0:00:02 > 0:00:03It's a tornado!
0:00:04 > 0:00:06Look at that!
0:00:11 > 0:00:15Our planet is home to some spectacular natural wonders.
0:00:17 > 0:00:19THUNDERCLAP
0:00:19 > 0:00:25Yet exactly how and why they form is still a mystery.
0:00:27 > 0:00:31But now, new camera technologies are revealing
0:00:31 > 0:00:34their inner workings in stunning detail.
0:00:38 > 0:00:41My name is Dr Helen Czerski
0:00:41 > 0:00:44and I'll be looking at how these extraordinary images
0:00:44 > 0:00:49are transforming our understanding of the natural world.
0:00:53 > 0:00:57In this programme, we look at the latest scientific insights
0:00:57 > 0:01:01into the hottest natural phenomenon on Earth - lightning.
0:01:04 > 0:01:08Lightning is one of the most dramatic natural spectacles
0:01:08 > 0:01:11on the planet but we still don't fully understand it.
0:01:11 > 0:01:15Exactly what triggers it and why there are more lightning strikes
0:01:15 > 0:01:17in some places than others is a mystery.
0:01:20 > 0:01:23Now, cameras are seeing what our eyes can't -
0:01:23 > 0:01:27from discovering the secrets of rare upward lightning
0:01:27 > 0:01:29in super high speed...
0:01:30 > 0:01:33..to capturing vast electrical bursts,
0:01:33 > 0:01:35spreading kilometres above thunderstorms.
0:01:37 > 0:01:40We can now capture, on camera, the complex processes
0:01:40 > 0:01:44crucial to understanding this unpredictable force of nature.
0:01:59 > 0:02:04Lightning strikes our planet over 30 times a second.
0:02:07 > 0:02:11And each strike is five times hotter than the surface of the sun.
0:02:14 > 0:02:17Now that so many of us carry cameraphones...
0:02:20 > 0:02:24..we're capturing just how dangerous this force of nature can be.
0:02:24 > 0:02:27LOUD THUNDERCLAP
0:02:27 > 0:02:30Oh, my God! Whoa!
0:02:30 > 0:02:32In Sydney, drivers had a lucky escape
0:02:32 > 0:02:36when a lightning-hit tree crashed into their path.
0:02:36 > 0:02:38- Is that guy OK? Whoa! - He's all right.
0:02:38 > 0:02:44In Newcastle, a lightning bolt set this house on fire.
0:02:46 > 0:02:51And these holidaymakers were lucky to walk away with their lives.
0:02:51 > 0:02:53THUNDERCLAP
0:02:53 > 0:02:56Oh! Oh, my God! I felt it! I felt it!
0:03:01 > 0:03:03So, how and why is the planet we live on
0:03:03 > 0:03:06plagued by these vast electrical bolts?
0:03:11 > 0:03:14Considering that lightning strikes somewhere on Earth
0:03:14 > 0:03:16three million times every single day,
0:03:16 > 0:03:19it's slightly surprising that we don't know very much
0:03:19 > 0:03:21about the details of what triggers it.
0:03:21 > 0:03:22But we do know the big picture,
0:03:22 > 0:03:27which is that lightning is basically a giant electric static discharge.
0:03:27 > 0:03:30It's just like when you touch a car, maybe on a cold, dry day,
0:03:30 > 0:03:32and you feel an electric shock.
0:03:32 > 0:03:35It's exactly the same thing but on a much bigger scale.
0:03:35 > 0:03:37And I can demonstrate the principle with this machine here,
0:03:37 > 0:03:41which is a Van de Graaff generator, one of the workhorses of physics.
0:03:41 > 0:03:42If I switch it on...
0:03:42 > 0:03:45There's a positive electric charge building up
0:03:45 > 0:03:47on the large metal sphere
0:03:47 > 0:03:50and a negative electric charge building up on the small one.
0:03:50 > 0:03:53And when that charge gets big enough...
0:03:53 > 0:03:55SPARK CRACKLES ..we get a spark,
0:03:55 > 0:03:58really clear spark. There we go. SPARKS CONTINUE TO CRACKLE
0:03:58 > 0:04:02And this is what's happening when there's a bolt of lightning.
