0:00:10 > 0:00:11Over the last few years,
0:00:11 > 0:00:13Britain's weather has become more extreme...
0:00:17 > 0:00:19..especially our winters.
0:00:25 > 0:00:28Last winter was the wettest on record.
0:00:28 > 0:00:30Dad, look out behind you!
0:00:31 > 0:00:35Deadly storms battered Britain for months,
0:00:35 > 0:00:37causing misery for millions.
0:00:41 > 0:00:45Before that, we had a run of cold winters,
0:00:45 > 0:00:49record-breaking temperatures with bitter lows of minus 22.
0:00:52 > 0:00:56Now there are big questions everyone wants answered.
0:00:56 > 0:00:59Why is our weather getting more extreme?
0:00:59 > 0:01:01Can we expect more of it in the future?
0:01:03 > 0:01:06And has it got anything to do with climate change?
0:01:27 > 0:01:29- I'm Helen Czerski. - I'm John Hammond.
0:01:29 > 0:01:32Together we're going to try and make sense
0:01:32 > 0:01:34of Britain's recent extreme weather.
0:01:34 > 0:01:37And find out what's behind these unusual events and
0:01:37 > 0:01:40is there more extreme weather on the way?
0:01:43 > 0:01:46Hi, Studio E, can you hear me? It's John in the weather centre.
0:01:46 > 0:01:48Yeah, got two and a half minute weather for you.
0:01:48 > 0:01:50How long until me?
0:01:50 > 0:01:5230 seconds. OK, fine.
0:01:52 > 0:01:54'I'm a meteorologist.'
0:01:54 > 0:01:56Hello there, plenty of fine weather to come in the outlook
0:01:56 > 0:01:59but those temperatures, though, aren't high, are they?
0:01:59 > 0:02:01Five or six degrees in parts of the Midlands...
0:02:01 > 0:02:04'And I'm going to find out if there's anything that connects all
0:02:04 > 0:02:07'the different types of recent extreme weather we've had.'
0:02:07 > 0:02:09..regularly on our BBC weather website,
0:02:09 > 0:02:12I'll be back with more detail on UK weather in half an hour's time.
0:02:12 > 0:02:16- Lower, lower, lower, lower, that's it, that's it.- I can't go any lower.
0:02:16 > 0:02:17'And I'm a physicist.'
0:02:17 > 0:02:18Off we go.
0:02:18 > 0:02:21'I'm going to investigate the underlying causes
0:02:21 > 0:02:23'of our extreme weather.'
0:02:23 > 0:02:24Wow, that's fast.
0:02:25 > 0:02:30Together we want to find out if our recent extreme weather
0:02:30 > 0:02:32will become our normal weather in the future.
0:02:36 > 0:02:40To get to grips with Britain's recent extreme winters,
0:02:40 > 0:02:42you need to understand what makes our weather so unusual
0:02:42 > 0:02:45in the first place.
0:02:45 > 0:02:48I've been forecasting the weather now for over 20 years
0:02:48 > 0:02:51and, for me, it's the unpredictability of our weather
0:02:51 > 0:02:53which makes it so interesting.
0:02:54 > 0:03:00Here in the UK, it can change hour by hour, minute by minute sometimes.
0:03:00 > 0:03:04It's such a challenge to accurately predict
0:03:04 > 0:03:06what the weather is going to do next.
0:03:08 > 0:03:10We all like to moan about our weather.
0:03:10 > 0:03:13It changes constantly and it's very hard to predict.
0:03:14 > 0:03:16But there's a good reason for that.
0:03:17 > 0:03:21It's all down to Britain's unique position on the planet.
0:03:21 > 0:03:24Above our heads is a battleground,
0:03:24 > 0:03:28a constant struggle for supremacy between different types of air.
0:03:28 > 0:03:30Now most places in the world aren't like this.
0:03:30 > 0:03:34They're dominated by one or maybe two air masses
0:03:34 > 0:03:36but here in the UK we have to cope with four.
0:03:39 > 0:03:43During winter, these four major air masses are the Arctic air mass
0:03:43 > 0:03:46bringing cold, snowy weather from the Arctic...
0:03:47 > 0:03:48..the polar continental air mass
0:03:48 > 0:03:52dragging bitter winds in from Siberia,
0:03:52 > 0:03:54the maritime air mass tracking over the Atlantic
0:03:54 > 0:03:57bringing mild, wet weather,
0:03:57 > 0:04:01and the tropical air mass bringing warm air up from the south.
0:04:04 > 0:04:08No air mass dominates our weather for long which is one reason
0:04:08 > 0:04:10why it's constantly changing.
0:04:11 > 0:04:14But which air mass dominates isn't just down to chance.
0:04:16 > 0:04:19There is one factor that plays a major role in controlling
0:04:19 > 0:04:21which air mass sits over Britain.
0:04:23 > 0:04:26What determines which air mass dominates
0:04:26 > 0:04:30and the type of weather we get in the UK is a phenomenon
0:04:30 > 0:04:33which lies around 10 kilometres up in the atmosphere.
0:04:33 > 0:04:37It's called the jet stream, a high speed river of air
0:04:37 > 0:04:42which circles the globe at speeds of well over 100mph.
0:04:44 > 0:04:49Because the jet stream dictates the type of weather we get in Britain,
0:04:49 > 0:04:52it is the main suspect behind our recent extreme winters.
0:04:57 > 0:05:00Last winter was wet and stormy...
0:05:02 > 0:05:06..because the jet stream brought in the maritime air mass and with it,
0:05:06 > 0:05:08wet and windy weather.
0:05:11 > 0:05:14What was unusual was the persistence of this weather pattern...
0:05:16 > 0:05:19..as it dominated for weeks on end.
0:05:22 > 0:05:25But the previous winters brought bitterly cold weather.
0:05:28 > 0:05:31Between 2008 and 2011,
0:05:31 > 0:05:35the jet stream brought air masses in from the north and east,
0:05:35 > 0:05:39so Britain shivered under cold Arctic and Siberian winds.
0:05:43 > 0:05:46And again what was unusual was how long this cold air
0:05:46 > 0:05:48stayed over Britain.
0:05:53 > 0:05:56Whether our winters were wet or cold,
0:05:56 > 0:05:59they all had one thing in common -
0:05:59 > 0:06:03one of the four major air masses got stuck over Britain,
0:06:03 > 0:06:05resulting in extreme weather.
0:06:10 > 0:06:14It suggested the jet stream was doing something strange.
0:06:15 > 0:06:18Scientists wanted to understand more about its behaviour...
0:06:20 > 0:06:23..but it was a challenge because it's an elusive phenomenon.
0:06:23 > 0:06:25Just finding it can be a struggle.
0:06:28 > 0:06:30There are four different jet streams
0:06:30 > 0:06:33all snaking their way around the planet.
0:06:35 > 0:06:38The one that affects us is the polar front jet,
0:06:38 > 0:06:42seen here in red and orange lines sweeping over the country.
