret of what's going on underneath
0:52:08 > 0:52:10that powder cloud.
0:52:10 > 0:52:12Walter offers to show me how.
0:52:16 > 0:52:19OK, Walter, this is like an avalanche, how?
0:52:19 > 0:52:23Well, you imagine an avalanche is moving down a slope,
0:52:23 > 0:52:26it's going to pick up snow like you're doing now
0:52:26 > 0:52:29and it's going to put it in motion, as in our tumbler here.
0:52:31 > 0:52:33It seems you're losing your motivation, come on!
0:52:33 > 0:52:36Keep on going, one more, you can do it, you can do it! Come on, Richard!
0:52:36 > 0:52:40Perfect, I think we're good there. You can see already it's compacting,
0:52:40 > 0:52:43that it's breaking apart again, that it's compacting again.
0:52:43 > 0:52:47And at some point, you will end up with ball-shaped features.
0:52:47 > 0:52:50It is magically making snowballs, a cement mixer full of snowballs.
0:52:50 > 0:52:52We make snowballs.
0:52:52 > 0:52:55Of course, in an avalanche, this is happening much faster
0:52:55 > 0:52:58and it's a much more violent process going on there.
0:52:58 > 0:53:00But this is a slowed-down version of exactly the same process
0:53:00 > 0:53:03and you can see that kind of grinding, rolling motion
0:53:03 > 0:53:06- that you can imagine happening in an avalanche.- Perfect.
0:53:06 > 0:53:08That's exactly the case, true.
0:53:09 > 0:53:12So, understanding this will allow you to understand more about how
0:53:12 > 0:53:16fast it might go, where it might go, how it will behave?
0:53:16 > 0:53:18Absolutely. I would say they are quite done, yeah, yeah.
0:53:18 > 0:53:20- Turn it off? - Yes, turn it off, please.
0:53:22 > 0:53:25So in here, snowballs.
0:53:25 > 0:53:27Perfect snowballs, right? Aren't they?
0:53:27 > 0:53:28I mean, that's seriously packed.
0:53:28 > 0:53:30It's quite hard, right?
0:53:30 > 0:53:33I mean, it wouldn't be that nice to throw it at a person.
0:53:33 > 0:53:36Are you looking over there and thinking targets? Cos I was.
0:53:36 > 0:53:39Those cross-country runners? Come on, do it, do it!
0:53:40 > 0:53:43He was scared for a second. Did you see?
0:53:45 > 0:53:49Walter wants to excavate the avalanche to see how much
0:53:49 > 0:53:53snow it contained, and I follow him into, well,
0:53:53 > 0:53:55a big hole, because I want to
0:53:55 > 0:53:58be sure whether it's these snowballs
0:53:58 > 0:54:01that make the avalanche move so fast.
0:54:01 > 0:54:05This is not easy to answer because it's still ongoing research.
0:54:05 > 0:54:08But, for sure, it defines the motion of the avalanche.
0:54:08 > 0:54:12So, you can't say for definite yet as scientists, and I love it
0:54:12 > 0:54:14when you guys can't give a definitive answer...
0:54:14 > 0:54:16Yes, I cannot, because it's my research.
0:54:16 > 0:54:19And if I say it now... I mean, I have to publish this stuff first.
0:54:19 > 0:54:22So would you ever end up with your avalanche effectively
0:54:22 > 0:54:26rolling along on ball bearings? Or like when they used to build...
0:54:26 > 0:54:29they'd get a huge stone and move it to one place to put it up as a monument,
0:54:29 > 0:54:33they'd roll it along on logs, wouldn't they? Is it like that?
0:54:33 > 0:54:36I think you can kind of say it like that, yeah.
0:54:36 > 0:54:39From a scientific point of view, I'm not 100% sure.
0:54:39 > 0:54:41- You think that's rubbish don't you?- No, no, no.
0:54:41 > 0:54:44Be honest! Come on, you're being all like scientific...
0:54:44 > 0:54:46just say it's rubbish!
0:54:46 > 0:54:50No, there are studies that say that really it's the ratio between
0:54:50 > 0:54:54the bigger grains or the bigger balls to the smaller balls that can
0:54:54 > 0:54:59significantly influence the speed and the motion of the avalanche.
0:54:59 > 0:55:01So, you're not that far off, actually.
0:55:01 > 0:55:04- You're just jealous because it was my idea.- Yes, but, you know...
0:55:04 > 0:55:07You can publish that, actually, it would be something for you.
0:55:07 > 0:55:08Would I have to write it up?
0:55:08 > 0:55:10Yeah, but you could do research.
0:55:10 > 0:55:14I can't be bothered, it'll take ages, you can have it, it's yours.
0:55:16 > 0:55:21Put something on wheels and it can accelerate quicker than
0:55:21 > 0:55:22if you simply drop it.
0:55:24 > 0:55:29And these snowballs may be the wheels of a dry powder avalanche.
0:55:36 > 0:55:41Snow is the softest, lightest way that water can fall to earth.
0:55:43 > 0:55:49But an avalanche can move faster than any other type of water.
0:55:49 > 0:55:55Four times faster than the fastest flash flood ever measured,
0:55:55 > 0:55:58and it seems snowballs might well be the secret.
0:56:03 > 0:56:08Of all the water on our blue planet, only a tiny fraction is
0:56:08 > 0:56:10actually in the atmosphere.
0:56:12 > 0:56:15Yet water's incredible powers of transformation mean that
0:56:15 > 0:56:21that's enough to bring us all our clouds, rain, hail and snow...
0:56:23 > 0:56:27..and with it, all the everyday weather on Earth.
0:56:32 > 0:56:34In the final episode,
0:56:34 > 0:56:38I investigate the one thing that drives all our weather...
0:56:38 > 0:56:41temperature.
0:56:41 > 0:56:43I discover how you can be struck by lightning...
0:56:43 > 0:56:46but you can also be hit by thunder.
0:56:46 > 0:56:47- BOOM - Ohhh!
0:56:49 > 0:56:52I witness the mystery of an ice storm.
0:56:52 > 0:56:55This is strangely addictive!
0:56:55 > 0:56:59And I start my very own dust storm...
0:56:59 > 0:57:01I hope I don't trigger an international incident!
0:57:01 > 0:57:04..to find out how it's possible for
0:57:04 > 0:57:08sand to travel halfway round the globe.
0:57:08 > 0:57:10Seriously, it's gone!
0:57:12 > 0:57:15You can find out more about Wild Weather with
0:57:15 > 0:57:18The Open University's free wall poster.
0:57:18 > 0:57:23Call 0845 030 3045 or go to...
0:57:27 > 0:57:30..and follow the links to The Open University.