0:04:02 > 0:04:03THUNDERCLAP
0:04:03 > 0:04:07In both cases, the spark we're seeing is an electrical discharge
0:04:07 > 0:04:10travelling through the air.
0:04:10 > 0:04:15And the sound is lightning's own sonic boom - thunder...
0:04:15 > 0:04:17THUNDERCLAP
0:04:17 > 0:04:20..created by the hot air rapidly expanding.
0:04:22 > 0:04:26In the Van de Graaff generator, I'm creating the charge artificially.
0:04:27 > 0:04:31In a thundercloud, scientists think that the charge builds up
0:04:31 > 0:04:35when ice crystals of different sizes collide.
0:04:35 > 0:04:38The heavier, usually negatively-charged crystals,
0:04:38 > 0:04:40move to the bottom of the cloud.
0:04:45 > 0:04:48Just like in a battery, opposite charges are trapped.
0:04:49 > 0:04:53So, on the ground below the cloud, a positive charge builds up.
0:04:57 > 0:05:00Here, the two steel balls represent the positive Earth
0:05:00 > 0:05:03and the negative base of the cloud.
0:05:03 > 0:05:07Between the two, the electric field grows stronger and stronger.
0:05:09 > 0:05:12The problem is, air is an electrical insulator.
0:05:12 > 0:05:15Charge can't travel through it very easily.
0:05:15 > 0:05:20However, if you get enough negative charge and enough positive charge,
0:05:20 > 0:05:25eventually, it builds up so much that the air actually breaks down.
0:05:25 > 0:05:28The air molecules break down and form a little tube
0:05:28 > 0:05:30that the electric charge can travel through.
0:05:30 > 0:05:32SPARKS CRACKLE Ooh! So, on a small scale,
0:05:32 > 0:05:35this is almost exactly what's happening
0:05:35 > 0:05:37in these big thunderclouds,
0:05:37 > 0:05:39where you get these huge bolts of lightning.
0:05:39 > 0:05:42THUNDERCLAPS
0:05:44 > 0:05:47The detail of exactly how the electrical charge travels
0:05:47 > 0:05:49through the air is still being worked out.
0:05:52 > 0:05:57Lightning moves at speeds of over 150,000 kilometres a second,
0:05:57 > 0:06:01so it's only with the invention of new camera techniques
0:06:01 > 0:06:04that scientists have really been able to see what's going on.
0:06:06 > 0:06:08Rapid City, South Dakota,
0:06:08 > 0:06:11is in the heart of America's thunderstorm zone.
0:06:13 > 0:06:18Here, meteorologist Tom Warner uses specialist high-speed cameras
0:06:18 > 0:06:24to film lightning at up to 100,000 frames per second.
0:06:24 > 0:06:26When you view a flash in real time,
0:06:26 > 0:06:29it's like seeing a title of a book. You can see there was a flash there
0:06:29 > 0:06:33that reached the ground and maybe it flickered a little bit,
0:06:33 > 0:06:34but that's all you know.
0:06:36 > 0:06:39You come and record the same flash with these high-speed cameras,
0:06:39 > 0:06:41it's like a novel.
0:06:41 > 0:06:45It tells a unique story every time you play it back. Incredible.
0:06:45 > 0:06:50Tom keeps the cameras constantly recording during a storm.
0:06:50 > 0:06:54Once he triggers, it saves just the previous few seconds of footage,
0:06:54 > 0:06:56but that's enough.
0:06:56 > 0:06:59THUNDERCLAP
0:06:59 > 0:07:02These images show each stage of a lightning strike.
0:07:02 > 0:07:06It starts with a fleeting stroke, called a stepped leader,
0:07:06 > 0:07:09normally invisible to the naked eye.
0:07:09 > 0:07:12Here, we see it branching downwards
0:07:12 > 0:07:15until one of the forks makes contact with the ground,
0:07:15 > 0:07:19producing the far brighter and faster return stroke.
0:07:22 > 0:07:25This is the powerful channel of electrical current
0:07:25 > 0:07:27we usually see as lightning.
0:07:28 > 0:07:32The entire process takes around one-hundredth of a second.