0:06:43 > 0:06:46Its path constantly changes,
0:06:46 > 0:06:50getting weaker or stronger from one day to the next
0:06:50 > 0:06:54and that makes it hard to predict its behaviour beyond a few days ahead.
0:06:57 > 0:07:00But there is a way to track its location and speed.
0:07:02 > 0:07:05We live in a world full of sophisticated technology
0:07:05 > 0:07:07for monitoring our weather
0:07:07 > 0:07:09and keeping an eye on things like the jet stream.
0:07:09 > 0:07:13So, for example, we have satellites and radar but there's no substitute
0:07:13 > 0:07:15for actually being up there at the place in the sky where
0:07:15 > 0:07:18the weather's happening and the piece of kit that gets you there is
0:07:18 > 0:07:21crucial for meteorologists
0:07:21 > 0:07:25and it's this - a very, very large balloon.
0:07:28 > 0:07:30So that just clips on like that.
0:07:30 > 0:07:32There you go, let's throw it out.
0:07:32 > 0:07:34So attach the balloon to the line.
0:07:35 > 0:07:37'Today I'm launching a weather balloon
0:07:37 > 0:07:40'with the help of Sam Howett from the Met Office.'
0:07:40 > 0:07:42Right, and then just inflate it.
0:07:48 > 0:07:51'In Britain, weather balloons are manually launched twice a day,
0:07:51 > 0:07:55'one of them from here in Camborne in Cornwall.'
0:07:56 > 0:07:59- So, yeah, that's the parachute. - It does look a tiny bit like
0:07:59 > 0:08:00the sort of thing I used to make as a kid
0:08:00 > 0:08:03when I was dropping things off the balcony at the top of the stairs.
0:08:03 > 0:08:07Absolutely, yeah. I'll just lay that out like that.
0:08:07 > 0:08:09The magic bit of string.
0:08:11 > 0:08:15'The data collected by these balloons is vital for forecasting
0:08:15 > 0:08:18'the daily weather across the whole of Britain.'
0:08:18 > 0:08:20In you get.
0:08:20 > 0:08:22Should stop laughing at your parachute here.
0:08:22 > 0:08:24I know, it's great, isn't it?
0:08:25 > 0:08:27The balloon is made of latex
0:08:27 > 0:08:30and it's filled with helium which is making it buoyant and that buoyancy
0:08:30 > 0:08:33will carry it upwards when it leaves the ground
0:08:33 > 0:08:35at five to six metres every second
0:08:35 > 0:08:37so it's going to go up really, really quickly.
0:08:37 > 0:08:41And as it goes up, it'll expand because there's less air higher up
0:08:41 > 0:08:46and somewhere way up there, some little flaw in the latex
0:08:46 > 0:08:49will give way and it will pop and the parachute will carry the payload
0:08:49 > 0:08:54back down to Earth but by that time we'll already have the data back here
0:08:54 > 0:08:58and we'll know whether the jet stream is over the top of us today.
0:08:58 > 0:09:00Right, it's all yours.
0:09:00 > 0:09:02Don't let go.
0:09:02 > 0:09:05- Lower, lower, lower, lower, that's it, that's it.- I can't go any lower.
0:09:05 > 0:09:08OK, go on. Go on, keep going. That's it, OK.
0:09:08 > 0:09:12OK. So grip it quite tightly and come out this way.
0:09:14 > 0:09:16Keep coming, keep coming. OK.
0:09:16 > 0:09:20- OK, let it go.- OK, ready? - Yeah, go!- Off we go.
0:09:27 > 0:09:30And off it goes. Wow, that's fast.
0:09:34 > 0:09:35It shows how strong the wind is
0:09:35 > 0:09:38- because it's basically gone off at 45 degrees.- Yeah, absolutely.
0:09:38 > 0:09:40It hasn't gone straight up at all.
0:09:47 > 0:09:49Right now, the balloon's rising through the troposphere
0:09:49 > 0:09:51which is the lower level of the atmosphere
0:09:51 > 0:09:55where most of our weather happens and up near the top of that layer,
0:09:55 > 0:09:58that's where the jet stream runs, about 10 kilometres up.
0:09:58 > 0:10:01And as the balloon keeps going up beyond that,
0:10:01 > 0:10:04it'll hit the next layer of the atmosphere which is the stratosphere.
0:10:18 > 0:10:20The troposphere is around 10 kilometres thick.
0:10:20 > 0:10:25It's turbulent and this is where most of our weather happens
0:10:25 > 0:10:27but this layer, the one above, the stratosphere,
0:10:27 > 0:10:33is much more stable because the air's thin and dry
0:10:33 > 0:10:38but what's really critical is the boundary between these two layers
0:10:38 > 0:10:42because this is where the jet stream can be found.
0:10:48 > 0:10:51But it's hard to predict the exact route that the jet stream
0:10:51 > 0:10:54is taking along that boundary on any given day.
0:10:55 > 0:10:58'So did the jet stream pass over us today?'
0:10:59 > 0:11:02Well, this is the trace so far.
0:11:02 > 0:11:07You can just see the balloon is giving us data every two seconds.
0:11:07 > 0:11:10- That's these new little green dots coming in?- That's right,
0:11:10 > 0:11:13and it's just about approaching 20 kilometres at the moment.
0:11:13 > 0:11:16So, right now, where the balloon is is above where we'd expect
0:11:16 > 0:11:18the jet stream to be, so we've gone right through that region
0:11:18 > 0:11:21- and not seen anything? - Unfortunately not.
0:11:21 > 0:11:23If we look at about 10 kilometres, we can see there's not that
0:11:23 > 0:11:26much of a variation so the jet stream's not above us today.
0:11:26 > 0:11:29If this was a day when the weather balloon did go through
0:11:29 > 0:11:32the jet stream, we'd expect to see really high wind speeds out here?
0:11:32 > 0:11:35That's right. As we can see here, the wind speed,
0:11:35 > 0:11:40it's fairly constant between five and 23 metres a second
0:11:40 > 0:11:42and with the jet stream you'd expect
0:11:42 > 0:11:45a wind speed of between 40 and 50 metres a second.
0:11:48 > 0:11:51The weather balloon we launched today didn't go through the jet stream.
0:11:51 > 0:11:55It went just to one side of it, but it did go really high up
0:11:55 > 0:11:59in the atmosphere, 35 kilometres up into the sky before it popped
0:11:59 > 0:12:02and the data that it sent back is already at the Met Office.
0:12:02 > 0:12:05It's only three hours since it was launched
0:12:05 > 0:12:08but the data is already being incorporated into the models
0:12:08 > 0:12:11and it'll be used for the weather forecasts that go out tonight.
0:12:13 > 0:12:15The fact that I didn't see the jet stream today
0:12:15 > 0:12:18just goes to show how fickle its path is.
0:12:20 > 0:12:24But knowing what it's doing is absolutely critical because
0:12:24 > 0:12:28this is the key suspect behind last winter's extreme weather.
0:12:34 > 0:12:37There were at least 12 major storms last winter.
0:12:39 > 0:12:44The first on the 5th of December had wind gusts reaching 142mph.
