How to see thunder
NARRATION: Wherever you live on the planet weather shapes your world. Yet for most of us how it works is a mystery. So, I鈥檓 going to strip weather back to basics, uncovering its secrets in a series of brave ambitious and sometimes just plain unlikely experiments - to show you weather like you鈥檝e never seen it before.
For most of us the only extreme weather we鈥檙e likely to encounter is thunder and lightning. We鈥檝e all heard thunder and seen lightning, but believe it or not it鈥檚 actually possible to see thunder too.
To find out more I鈥檓 visiting one of the few places in the world capable of creating full-blown thunder indoors. They do it by firing 200,000 amps of electrical current down this narrow copper wire. Exactly the same amount as in real lightning.
RICHARD: So this is it, this is where it鈥檚 all controlled?
DAN MORGAN: Yep, so you鈥檒l need a pair of these.
RICHARD: Will I?
DAN: As it鈥檚 going to be quite loud.
RICHARD: Is it?
DAN: As we鈥檙e going to be producing thunder.
RICHARD: OK.
DAN: So don鈥檛 look directly at the arc because it鈥檚 very bright.
RICHARD: Right, so I鈥檝e come quite a long way to see something that I can鈥檛 look at or listen to.
DAN: Pretty much!
RICHARD: Good, OK, brilliant.
DAN: So now we can see the voltage on the capacitors.
RICHARD: Oh, yeah! So essentially this is going to build up a colossal charge and then discharge it?
DAN: Yeah.
RICHARD: Dan?
DAN: Yeah.
RICHARD: I can hear what you鈥檙e saying.
DAN: Yeah, when the shot goes through you might want to put your hands over the defenders too as it鈥檚 quite loud.
RICHARD: Shall I just cower under the table? I don鈥檛 mind.
DAN: Or you could cower under the table.
RICHARD: Right it says 25.
DAN: Yeah we鈥檙e nearly there. OK, we鈥檙e ready so we can fire.
RICHARD: But I can鈥檛 look?
DAN: Yeah, we can鈥檛 look.
RICHARD: Or listen. I imagine 鈥 BANG! Oh-ho! That, in fact, was quite staggeringly loud.
DAN: Yeah.
RICHARD: I mean really amazingly loud.
DAN: Yeah.
RICHARD: So, was that thunder or was it just, sort of, the discharge of the electricity leaving and arriving?
DAN: No, that was thunder.
NARRATION: But it still doesn鈥檛 explain how we might see thunder. Luckily Dan has a way to show us 鈥 using slow-motion cameras and a line of lit candles. [ALARM BEEPS, ALARM BLARES, BANG!] The candles are all blown out. And if you watch carefully, you can see that they鈥檙e blown out one by one. So what is going on? Well, it鈥檚 all to do with temperature.
TO CAMERA: A typical bolt of lightning is somewhere between two and five centimetres wide so something close to that. These by the way are not for style reasons they鈥檙e for protection because effectively I鈥檓 taking a shaft of sunlight as wide as this screen and focusing it down to something roughly the size of a bolt of lightning. It is hot but it鈥檚 nothing compared with lightning. A typical bolt will reach 20 000 degrees Celsius - that鈥檚 well over three times the temperature of the surface of the sun itself. Thankfully it only lasts for about one ten-thousandth of a second, but that鈥檚 still enough for something quite amazing to happen.
NARRATION: Because lightning is so ferociously hot it explodes the air around it鈥ausing it to rush outwards. What we see with the candles is that air moving away from the lightning bolt in a shock wave. But just how powerful is this wave? Time for another experiment. Can thunder break these glass light bulbs?
DAN: One way to find out.
RICHARD: Zap 鈥榚m!
DAN: Yes.
NARRATION: The thunder blows them apart. And it鈥檚 so dramatic that I want to watch it again in slow motion.
RICHARD: I really want to see this one cos I still can鈥檛 believe it was strong enough. That鈥檚 a heck of a wave!
NARRATION: Notice it鈥檚 not the lightning destroying the light bulbs. The arc never even touches them. It鈥檚 the shock wave after the flash that does the damage. But I want to see more.
TO CAMERA: What we鈥檝e been looking at impressive though it is is the effect of thunder. I want to look at the thunder itself.
NARRATION: With very specialised cameras we can actually attempt to capture that shock wave on screen. Not the effects, but the actual shock wave itself.
RICHARD: That鈥檚 fantastic, that鈥檚 absolutely brilliant!
NARRATION: That is the air exploding away from the hot lightning bolt at over 700 mph.
RICHARD: I think we can count that one a definite success.
NARRATION: So it is possible to see thunder. And if you鈥檙e unlucky enough to be stood near 鈥 you can feel it too.
Richard Hammond visits Lightning Testing and Consultancy in Oxfordshire to take part in laboratory experiments that demonstrate the effect of thunder within controlled conditions.
There are four stages to the demonstration. The first is a test firing of the experiment, where the volume of the thunder can be heard.
Next, a series of candles and slow motion cameras are used to show how thunder鈥檚 shock wave travels, extinguishing the candles. An explanation of how temperature differences cause thunder is given outside using a lens and wood.
A typical bolt of lightning will reach 20,000 degrees Celsius. Because of this heat, the air around it expands into a shock wave.
A third iteration of the demonstration sees thunder breaking light-bulbs, and slow motion allows viewers to distinguish between the shock wave (thunder) and the lighting.
The final demonstration allows the shock wave itself to be seen.
Teacher Notes
This short film provides useful background information for KS3 classes studying storms. For KS4 / GCSE physics, this short film can form an introduction to forces, where students can consider experiment design and the practical application of laboratory science.
Curriculum Notes
This topic appears in geography and physics at KS3 and KS4 in England, Wales and Northern Ireland and National 5 in Scotland. At GCSE it appears in AQA, OCR A, EDEXCEL, EDUQAS, WJEC and CCEA, in SQA at National 5.
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