0:07:33 > 0:07:36So, it's only by slowing it down many hundreds of times
0:07:36 > 0:07:38that it becomes visible.
0:07:42 > 0:07:46Lightning is so fast that studying its form is only possible
0:07:46 > 0:07:48with photographic advances.
0:07:50 > 0:07:52Historically, our understanding of it
0:07:52 > 0:07:54has gone hand in hand with camera technology.
0:07:54 > 0:07:56Before photography,
0:07:56 > 0:07:59the most common way of depicting lightning
0:07:59 > 0:08:01was in the form of a zigzag.
0:08:01 > 0:08:06The problem for meteorologists was that they didn't believe
0:08:06 > 0:08:08that zigzag lightning appeared in nature.
0:08:09 > 0:08:15This is the first known photograph of lightning, dated 1847.
0:08:15 > 0:08:19Early pictures like this reveal that each split-second bolt
0:08:19 > 0:08:22was a complex pattern of different shapes.
0:08:24 > 0:08:27One of the first photographers to use the camera
0:08:27 > 0:08:31for the scientific investigation of lightning
0:08:31 > 0:08:33was a railroad photographer named William Jennings.
0:08:33 > 0:08:38He suspected that zigzag lightning was not the true form of lightning
0:08:38 > 0:08:41and he sought to show that lightning comes in a range
0:08:41 > 0:08:43of different forms and shapes.
0:08:43 > 0:08:47This one shows lightning behind the clouds.
0:08:47 > 0:08:49In this case, it wanders across the sky
0:08:49 > 0:08:51and doesn't even touch the ground.
0:08:51 > 0:08:54And in this one, we have parallel discharges.
0:08:56 > 0:08:58But normal photography couldn't capture
0:08:58 > 0:09:01how a split-second stroke changed over time.
0:09:03 > 0:09:06A big problem with early lightning photography
0:09:06 > 0:09:10was capturing the high-speed movement of the lightning flash.
0:09:10 > 0:09:14To solve this problem, a British physicist,
0:09:14 > 0:09:17Charles Vernon Boys, invented a lightning camera.
0:09:17 > 0:09:21The concept was ingenious. It was to move the lenses.
0:09:21 > 0:09:24They didn't have film that was high-speed enough to capture,
0:09:24 > 0:09:26but with the system of rotating lenses,
0:09:26 > 0:09:30it was possible to capture the evolution of the flash
0:09:30 > 0:09:33as it evolved, and spread it across the film.
0:09:33 > 0:09:38This 1941 movie shows how this innovative camera
0:09:38 > 0:09:41revealed lightning's multiple flashes.
0:09:43 > 0:09:46- MOVIE SOUNDTRACK:- Our eyes saw only this.
0:09:46 > 0:09:49- THUNDERCLAP - But here are some surprises,
0:09:49 > 0:09:53revealed by the Thunderbolt Hunters' ultra high-speed cameras.
0:09:53 > 0:09:55What appeared as a single flash
0:09:55 > 0:09:57was, in reality, a series of strokes.
0:10:00 > 0:10:03High-speed photography has come a long way since then,
0:10:03 > 0:10:07revealing the evolution of a lightning strike beat by beat.
0:10:11 > 0:10:16But Tom Warner's cameras have also captured something unexpected -
0:10:16 > 0:10:21remarkable images of a rare form of lightning that travels upward.
0:10:24 > 0:10:26Most lightning discharges down from the sky
0:10:26 > 0:10:30through the pathways of ionised air, called leaders.
0:10:30 > 0:10:35But this footage reveals lightning travelling up into the clouds.
0:10:35 > 0:10:38When I played this for the first time, I was just blown away.
0:10:38 > 0:10:41It was just amazing to see it.
0:10:48 > 0:10:51What's extraordinary is that most of this upward lightning
0:10:51 > 0:10:53is caused by us...
0:10:58 > 0:11:02..set off by the transmission towers, wind turbines and high-rises
0:11:02 > 0:11:05that now litter our urban landscapes.
0:11:08 > 0:11:11Naturally occurring upward lightning is rare.
0:11:14 > 0:11:18But these tall structures create intense electric fields,
0:11:18 > 0:11:22concentrated at their tips, whenever there's a storm cloud overhead.