0:12:47 > 0:12:50But it wasn't just strong winds we had to battle with.
0:12:52 > 0:12:54These rain radar images show the extent
0:12:54 > 0:12:56and intensity of the wet weather.
0:13:02 > 0:13:04In January, the south of England
0:13:04 > 0:13:09received almost three times its normal rainfall
0:13:09 > 0:13:12and as the frequency of storms increased,
0:13:12 > 0:13:15it led to the wettest winter on record.
0:13:21 > 0:13:25'So why were there so many storms with so much rain?
0:13:27 > 0:13:30'And how did they get so powerful?'
0:13:30 > 0:13:32So the jet's been driving this active cold front
0:13:32 > 0:13:34across the country today but then...
0:13:34 > 0:13:37'The answers lie with what the jet stream was doing.'
0:13:39 > 0:13:41This is the jet stream from last winter.
0:13:41 > 0:13:43The most important thing to see is that it's heading
0:13:43 > 0:13:45straight across the Atlantic.
0:13:45 > 0:13:49Now, normally the wind speeds within the jet stream
0:13:49 > 0:13:52are around 100 to 150mph but last winter,
0:13:52 > 0:13:54the speeds reached almost 300mph
0:13:54 > 0:13:58so that's twice the normal speed and the jet stream
0:13:58 > 0:14:01was heading straight towards us and it was this which delivered
0:14:01 > 0:14:06storm after storm after storm so quickly, one after the other.
0:14:08 > 0:14:11The key to understanding last winter's stormy weather
0:14:11 > 0:14:15was figuring out why the jet stream had got so fast.
0:14:30 > 0:14:32One of the first clues began to emerge
0:14:32 > 0:14:36when scientists realised that something strange had been going on
0:14:36 > 0:14:38in the Atlantic before the winter began.
0:14:48 > 0:14:52It concerned one of nature's most deadly and powerful weapons...
0:14:54 > 0:14:56..hurricanes.
0:15:02 > 0:15:05I'm here in Miami and I've come to visit Eric Uhlhorn
0:15:05 > 0:15:09who's been studying hurricanes for some 15 years or so
0:15:09 > 0:15:11and I want to speak to him because something very unusual
0:15:11 > 0:15:14happened with the hurricane season of 2013.
0:15:14 > 0:15:16I want to find out more.
0:15:22 > 0:15:26'Inside this laboratory, I'm hoping to find the first clue
0:15:26 > 0:15:29'behind last winter's fast jet stream.
0:15:30 > 0:15:34'It all starts with the unique way that hurricanes interact with
0:15:34 > 0:15:37'the sea as they track over its surface.'
0:15:39 > 0:15:42As hurricanes come across the Atlantic,
0:15:42 > 0:15:43typically they mix up the cold water
0:15:43 > 0:15:47and behind the storm, as the storm tracks across the ocean,
0:15:47 > 0:15:51you'd typically see what we call a cold wake.
0:15:51 > 0:15:53What you're looking at here is a sea surface temperature map
0:15:53 > 0:15:55of the North Atlantic.
0:15:55 > 0:15:57You can see the United States here, here's Florida,
0:15:57 > 0:16:00these are warm ocean waters in orange
0:16:00 > 0:16:02and these are the colder ocean waters.
0:16:02 > 0:16:04And what you see is a hurricane
0:16:04 > 0:16:06tracking across the Atlantic right here
0:16:06 > 0:16:09and you can see that it leaves a scar of cool water
0:16:09 > 0:16:12behind the storm as it mixes up that cold water below the surface.
0:16:14 > 0:16:18And it's typically about 200 to 300 kilometres or so across
0:16:18 > 0:16:21and it can last for several weeks after the storm.
0:16:22 > 0:16:26So how much cooler does it actually get within that scarring,
0:16:26 > 0:16:28that track behind the hurricane?
0:16:28 > 0:16:31Typically we see ocean temperatures cool
0:16:31 > 0:16:34about three to five degrees Celsius behind the storm.
0:16:34 > 0:16:38If just one hurricane can have such a dramatic effect
0:16:38 > 0:16:42on the upper ocean, what happens when a whole season of hurricanes
0:16:42 > 0:16:45power their way across the Atlantic?
0:16:45 > 0:16:48In 2012, we had a very active hurricane season with
0:16:48 > 0:16:52ten hurricanes which helped to cool the water across the North Atlantic.
0:16:54 > 0:16:58But last year, there weren't many hurricanes.
0:16:58 > 0:17:01In 2013, we only saw two hurricanes.
0:17:01 > 0:17:04What we see are significantly warmer temperatures
0:17:04 > 0:17:06compared to average than we saw in 2012.
0:17:13 > 0:17:17So a lack of hurricanes may have resulted in areas of the Atlantic
0:17:17 > 0:17:19being warmer than average.
0:17:24 > 0:17:27But what's puzzling is how could warmer waters in the Atlantic
0:17:27 > 0:17:30produce last winter's super-fast jet?
0:17:36 > 0:17:39It turns out that the speed of the jet stream
0:17:39 > 0:17:42is driven in part by temperature differences between cold air
0:17:42 > 0:17:46over the poles and warm air over the tropics.
0:17:50 > 0:17:52When the temperature difference between these two regions
0:17:52 > 0:17:56is very big, the jet stream tends to travel very fast.
0:18:01 > 0:18:05So last year's warmer than average temperatures in the Atlantic
0:18:05 > 0:18:08may have increased this temperature gradient...
0:18:12 > 0:18:15..which could have produced a fast jet.
0:18:17 > 0:18:20We had warmer temperatures in 2013
0:18:20 > 0:18:23so you may see some large temperature gradient
0:18:23 > 0:18:26between the North Atlantic and the Arctic region
0:18:26 > 0:18:28which may then impact the atmosphere
0:18:28 > 0:18:31and therefore develop a large temperature gradient
0:18:31 > 0:18:34which then can potentially drive a stronger jet stream.
0:18:42 > 0:18:46Well, it makes sense to me as a meteorologist that
0:18:46 > 0:18:50something as hugely energetic as a hurricane can have a big influence
0:18:50 > 0:18:54on the system, if you like, the atmosphere and the ocean's system.
0:18:54 > 0:18:56Of course there are so many other factors involved
0:18:56 > 0:18:58and that's the challenge of meteorology.
0:18:58 > 0:19:01We can't make a direct link but it's very intriguing.
0:19:01 > 0:19:06And it poses the question, could the lack of hurricanes in 2013
0:19:06 > 0:19:10have played a role in producing such a strong jet stream?
0:19:12 > 0:19:14A few weeks after I met up with Eric,
0:19:14 > 0:19:16scientists discovered the answer.
0:19:19 > 0:19:23It turned out that the impact of a lack of hurricanes last year
0:19:23 > 0:19:26wasn't big enough to have turbo-charged the jet stream.
0:19:27 > 0:19:30So the abnormal hurricane season was a red herring.
0:19:33 > 0:19:37Although the jet stream still remained the number one suspect,
0:19:37 > 0:19:41the hunt to discover why it got so fast would have to start again.