0:11:25 > 0:11:28Tom's videos have revealed that during a storm,
0:11:28 > 0:11:31these intense electric fields discharge
0:11:31 > 0:11:33as ground to cloud strikes.
0:11:36 > 0:11:40These towers experience up to 100 times more
0:11:40 > 0:11:41of these upward lightning strikes
0:11:41 > 0:11:44than they do regular downward lightning,
0:11:44 > 0:11:47so there's a far greater chance of damage.
0:11:56 > 0:12:00Lightning that's caused by human activity is nothing new
0:12:00 > 0:12:03and it can have near-catastrophic consequences.
0:12:10 > 0:12:12Kennedy Space Center sits in the heart
0:12:12 > 0:12:15of America's lightning capital, Florida.
0:12:15 > 0:12:17THUNDERCLAP
0:12:17 > 0:12:23On November 14th, 1969, they launched the Apollo 12 moon mission.
0:12:27 > 0:12:28It had been a stormy day,
0:12:28 > 0:12:31but the countdown proceeded as normal to blast-off.
0:12:35 > 0:12:37- FLIGHT COMMS:- 'Houston, good trajectory...'
0:12:37 > 0:12:42But just seconds after takeoff, warning lights started flashing
0:12:42 > 0:12:46and electrical circuits began to malfunction.
0:12:46 > 0:12:49- COMMS:- 'What happened here, we had everything in the world drop out.
0:12:49 > 0:12:51'I'm not sure if we've been hit by lightning.'
0:12:51 > 0:12:54Well, power went up through
0:12:54 > 0:12:57what was a fairly weak electric field
0:12:57 > 0:13:00and actually magnified that electric field as it went up through it.
0:13:00 > 0:13:03THUNDERCLAPS
0:13:04 > 0:13:08By magnifying the small electric field of the storm cloud,
0:13:08 > 0:13:12Apollo 12 triggered its own bolt of lightning.
0:13:12 > 0:13:16Only quick thinking and a backup computer saved the mission.
0:13:16 > 0:13:18It almost shut Apollo down.
0:13:18 > 0:13:22It was only because they had lots of redundant systems
0:13:22 > 0:13:23that they were able to escape.
0:13:23 > 0:13:26It was very, very close to losing the mission
0:13:26 > 0:13:28and losing the astronauts.
0:13:31 > 0:13:35Space rockets aren't the only aircraft at risk from lightning.
0:13:36 > 0:13:38We all come a lot closer to lightning strikes
0:13:38 > 0:13:40than we might imagine.
0:13:45 > 0:13:47Whoa!
0:13:47 > 0:13:52Because every commercial airliner is struck by lightning
0:13:52 > 0:13:54on average once a year.
0:13:57 > 0:14:00This clip, filmed by an eyewitness in London,
0:14:00 > 0:14:02shows the hair-raising moment
0:14:02 > 0:14:06a bolt of lightning seems to travel right through the plane.
0:14:09 > 0:14:11So, how do planes withstand
0:14:11 > 0:14:14being struck by millions of volts of electricity?
0:14:17 > 0:14:21It turns out they're behaving like giant Faraday cages.
0:14:24 > 0:14:27Faraday cages are named after Michael Faraday
0:14:27 > 0:14:29because he did the experiments, back in 1836,
0:14:29 > 0:14:32that demonstrated this principle.
0:14:32 > 0:14:35And it's all to do with what happens when you have a hollow object
0:14:35 > 0:14:37that's made of a conducting material,
0:14:37 > 0:14:39so that's normally a metal.
0:14:39 > 0:14:42If you bring that close to an electric field,
0:14:42 > 0:14:46the charges can travel right around the outside of your hollow container
0:14:46 > 0:14:48without ever going near the inside.
0:14:49 > 0:14:52It's a bit of physics used to dramatic effect
0:14:52 > 0:14:55by high-voltage entertainers, like this.
0:15:00 > 0:15:02This performer is wearing a Faraday cage
0:15:02 > 0:15:04and that's why he can reach out
0:15:04 > 0:15:07and touch these strong electric discharges
0:15:07 > 0:15:09without it hurting him at all.
0:15:10 > 0:15:13So, the electric field is very strong here
0:15:13 > 0:15:16but, instead of going through the performer,
0:15:16 > 0:15:20it travels around the outside of his conducting suit
0:15:20 > 0:15:23and will reach the ground without ever touching his body.
0:15:23 > 0:15:25So, he's completely protected
0:15:25 > 0:15:26because he's inside the Faraday cage.
0:15:26 > 0:15:28And this is what happens in a plane.
0:15:28 > 0:15:31The metal tube of the plane acts as a Faraday cage
0:15:31 > 0:15:33so, that when it gets struck by lightning,
0:15:33 > 0:15:36the electric charge travels around the outside and onwards,
0:15:36 > 0:15:39without ever going anywhere near the passengers,
0:15:39 > 0:15:42the flight controls or anything else inside the plane.
0:15:43 > 0:15:46If we look back at the clip of the London flight,
0:15:46 > 0:15:49you can see how the lightning travels harmlessly
0:15:49 > 0:15:53from wing tip to wing tip and the plane keeps flying.
0:15:57 > 0:16:00The planes we fly on are safe because they're made of metal
0:16:00 > 0:16:04but now, to make aircraft lighter and more efficient,
0:16:04 > 0:16:08manufacturers are starting to move over to carbon fibre.
0:16:10 > 0:16:13The problem is that carbon fibre doesn't conduct electricity
0:16:13 > 0:16:18nearly as well, making the planes more vulnerable to lightning.
0:16:20 > 0:16:23At the Cobham Research Centre in Oxfordshire,
0:16:23 > 0:16:26scientist Stephen Haigh is testing a solution.
0:16:27 > 0:16:29If you design an aircraft,
0:16:29 > 0:16:31you have to demonstrate it's safe from lightning.
0:16:31 > 0:16:34It's a real threat and you make sure you're protected it against it.
0:16:34 > 0:16:39Stephen and a team of engineers shoot artificial lightning bolts
0:16:39 > 0:16:40through different materials
0:16:40 > 0:16:43to see just what happens when a plane is struck.
0:16:47 > 0:16:50First, they test a panel made of aluminium,
0:16:50 > 0:16:53the material most planes in the air today are still made of.
0:16:55 > 0:16:57Primarily, the concern is to make sure
0:16:57 > 0:16:59that we don't have any puncture of this skin
0:16:59 > 0:17:01during the lightning attachment.
0:17:01 > 0:17:05Some of these panels are, effectively, wing skin panels
0:17:05 > 0:17:07covering the fuel tank, so what you don't want
0:17:07 > 0:17:12is any possibility of ignition of a fuel vapour within the tank.
0:17:12 > 0:17:15With the help of a massive high-current generator,
0:17:15 > 0:17:19they're able to recreate the worst that nature might throw at it.
0:17:19 > 0:17:22This is mimicking a severe lightning strike, actually.
0:17:22 > 0:17:25Much more severe than that you'd normally expect
0:17:25 > 0:17:28but that's the way you qualify an aircraft -
0:17:28 > 0:17:30you know, go for the worst case.
0:17:30 > 0:17:31- OK, good to go?- Yep.
0:17:35 > 0:17:36OK.
0:17:36 > 0:17:39MACHINE WHIRRS
0:17:39 > 0:17:41EXPLOSION
0:17:44 > 0:17:49100,000 amps causes quite a strike, but has it punctured the panel?
0:17:50 > 0:17:55So, you can see a little bit of surface damage, loss of the paint.
0:17:55 > 0:17:58No puncture though. It's not a problem. That's a good result.
0:17:58 > 0:18:02Next, they put in one of the new carbon fibre panels.
0:18:02 > 0:18:05MACHINE WHIRRS
0:18:06 > 0:18:07EXPLOSION
0:18:09 > 0:18:10It seems to be much more damaged,
0:18:10 > 0:18:12but is it?
0:18:12 > 0:18:15Much of the damage is due to the fact
0:18:15 > 0:18:18there's a really thick paint layer on this panel,
0:18:18 > 0:18:21so you can see it's just been peeled off.