0:19:51 > 0:19:54The next clue was hidden away in an event
0:19:54 > 0:19:56that happened over 130 years ago.
0:20:02 > 0:20:07In 1883, the Krakatoa volcano in Indonesia erupted.
0:20:09 > 0:20:13Around 40,000 people died
0:20:13 > 0:20:17and ash and dust were hurled 35 kilometres into the air.
0:20:24 > 0:20:27But there was an unexpected side effect to this cataclysm,
0:20:27 > 0:20:29felt around the world.
0:20:43 > 0:20:45These spectacular crayon sketches
0:20:45 > 0:20:49were done on the banks of the Thames in Chelsea in London
0:20:49 > 0:20:53and I've never seen the sky over London look like this.
0:20:53 > 0:20:56These were done on a very specific date in 1883
0:20:56 > 0:20:58by the painter William Ashcroft.
0:20:59 > 0:21:04They look like the sky's on fire, really bright red,
0:21:04 > 0:21:07and the events that caused it were on the other side of the world,
0:21:07 > 0:21:10the eruption of the volcano Krakatoa.
0:21:16 > 0:21:19While artists were inspired to paint,
0:21:19 > 0:21:23scientists wanted to understand how the volcanic dust had spread
0:21:23 > 0:21:27so quickly across the globe,
0:21:27 > 0:21:30producing these extraordinary sunsets.
0:21:35 > 0:21:40So the Royal Society put an advert in newspapers asking the public
0:21:40 > 0:21:44to send in any unusual observations in connection with the volcano.
0:21:47 > 0:21:53These are just some of the letters that came in and they're so varied.
0:21:53 > 0:21:54They're also really hard to read
0:21:54 > 0:21:56because the handwriting's almost illegible.
0:21:56 > 0:22:01People are describing the sky all of a flare at sunset, fall of ashes,
0:22:01 > 0:22:06explosions heard and on the back there's these colour pictures of
0:22:06 > 0:22:11a sunset and afterglow, the rings, it says, as they formed in succession.
0:22:13 > 0:22:16So imagine sitting in the Royal Society in London and getting
0:22:16 > 0:22:21these letters and the huge amount of information that's in them.
0:22:21 > 0:22:24And that information let them build up a picture of what had
0:22:24 > 0:22:28happened all around the globe in the months following the eruption.
0:22:33 > 0:22:37That picture revealed something that had never been observed before.
0:22:37 > 0:22:41What these observations show is that the dust and the aerosols
0:22:41 > 0:22:46that were carried from the explosion spread westwards around the globe
0:22:46 > 0:22:50at the equator, almost as if there was a sort of river of wind
0:22:50 > 0:22:52running westwards up in the atmosphere
0:22:52 > 0:22:54that was carrying it along.
0:23:00 > 0:23:03These winds are similar to the jet stream but they travel much
0:23:03 > 0:23:07higher up in the stratosphere
0:23:07 > 0:23:10and they're only found near the equator.
0:23:12 > 0:23:14What's fascinating about these winds
0:23:14 > 0:23:17is that they don't always flow in the same direction.
0:23:20 > 0:23:22Up there in the stratosphere, above the equator,
0:23:22 > 0:23:26there are winds that either travel to the west or to the east
0:23:26 > 0:23:30and it switches direction every 14 months or so.
0:23:30 > 0:23:33It's known as the quasi-biennial oscillation,
0:23:33 > 0:23:35sometimes called the QBO for short.
0:23:42 > 0:23:45Back in the 1970s, scientists discovered that
0:23:45 > 0:23:49when these winds flow towards the east, they strengthen the jet stream.
0:23:54 > 0:23:57And it's intriguing that last winter these same winds were flowing
0:23:57 > 0:23:59towards the east.
0:24:00 > 0:24:03Finally, scientists had the first concrete piece of evidence
0:24:03 > 0:24:07that something was helping to speed up the jet stream
0:24:07 > 0:24:09but there was a problem.
0:24:11 > 0:24:15These winds have travelled towards the east many times in the past
0:24:15 > 0:24:19without producing a record-breaking winter like last year.
0:24:19 > 0:24:21So whilst they probably played a small role
0:24:21 > 0:24:25in strengthening the jet stream, on their own, they weren't enough.
0:24:33 > 0:24:35As the winter ended,
0:24:35 > 0:24:38scientists began to develop a third explanation.
0:24:44 > 0:24:47They knew the perfect conditions for a fast jet stream involve
0:24:47 > 0:24:52a big temperature difference between the poles and the tropics
0:24:52 > 0:24:56so they looked for any signs that showed that this temperature gradient
0:24:56 > 0:24:58increased last winter.
0:25:01 > 0:25:03Throughout the next few days,
0:25:03 > 0:25:07temperatures will fall to the low, around minus 13 -
0:25:07 > 0:25:10everyday activities may not be feasible.
0:25:11 > 0:25:14It led them to investigate the unusual weather conditions
0:25:14 > 0:25:16in North America.
0:25:19 > 0:25:22These bone-chilling temperatures normally stay locked up over
0:25:22 > 0:25:26the Arctic but last winter this freezing air was dragged
0:25:26 > 0:25:28southwards over North America.
0:25:35 > 0:25:39Professor Dame Julia Slingo, the Met Office chief scientist,
0:25:39 > 0:25:42has been looking at what might have caused this cold air
0:25:42 > 0:25:44to be dragged south.
0:25:46 > 0:25:49Surprisingly her search began with a deadly flood
0:25:49 > 0:25:52that happened on the other side of the world in Indonesia.
0:25:58 > 0:26:02Last December, unusually intense rain persisted for weeks.
0:26:07 > 0:26:12The fatal floods that followed displaced 60,000 people
0:26:12 > 0:26:15and left areas under more than two metres of water.
0:26:17 > 0:26:23The Indonesian region has been a large part of what's been happening.
0:26:23 > 0:26:28You might say well that's an awfully long way from the UK and it is,
0:26:28 > 0:26:32but what happens in Indonesia affects profoundly
0:26:32 > 0:26:35the weather patterns around the world.
0:26:39 > 0:26:43This extraordinary amount of rain triggered off a sequence of events
0:26:43 > 0:26:47that would ultimately contribute to a super-fast jet stream.
0:26:50 > 0:26:54First, the intense rainfall in Indonesia helped to dramatically
0:26:54 > 0:26:58alter the normal path taken by a different jet stream,
0:26:58 > 0:26:59the Pacific jet.
0:27:02 > 0:27:06That usually follows its path across and well north of California.
0:27:06 > 0:27:10In this year, it's gone a very long way north
0:27:10 > 0:27:15and then made a very deep curve down over the US and Canada,
0:27:15 > 0:27:18what we call a great "buckle" in the jet stream.
0:27:19 > 0:27:23This buckle in the Pacific jet stream helped drag
0:27:23 > 0:27:27the freezing cold air from the Arctic down over Canada and America.