0:18:21 > 0:18:25But, actually, the physical damage to the panel is quite small.
0:18:25 > 0:18:28In order to protect the plane,
0:18:28 > 0:18:31a layer of conductive material has been added.
0:18:31 > 0:18:33To protect the carbon,
0:18:33 > 0:18:35there's a very thin weave of copper mesh over here,
0:18:35 > 0:18:37which gives it the lightning protection.
0:18:37 > 0:18:40That's protected the carbon fibre underneath.
0:18:42 > 0:18:46Just like the aluminium planes, this thin layer of metal mesh,
0:18:46 > 0:18:50embedded in the carbon fibre, acts as a Faraday cage,
0:18:50 > 0:18:53keeping the inside of the plane safe from harm.
0:18:55 > 0:18:58Work like this ensures all the planes in our skies
0:18:58 > 0:19:02will keep flying, even if they're struck by lightning.
0:19:03 > 0:19:05THUNDERCLAP
0:19:12 > 0:19:15A lightning strike can be up to one billion volts
0:19:15 > 0:19:19and when there's no Faraday cage to conduct the heat away,
0:19:19 > 0:19:22the effect can be devastating.
0:19:23 > 0:19:27These are as close as you can get to actually holding lightning.
0:19:27 > 0:19:31It's called a fulgurite and this was formed in the Western Sahara
0:19:31 > 0:19:34when a lightning bolt struck the desert sand
0:19:34 > 0:19:38and it heated the sand up so much, it fused it into glass,
0:19:38 > 0:19:40so at least 2,000 degrees C.
0:19:40 > 0:19:44And the lovely thing about it is all these stunning details on here.
0:19:44 > 0:19:47You can see the shape of the lightning bolt,
0:19:47 > 0:19:49because that's what's left behind in glass.
0:19:49 > 0:19:52And if lightning can do this to sand,
0:19:52 > 0:19:54you can only imagine what it would do
0:19:54 > 0:19:56to something much more vulnerable...
0:19:58 > 0:20:00..like a human being.
0:20:02 > 0:20:06Around one in ten people struck by lightning are killed,
0:20:06 > 0:20:08most from cardiac arrest.
0:20:09 > 0:20:12But a surprising number survive the intense heat.
0:20:13 > 0:20:16I picked up the backpack with one hand
0:20:16 > 0:20:20and went to grab the pitchfork and the lights went out.
0:20:20 > 0:20:23Where I laid, the grass was totally burned.
0:20:24 > 0:20:26The whole length of my body was burned.
0:20:26 > 0:20:30I felt as though I was completely on fire.
0:20:30 > 0:20:33I didn't see any fire or smoke or anything,
0:20:33 > 0:20:36but just from the inside out of me,
0:20:36 > 0:20:39it just felt like I was just...on fire.
0:20:39 > 0:20:41THUNDERCLAP
0:20:41 > 0:20:45So, what does happen when lightning hits a human body
0:20:45 > 0:20:48and why do most people survive?
0:20:49 > 0:20:52The answer may lie with the surface of the human skin.
0:20:54 > 0:20:58It's thought that people who survive lightning may be partly protected
0:20:58 > 0:21:01if there's a thin layer of sweat or rainwater on their skin.
0:21:05 > 0:21:07By coating this mannequin in water
0:21:07 > 0:21:12and striking it with 30,000 amps of lab-generated lightning,
0:21:12 > 0:21:14we can replicate the effect.
0:21:14 > 0:21:16MACHINE BUZZES
0:21:18 > 0:21:20ELECTRICAL CRACKLE
0:21:20 > 0:21:22MACHINE BUZZES
0:21:22 > 0:21:23ELECTRICAL CRACKLE
0:21:23 > 0:21:26The layer of water conducts the lightning
0:21:26 > 0:21:28around the surface of the body,
0:21:28 > 0:21:30protecting the internal organs from harm.
0:21:32 > 0:21:36It may save your life, but the resulting burns can be severe.