0:27:28 > 0:27:33This cold air then shoved up against the warm air over the Atlantic
0:27:33 > 0:27:36which produced a big temperature gradient,
0:27:36 > 0:27:40the perfect conditions for a fast jet stream.
0:27:41 > 0:27:44The end result was a whole series of storms
0:27:44 > 0:27:47so, in a sense, you've had a double whammy if you like.
0:27:47 > 0:27:51You've had the cold air coming down and setting up things
0:27:51 > 0:27:54on the north side of the jet, but you've had disturbances
0:27:54 > 0:27:57also coming into the south side of the jet.
0:27:57 > 0:28:00It is a bit like a row of dominoes.
0:28:00 > 0:28:03I mean, you know it takes about a week for something
0:28:03 > 0:28:07that happens in Indonesia to have its domino effect, if you like,
0:28:07 > 0:28:10and we see it in our weather over the UK.
0:28:10 > 0:28:13But the connections are so clear this year.
0:28:24 > 0:28:27Last winter, there were many different factors at play.
0:28:29 > 0:28:33These all worked together to produce a fast jet stream.
0:28:37 > 0:28:39The QBO was powering along towards the east...
0:28:43 > 0:28:45..and intense rain in Indonesia
0:28:45 > 0:28:48knocked the Pacific jet stream off its normal path.
0:28:51 > 0:28:53Dad, look out behind you!
0:28:53 > 0:28:55This helped increase the temperature gradient...
0:28:59 > 0:29:03..which led to our jet stream thundering its way towards Britain.
0:29:15 > 0:29:18In many ways, last winter was the perfect storm.
0:29:18 > 0:29:21Everything that could have come together
0:29:21 > 0:29:23to change the jet stream did.
0:29:25 > 0:29:27And it's incredible to think that
0:29:27 > 0:29:30so many different factors could have affected our weather back here
0:29:30 > 0:29:35in the UK but that's exactly what happened and it just goes to show
0:29:35 > 0:29:39how many pieces there are in this giant jigsaw puzzle.
0:29:43 > 0:29:47But there is one more piece to this puzzle.
0:29:47 > 0:29:50It is perhaps the most controversial and complex piece of all...
0:29:52 > 0:29:53..climate change.
0:30:04 > 0:30:06How much of an impact did climate change have on
0:30:06 > 0:30:09last winter's stormy weather?
0:30:14 > 0:30:17It's one of the hardest questions to answer because our climate is
0:30:17 > 0:30:21so complex and because there are so many competing factors
0:30:21 > 0:30:23that influence our weather.
0:30:26 > 0:30:30But we have one very effective tool for understanding
0:30:30 > 0:30:33the role of climate change - computer models.
0:30:34 > 0:30:38They incorporate the best of our current understanding.
0:30:38 > 0:30:41They represent the collective work of thousands of scientists.
0:30:41 > 0:30:43They're an amazing achievement
0:30:43 > 0:30:45and when they get as good as they are now,
0:30:45 > 0:30:49it's possible to use them like a sort of flight simulator for a planet.
0:30:49 > 0:30:51It's an amazing tool to have.
0:30:59 > 0:31:03Today, super-computers like this one at the Met Office
0:31:03 > 0:31:09can do more than 100 trillion calculations every second
0:31:09 > 0:31:12and can look at the impact climate change may have
0:31:12 > 0:31:13on our future weather.
0:31:18 > 0:31:24I think today the incredible complexity and power and skill
0:31:24 > 0:31:28of these models that we use, they are one of the great achievements
0:31:28 > 0:31:32of modern science and you realise that we're entering,
0:31:32 > 0:31:35I think, a golden age for climate science and it's good that
0:31:35 > 0:31:40we are because we have some really, really big questions to answer
0:31:40 > 0:31:45for the world in terms of what climate change will mean for us all.
0:31:47 > 0:31:49Models have predicted that, in the future,
0:31:49 > 0:31:53climate change will lead to an increase in extreme weather.
0:31:54 > 0:31:59So was last year's extreme winter an early sign of this becoming true?
0:32:01 > 0:32:04There can't be a definitive answer on that just yet
0:32:04 > 0:32:07because there's quite a lot of research that needs to be done.
0:32:07 > 0:32:10That being said,
0:32:10 > 0:32:15I think there are various factors that we understand
0:32:15 > 0:32:18from the science of climate change
0:32:18 > 0:32:24that again would suggest that it's been an additional factor.
0:32:39 > 0:32:43To fully understand the impact that climate change had last winter,
0:32:43 > 0:32:45more research needs to be done.
0:32:50 > 0:32:54But perhaps that's missing the point
0:32:54 > 0:32:59because we may never be able to say that one particular weather event
0:32:59 > 0:33:02or one unusual season is because of climate change.
0:33:06 > 0:33:10But it seems likely that one consequence of climate change
0:33:10 > 0:33:12will be more intense rain.
0:33:15 > 0:33:18I think it's important to remember on top of all this discussion
0:33:18 > 0:33:23of global weather patterns that there is this basic bit of physics
0:33:23 > 0:33:29that says that in a warmer world rainfall will be more intense.
0:33:29 > 0:33:31No-one's produced any evidence to counter that idea
0:33:31 > 0:33:34and it's widely accepted
0:33:34 > 0:33:37and so it's reasonable to expect that in the future,
0:33:37 > 0:33:41as the world warms, we will get more intense rainfall.
0:33:49 > 0:33:54And more intense rain will increase the potential for flooding.
0:33:54 > 0:33:58So regardless of whether last winter was made worse by climate change,
0:33:58 > 0:34:01flooding is something we may have to get used to.
0:34:10 > 0:34:15But our extreme winters haven't just been about rain and storms...
0:34:17 > 0:34:21..because previous winters have sent Britain into a deep freeze.
0:34:25 > 0:34:30It started in 2008 when temperatures dropped to minus 12...
0:34:32 > 0:34:36..as cold Siberian air from the east brought snow across the country.
0:34:40 > 0:34:44A year later, Northern Scotland had the coldest winter on record
0:34:44 > 0:34:48as once again Britain shivered under cold Siberian winds
0:34:48 > 0:34:49for weeks on end.
0:34:51 > 0:34:53The following winter,
0:34:53 > 0:34:57we had the coldest December in 100 years,
0:34:57 > 0:35:00as bitterly cold air from the Arctic
0:35:00 > 0:35:02brought a blanket of snow across Britain.
0:35:09 > 0:35:13And there is one thing all these recent cold winters had in common.
0:35:16 > 0:35:20Once again, the main suspect was the behaviour of the jet stream.
0:35:21 > 0:35:25The track of the jet stream varies a lot but, during a typical winter,
0:35:25 > 0:35:28it takes this sort of path, straight across the UK.
0:35:30 > 0:35:33But in recent cold winters it's done something rather peculiar.
0:35:33 > 0:35:37It's taken a meander and, instead, it's moved its way northwards
0:35:37 > 0:35:40and then dived southwards which has meant that the UK
0:35:40 > 0:35:43has been very much on the northern side of the jet and
0:35:43 > 0:35:46that's exposed us to particularly cold air in recent winters.
0:35:49 > 0:35:51These big meanders dragged in
0:35:51 > 0:35:54either the Arctic air mass from the north
0:35:54 > 0:35:58or the polar continental air mass from the east.
0:35:58 > 0:36:01Both brought bitterly cold winds and snow.
0:36:03 > 0:36:06So what caused these big meanders in the jet stream?
0:36:07 > 0:36:11The search for answers soon became an international one
0:36:11 > 0:36:15because big meanders in the jet stream also played a role
0:36:15 > 0:36:18in one of America's most deadly storms.
0:36:30 > 0:36:32The ferocious power of Hurricane Sandy
0:36:32 > 0:36:36was the most destructive hurricane of 2012.
0:36:39 > 0:36:41On the 29th of October,
0:36:41 > 0:36:45it collided head on with the coast of New Jersey in America.
0:36:48 > 0:36:53Over 70 died, half a million buildings were ripped apart
0:36:53 > 0:36:57and the clean-up bill cost over 50 billion.
0:37:10 > 0:37:14Dr Jennifer Francis has been investigating this hurricane.
0:37:16 > 0:37:19She's taking me to a part of the New Jersey coastline
0:37:19 > 0:37:20that was badly damaged.
0:37:22 > 0:37:25So just how unusual was Hurricane Sandy?
0:37:25 > 0:37:28Sandy was a very unusual storm.
0:37:28 > 0:37:32This part of the coast of New Jersey was one of the worst hit.
0:37:32 > 0:37:35In fact the ocean, which is on our left here,
0:37:35 > 0:37:37came right across the sea wall.
0:37:37 > 0:37:40So this whole area was under, what, five foot of water?
0:37:40 > 0:37:43Something like that and you know, of course, with waves on top
0:37:43 > 0:37:46and the roadway was covered with one or two feet of sand
0:37:46 > 0:37:50after the storm, I can't even imagine what it looked like.
0:37:58 > 0:38:01What made Hurricane Sandy so devastating
0:38:01 > 0:38:03was the unusual path it took.
0:38:06 > 0:38:10Well, normally hurricanes tend to steer right out into the Atlantic,
0:38:10 > 0:38:14out to the east, but Hurricane Sandy did something completely different.
0:38:17 > 0:38:20It encountered the jet stream
0:38:20 > 0:38:25and that created the winds that blew her onto her very unusual path,
0:38:25 > 0:38:29taking a sharp left turn right into New Jersey.
0:38:29 > 0:38:33And the shape of the jet stream was really critical for steering
0:38:33 > 0:38:35Sandy into the coast.
0:38:42 > 0:38:45The jet stream had taken a big meander which helped push
0:38:45 > 0:38:48Hurricane Sandy off its normal path.
0:38:53 > 0:38:56But why had the jet stream developed this large meander
0:38:56 > 0:38:58over such a vast area?
0:39:04 > 0:39:07To find out, you need to understand what causes the jet stream
0:39:07 > 0:39:10to change shape in the first place.
0:39:13 > 0:39:16And you can look in the most unlikely of places for the answer.
0:39:19 > 0:39:21The jet stream's a bit like a river in the sky
0:39:21 > 0:39:24but it's 10 kilometres up and invisible so we can't see it
0:39:24 > 0:39:28and it can be hard to understand its behaviour but something that
0:39:28 > 0:39:31can help us understand what it's doing is to think about rivers
0:39:31 > 0:39:35down here on Earth, like this one, the River Cuckmere in Sussex.
0:39:40 > 0:39:43The reason that the water in rivers moves
0:39:43 > 0:39:45is that it's flowing down a gradient.
0:39:45 > 0:39:48In places where the ground is steep, the gradient is steep
0:39:48 > 0:39:52and water flows quickly and it usually flows in a straight line.
0:39:55 > 0:39:58But down here on the flood plain, it's a little bit different.
0:40:01 > 0:40:05Here I'm down near the end of the river where the land is almost flat.
0:40:05 > 0:40:09There's only a really shallow gradient in height
0:40:09 > 0:40:11but that gradient is enough to keep the river flowing and I can
0:40:11 > 0:40:15measure how fast it's flowing using this and this is a flow meter.
0:40:21 > 0:40:28And it's come out at 26 centimetres a second so that's relatively slow.
0:40:28 > 0:40:30There's only a shallow height gradient here
0:40:30 > 0:40:33and the river's running slowly
0:40:33 > 0:40:36and to see the effect of that, I need to go up there on the hillside.
0:40:43 > 0:40:46From up here, we can see what we couldn't see down below.
0:40:53 > 0:40:55This river isn't running in a straight line.
0:40:55 > 0:40:59It's got these big loops in it called meanders
0:40:59 > 0:41:02and they develop when rivers run more slowly.
0:41:02 > 0:41:05The reason it's useful to look at this is that the same thing
0:41:05 > 0:41:08happens up in the sky with the jet stream.
0:41:08 > 0:41:10When it slows down, it changes shape
0:41:10 > 0:41:13and develops meanders just like this.
0:41:25 > 0:41:30The jet stream speed is also linked to a gradient
0:41:30 > 0:41:33but that gradient is different to that of a river.
0:41:39 > 0:41:42The river here is running because of a gradient in height.
0:41:42 > 0:41:45It's running from the ground, the higher ground, inland,
0:41:45 > 0:41:49out to the ocean but the jet stream is running because of a gradient
0:41:49 > 0:41:52in temperature and just like the river,
0:41:52 > 0:41:55when that temperature gradient becomes shallower,
0:41:55 > 0:41:58the jet stream slows down and starts to meander.
0:42:00 > 0:42:06So weak, lazy jet streams develop big meanders which can get stuck,
0:42:06 > 0:42:10resulting in one air mass sitting over Britain for weeks on end.
0:42:12 > 0:42:16Which is exactly what happened during our recent cold winters.
0:42:18 > 0:42:22So to understand what caused them, scientists needed to find out
0:42:22 > 0:42:26why the jet stream had slowed down and produced these big meanders.
0:42:31 > 0:42:34It's a search which has led scientists to some of the most
0:42:34 > 0:42:37contentious areas of climate research
0:42:37 > 0:42:40because Jennifer thinks the answer might be found with dramatic changes
0:42:40 > 0:42:43that are going on in the Arctic.
0:42:43 > 0:42:47I've been studying the Arctic my whole life and we started realising
0:42:47 > 0:42:53in the late 1990s that things were changing really fast up there.
0:43:04 > 0:43:08The Arctic is warming almost twice as fast as the rest of the world.
0:43:16 > 0:43:19It's a phenomenon known as Arctic amplification.
0:43:25 > 0:43:28Three-quarters of the volume of summer sea ice
0:43:28 > 0:43:31has disappeared in just 30 years.
0:43:35 > 0:43:38The scale of the ice loss is just truly breathtaking.
0:43:38 > 0:43:43In 2007, we had a new record for the least amount of ice
0:43:43 > 0:43:45in the Arctic ocean at the end of the summer
0:43:45 > 0:43:49and since then it's just been every year has been very low and then
0:43:49 > 0:43:55in 2012, five years later, we hit another new record low, much lower
0:43:55 > 0:44:00than even 2007, so it's just been a steady decline in the amount of ice.