0:21:36 > 0:21:40One of the things we see with the burns that do occur with lightning,
0:21:40 > 0:21:42are not really caused by the lightning
0:21:42 > 0:21:44so much as they're caused by what lightning is doing
0:21:44 > 0:21:47on the surface of the body -
0:21:47 > 0:21:49turning the rainwater or sweat into steam
0:21:49 > 0:21:52and then that causes a burn. So, for instance,
0:21:52 > 0:21:54if you've got a cotton T-shirt on,
0:21:54 > 0:21:56the steam can escape readily through that,
0:21:56 > 0:21:58but if you've got a leather jacket on,
0:21:58 > 0:22:01it will hold the steam in longer, so you'll end up with a deeper burn.
0:22:01 > 0:22:03One of the things that we see
0:22:03 > 0:22:06when the sweat or the rainwater's turned into steam,
0:22:06 > 0:22:08is a tremendous expansion, obviously,
0:22:08 > 0:22:10of the water into the steam.
0:22:10 > 0:22:14Shoes can actually be blown off because you've got wet socks,
0:22:14 > 0:22:17sweaty socks, and that turns into a vapour explosion
0:22:17 > 0:22:19within the closed space of the shoe
0:22:19 > 0:22:21and can actually blow the shoe apart.
0:22:21 > 0:22:24ELECTRICAL CRACKS
0:22:27 > 0:22:31Being struck by lightning can also leave behind feathery tattoos,
0:22:31 > 0:22:33known as Lichtenberg figures.
0:22:35 > 0:22:37Just as lightning branches out,
0:22:37 > 0:22:40searching for the most conductive path through the air,
0:22:40 > 0:22:44on a person, it creates these dramatic marks,
0:22:44 > 0:22:47as the discharge travels across the surface of the skin.
0:22:58 > 0:23:01A normal bolt of lightning is powerful enough,
0:23:01 > 0:23:06but what about an electrical discharge up to 80km wide?
0:23:08 > 0:23:12For years, pilots reported seeing vast flashes of light,
0:23:12 > 0:23:16spread across the sky but they were so faint and so brief
0:23:16 > 0:23:19that no-one was sure if they even existed.
0:23:20 > 0:23:23It wasn't until 1989, that a scientist,
0:23:23 > 0:23:25trying out a new low-light camera,
0:23:25 > 0:23:28accidentally caught the first ever image of one.
0:23:31 > 0:23:34What he'd captured was a rare event called a sprite.
0:23:34 > 0:23:38Like lightning, these are immense discharges of electricity,
0:23:38 > 0:23:41but they form high in the atmosphere, above storms.
0:23:42 > 0:23:46INDISTINGUISHABLE FLIGHT COMMUNICATIONS
0:23:51 > 0:23:55Now, this team of scientists are setting out on an expedition
0:23:55 > 0:23:58to try and capture them on high-speed cameras,
0:23:58 > 0:24:00in order to discover more.
0:24:03 > 0:24:06Sprites dwarf normal lightning
0:24:06 > 0:24:11and yet they're so faint and so fleeting, they're barely visible.
0:24:11 > 0:24:15The real difference is how brief the sprites last.
0:24:15 > 0:24:17Normal lightning can last,
0:24:17 > 0:24:21if you count all the strokes together, maybe half a second.
0:24:21 > 0:24:24Sprites are a lot shorter in duration -
0:24:24 > 0:24:26one one-thousandth of a second.
0:24:26 > 0:24:30If they succeed, these will be the first ever high-speed images
0:24:30 > 0:24:32of sprites from the air.
0:24:32 > 0:24:35But sprites are so faint,
0:24:35 > 0:24:38they have to keep their plane in near-total darkness
0:24:38 > 0:24:41and use low-light intensifiers on the cameras
0:24:41 > 0:24:43as they approach the thunderstorm.
0:24:43 > 0:24:47We're headed to Mississippi, right now, and then, past that,
0:24:47 > 0:24:50we're going to head over toward Little Rock, Arkansas,
0:24:50 > 0:24:53and then we're going to go straight out toward Des Moines, Iowa.
0:24:53 > 0:24:55About two hours to get there.
0:24:55 > 0:24:59We'll have four hours to loiter around the storms
0:24:59 > 0:25:03for the sprite to... Pictures at the back, then two hours back home,
0:25:03 > 0:25:06so we'll be airborne about eight hours tonight.
0:25:06 > 0:25:11At 15,000 metres up, their plane is now high in the atmosphere.
0:25:11 > 0:25:16With me flying at this altitude, it's the clarity of the air,
0:25:16 > 0:25:19because the visibility is forever.
0:25:19 > 0:25:22With a thunderstorm in full swing,
0:25:22 > 0:25:25these should be perfect sprite conditions.
0:25:26 > 0:25:30It looks to me like we're getting all the lightning on our backside
0:25:30 > 0:25:34right now, so is it possible to turn, say, ten degrees to the left?
0:25:34 > 0:25:38The cameras are ready to trigger at super-high speed.
0:25:38 > 0:25:4210,000 frames a second, 50 microsecond inauguration time.
0:25:46 > 0:25:48Gain is 60,500.
0:25:48 > 0:25:51- Elevation is minus four.- Go again.
0:25:53 > 0:25:55Finally, they see something.
0:25:56 > 0:25:57Sprite.
0:25:57 > 0:26:00I think it was probably outside the field of view, I'm not sure.
0:26:00 > 0:26:04But it slips through their fingers. They set up to try again.
0:26:05 > 0:26:08We're now at minus four degrees elevation.
0:26:11 > 0:26:15- Sprite.- We're looking. Yeah, we got it.
0:26:17 > 0:26:20It looks like there's two of them I think he got.
0:26:20 > 0:26:22Finally, they have success.
0:26:22 > 0:26:25The high-speed cameras have caught this elusive phenomenon
0:26:25 > 0:26:27in all its glory.
0:26:27 > 0:26:32A vast column of light 30km tall, dwarfing the city below.
0:26:33 > 0:26:35Thanks to footage like this,
0:26:35 > 0:26:38scientists now know these astonishing displays
0:26:38 > 0:26:41almost always follow a powerful lightning strike.
0:26:43 > 0:26:45As the strike discharges,
0:26:45 > 0:26:49it sends huge amounts of electrical charge to the Earth,
0:26:49 > 0:26:51temporarily increasing the electrical field
0:26:51 > 0:26:56in the middle atmosphere and creating these giant sparks -
0:26:56 > 0:26:59an event so brief and so faint,
0:26:59 > 0:27:03it's almost invisible to the naked eye, finally revealed.
0:27:05 > 0:27:10You and I live on seconds or minutes or maybe even years of timescales,
0:27:10 > 0:27:13and sprites are one one-thousandth of a second.
0:27:13 > 0:27:17That makes you realise how different the world can be.
0:27:22 > 0:27:23To give you a sense of scale,
0:27:23 > 0:27:27a normal lightning bolt is about the diameter of my thumb.
0:27:27 > 0:27:30But a sprite can be up to 80km wide.
0:27:30 > 0:27:33New discoveries are being made all the time.
0:27:33 > 0:27:36I've got some very special footage here.
0:27:36 > 0:27:38It's taken from the International Space Station
0:27:38 > 0:27:42and it's video looking down on the top of a thunderstorm,
0:27:42 > 0:27:45so there's lots of flashes of lightning.
0:27:45 > 0:27:48If I stop it at just the right moment...
0:27:48 > 0:27:50we'll see something very special.
0:27:50 > 0:27:52And here it is.
0:27:52 > 0:27:56It's a blue jet. It's much brighter than a sprite.
0:27:56 > 0:27:59It's the first time it's been filmed from space.
0:27:59 > 0:28:02It's a huge electrical discharge,
0:28:02 > 0:28:06going from the top of the thunderstorm out into space.
0:28:06 > 0:28:10And we don't know very much about what these are or how they form,
0:28:10 > 0:28:12but now that we have this sort of footage,
0:28:12 > 0:28:15we stand a very good chance of answering those questions.
0:28:18 > 0:28:22Lightning is one of the deadliest natural forces on Earth,
0:28:22 > 0:28:26striking our planet at random millions of times every day.
0:28:26 > 0:28:30There are still many secrets to be revealed
0:28:30 > 0:28:32but, thanks to new camera technologies,
0:28:32 > 0:28:34we're getting closer than ever before
0:28:34 > 0:28:38to understanding this spectacular phenomenon.