0:44:08 > 0:44:12Arctic amplification has not only caused sea ice to retreat
0:44:12 > 0:44:14but to reduce in thickness too.
0:44:17 > 0:44:21Today, it's around 50% thinner compared to previous decades.
0:44:25 > 0:44:29And this loss of sea ice has a feedback on the climate system.
0:44:31 > 0:44:35Less ice means less sunlight is reflected back into space.
0:44:38 > 0:44:41Instead, the ocean surface absorbs more heat from the sun
0:44:41 > 0:44:44so the Arctic warms faster,
0:44:44 > 0:44:47resulting in yet more sea ice melting.
0:44:53 > 0:44:56The scale of the ice loss gave me the chills
0:44:56 > 0:45:00because it is such a huge change to such a fundamental part of
0:45:00 > 0:45:05the Earth's climate system, to see that change happening so rapidly.
0:45:05 > 0:45:09So it just got me thinking, how is this rapid warming
0:45:09 > 0:45:12in the Arctic affecting areas farther south?
0:45:15 > 0:45:19To find out, Jennifer looked back over the last 30 years,
0:45:19 > 0:45:21the period of major ice loss in the Arctic.
0:45:22 > 0:45:27She was looking for any changes in the size of the jet stream's waves,
0:45:27 > 0:45:29how loopy they got.
0:45:31 > 0:45:35It was a simple measure but her findings were dramatic.
0:45:35 > 0:45:38We found that in the last couple of decades,
0:45:38 > 0:45:42the waves actually do appear to be getting larger.
0:45:42 > 0:45:46They appear to be extending northward more often
0:45:46 > 0:45:48and particularly in the North Atlantic
0:45:48 > 0:45:51which is important for the UK,
0:45:51 > 0:45:55these very large swings in the jet stream are happening more often.
0:45:55 > 0:45:57It appears to be the case.
0:45:58 > 0:46:01Jennifer believes warming in the Arctic is reducing
0:46:01 > 0:46:05the temperature difference between the poles and the tropics.
0:46:05 > 0:46:08This is slowing down the flow of the jet stream,
0:46:08 > 0:46:11making it more prone to big meanders.
0:46:11 > 0:46:13As that difference in temperature
0:46:13 > 0:46:16between those two bands of the Earth gets smaller
0:46:16 > 0:46:19because the Arctic is warming so much faster,
0:46:19 > 0:46:21the jet stream is weakening and
0:46:21 > 0:46:25because those waves are what create the weather that we experience
0:46:25 > 0:46:29down here on the surface, if those waves are moving more slowly,
0:46:29 > 0:46:31then the weather patterns should change more slowly
0:46:31 > 0:46:33in any given place
0:46:33 > 0:46:37so it feels like the weather that you're experiencing is stuck.
0:46:37 > 0:46:38Stuck in a rut.
0:46:38 > 0:46:40Stuck in a rut and that's, you know,
0:46:40 > 0:46:44we've seen that happen over and over again in the last decade or so.
0:46:44 > 0:46:48It just seems to be happening more often now and when one of
0:46:48 > 0:46:53those big dips happens just south of the UK, then all that cold air from
0:46:53 > 0:46:58the Arctic can come down over that area and create a very cold winter.
0:47:00 > 0:47:04So Arctic amplification produces a smaller temperature gradient
0:47:04 > 0:47:07between the poles and the tropics
0:47:07 > 0:47:11which Jennifer believes produces more meanders in the jet stream.
0:47:14 > 0:47:17And when these meanders happen over Britain,
0:47:17 > 0:47:20they can drag in the Arctic air mass from the north
0:47:20 > 0:47:23bringing cold Arctic winds which produce bitter winters.
0:47:41 > 0:47:43It's really interesting what Jennifer had to say to me.
0:47:43 > 0:47:46I mean, there's no doubt that the Arctic is losing ice
0:47:46 > 0:47:49at an alarming rate.
0:47:49 > 0:47:51You don't really have to be a meteorologist,
0:47:51 > 0:47:54a climatologist, to conclude that such a fundamental
0:47:54 > 0:47:59and rapid change to the system is going to have knock-on effects
0:47:59 > 0:48:03to the atmosphere and to the weather which we experience.
0:48:03 > 0:48:06Also quite compelling on the face of it is the fact that we have
0:48:06 > 0:48:11gone through a run of prolonged spells of unusually severe weather.
0:48:15 > 0:48:17However, that could all be a red herring
0:48:17 > 0:48:19because it's a challenge to pick out
0:48:19 > 0:48:22what is actually natural variability,
0:48:22 > 0:48:27just fluff, just noise, from what is a genuine signal.
0:48:29 > 0:48:32And the challenge is made even harder by a lack of data
0:48:32 > 0:48:36for scientists to study
0:48:36 > 0:48:40because dramatic changes in the Arctic have only been seen
0:48:40 > 0:48:44in the last 30 or so years,
0:48:44 > 0:48:46which isn't long enough to know if the connection
0:48:46 > 0:48:50between a warming Arctic and a meandering jet stream is real.
0:48:57 > 0:49:00And it's particularly wise to be cautious about the role
0:49:00 > 0:49:03of Arctic amplification because there are other theories
0:49:03 > 0:49:05behind our recent cold winters...
0:49:07 > 0:49:09..one of which has its origins in a very different part
0:49:09 > 0:49:11of the climate system.
0:49:13 > 0:49:16For decades it was thought that all our weather happened in just
0:49:16 > 0:49:20one layer of the atmosphere, the troposphere,
0:49:20 > 0:49:23but scientists have discovered that the layer above,
0:49:23 > 0:49:26the stratosphere, is also fundamental to our weather.
0:49:33 > 0:49:36Professor Adam Scaife is investigating
0:49:36 > 0:49:38this important part of the atmosphere.
0:49:41 > 0:49:42It's only recently that
0:49:42 > 0:49:44the computer models that we use to make
0:49:44 > 0:49:47weather forecasts and climate predictions have properly
0:49:47 > 0:49:50started to take into account the full depth of the atmosphere
0:49:50 > 0:49:52and to properly include the stratosphere.
0:49:54 > 0:49:56So at first sight,
0:49:56 > 0:49:58the stratosphere seems very remote from the surface weather.
0:49:58 > 0:50:03We're talking about very thin tenuous air, 50 or so kilometres
0:50:03 > 0:50:07above the surface, but there is an important connection there.
0:50:08 > 0:50:11Adam believes something going on in the stratosphere
0:50:11 > 0:50:15could provide another explanation behind some of our cold winters.
0:50:19 > 0:50:23And he's brought me to Chesil Beach in Dorset to show me what it is.
0:50:23 > 0:50:26So the reason I've brought us here is because although what you
0:50:26 > 0:50:31see behind us, these breaking waves, might seem remote and completely
0:50:31 > 0:50:35irrelevant for the cold winters that we've had, there is actually
0:50:35 > 0:50:40a deep underlying similarity between the breaking waves here and
0:50:40 > 0:50:43breaking waves really high in the atmosphere
0:50:43 > 0:50:45during these cold winter events.
0:50:51 > 0:50:54Scientists have discovered that the thin air of the stratosphere
0:50:54 > 0:51:00is home to giant atmospheric waves which behave in a similar way
0:51:00 > 0:51:02to crashing waves in the sea.
0:51:07 > 0:51:09When one of these waves breaks,
0:51:09 > 0:51:13it generates something called sudden stratospheric warming.
0:51:17 > 0:51:19Adam's discovered that these events have occurred
0:51:19 > 0:51:22during some of our recent cold winters.
0:51:26 > 0:51:30So two out of three of the recent very cold winters that we've had
0:51:30 > 0:51:34have occurred in conjunction with sudden stratospheric warming.
0:51:34 > 0:51:37This initially occurs really high in the atmosphere,
0:51:37 > 0:51:4150 kilometres or 30 miles above the surface and it happens when a wave
0:51:41 > 0:51:45in the atmosphere breaks at really high altitude.
0:51:45 > 0:51:49That breaking wave actually pushes the wind opposite
0:51:49 > 0:51:50to its normal direction.
0:51:55 > 0:51:58Normally the winds in the stratosphere blow
0:51:58 > 0:52:02in the same direction as the jet stream, from west to east,
0:52:02 > 0:52:06but as this enormous stratospheric wave breaks,
0:52:06 > 0:52:10it pushes these winds in the opposite direction towards the west.
0:52:12 > 0:52:17These winds then burrow their way down through the stratosphere
0:52:17 > 0:52:20until they hit the jet stream.
0:52:21 > 0:52:24And because the jet stream flows in the opposite direction,
0:52:24 > 0:52:27these winds act like a brake - slowing it down.
0:52:31 > 0:52:34The whole process of this burrowing down through the atmosphere
0:52:34 > 0:52:38can occur on a timescale of a few days, maybe a week or two, until it
0:52:38 > 0:52:41reaches the jet stream and at this point, it kind of switches off the
0:52:41 > 0:52:46jet stream and blows cold air from Siberia in towards the UK and leads
0:52:46 > 0:52:50to those dramatic cold snaps that we've experienced in recent winters.
0:52:57 > 0:53:01So a sudden stratospheric warming slows the jet stream
0:53:01 > 0:53:04which can produce a big meander.
0:53:05 > 0:53:10As a result, the polar continental air mass is dragged in from the east
0:53:10 > 0:53:14across Britain and with it comes bitterly cold air from Siberia.
0:53:18 > 0:53:22Since these events can influence our winters so dramatically,
0:53:22 > 0:53:25scientists want to know if there's a pattern to their occurrence.
0:53:26 > 0:53:28But it isn't so simple.
0:53:29 > 0:53:34Now there is no regular pattern to when these events occur.
0:53:34 > 0:53:37On average, they're every two years but just like tossing a coin,
0:53:37 > 0:53:39you could get three heads in a row,
0:53:39 > 0:53:42sudden stratospheric warmings can occur in runs of winters
0:53:42 > 0:53:45or you can have long periods, like the 1990s, when there were
0:53:45 > 0:53:48no sudden stratospheric warmings for several years on end.
0:53:53 > 0:53:56The thing that's a bit frustrating about these sudden
0:53:56 > 0:54:00stratospheric warming events is that there's no clear pattern to them
0:54:00 > 0:54:02but as humans, we're always looking for patterns.
0:54:02 > 0:54:05If a coin falls heads lots of times in a row,
0:54:05 > 0:54:09we start to ask why but sometimes that's just the luck of the draw
0:54:09 > 0:54:12and so it may well be that we've had some cold winters
0:54:12 > 0:54:15and we're looking for a pattern, but really there isn't one.
0:54:27 > 0:54:31So it looks as if at least two factors could have caused
0:54:31 > 0:54:34some of our recent cold winters by weakening the jet stream.
0:54:36 > 0:54:38Sudden stratospheric warming...
0:54:42 > 0:54:44..and Arctic amplification.
0:54:54 > 0:54:57We still don't know which one will dominate in the future...
0:55:01 > 0:55:05..or indeed whether other factors like the behaviour of the oceans
0:55:05 > 0:55:09or changes in solar activity could play a role too
0:55:09 > 0:55:11in influencing our winter weather.
0:55:14 > 0:55:17All this makes it a challenge to know
0:55:17 > 0:55:22whether we face cold or wet, stormy winters in the future.
0:55:30 > 0:55:32But despite this uncertainty,
0:55:32 > 0:55:36there may be something we can say about our future winters.
0:55:38 > 0:55:41Once again, it all comes back to the jet stream.
0:55:43 > 0:55:45For me, the strongest signal to emerge
0:55:45 > 0:55:50as we struggle to understand the recent extreme weather is the idea
0:55:50 > 0:55:54that the jet stream can become stuck in certain configurations.
0:55:54 > 0:55:58At one end of the spectrum, a very straight, fast jet stream
0:55:58 > 0:56:01which brought the storms of last winter.
0:56:01 > 0:56:06At the other end of the spectrum, a much slower, meandering jet stream
0:56:06 > 0:56:09which has brought the recent run of particularly cold winters.
0:56:09 > 0:56:14But either end of the spectrum is capable of bringing prolonged
0:56:14 > 0:56:17and extreme weather and perhaps this is something
0:56:17 > 0:56:19we should expect more of in the near future.
0:56:40 > 0:56:45Scientists are continuing to improve their understanding of the jet stream
0:56:45 > 0:56:49but, even now, there's a lot we can do to prepare for our future.
0:56:57 > 0:57:00We don't control the weather but we're not helpless
0:57:00 > 0:57:04and being uncertain about the future isn't the same as knowing nothing.
0:57:05 > 0:57:08I think the science here is in a really good state.
0:57:08 > 0:57:11There's lots of debate, lots of different ideas, lots of evidence
0:57:11 > 0:57:16that's available for everyone to see and I feel optimistic.
0:57:16 > 0:57:18I think that we're really getting to grips
0:57:18 > 0:57:20with the science of these extreme weather events.
0:57:28 > 0:57:32And that means we can also begin to get to grips with how to deal
0:57:32 > 0:57:33with our changing climate.
0:57:39 > 0:57:42When I was a boy, I lived on the side of a hill
0:57:42 > 0:57:44and I'd sit overlooking the valley
0:57:44 > 0:57:47and I'd watch the weather coming my way and that was my world really
0:57:47 > 0:57:49and as far as I was concerned,
0:57:49 > 0:57:52the weather was contained within the valley.
0:57:57 > 0:58:01We all have a tendency to think locally but we have to look beyond
0:58:01 > 0:58:08the parochial confines of our valley, our country, our ocean even.
0:58:08 > 0:58:13We have absolutely no control over the weather but what we can do is
0:58:13 > 0:58:18understand it and adapt our society for the changes which lie ahead.