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fountains 3: shining a light on lightning

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3888430-thunderbolt-storm-lightningThis one’s for Courtney. Another transcript of a ‘fountains of good stuff’ podcast, the third. Haven’t finished recording the podcast yet: will put it up in the next day or two.

The other night, when we saw lightning in the sky, the two people I was with started counting, apparently timing the arrival of the sound of thunder. There followed a brief conversation about the connection between lightning and thunder, with one of my companions, a lively ten-year-old, wondering aloud about what lightning actually was. Not surprisingly, I didn’t have an adequate answer to hand, so I’ve visited the fountains to explore this everyday but intriguing phenomenon.

Although I’m not a fulminologist [that’s an expert on lightning], I do know, like most people, that it’s about electricity. Beyond that, as is usual with electricity, it gets complicated. The simple answer is that lightning is electrostatic discharge, within clouds, or between clouds, or between clouds and the ground. But what is electrostatic discharge, and what causes it?

Well, electrostatic discharge is a sudden flow of electric current between two bodies. We’ve all experienced this in a minor form – some of us much more than others – when touching car doors, or when wearing a nylon shirt, for example. What we call ‘static electricity’ is a charge that builds up on the surface of a body, as opposed to an electrical current directed through a wire. When two bodies of opposite polarity – that’s to say, positive and negative charge – come together, the electric potential between them is neutralized by an electrostatic discharge.

Lightning is obviously a very spectacular, and potentially dangerous, form of static discharge. The bright light we see in a lightning flash is a form of incandescence. The material, in this case the air, in the discharge channel is superheated to a glowing blue-white, similar to the glow of an incandescent globe. In lightning, this ionised air, or plasma, can on occasion, reach a temperature 5 times the surface of the sun.

So how is it that this polarisation of charge occurs in the first place? Well, in the case of everyday objects, this can be explained by the triboelectric effect, but I’ll leave that for another day. In the case of lightning, this polarisation, and the creation of an electric field or a charge separation between the poles, is not fully understood. Fulminology, and meteorology in general, are not exact sciences.

One key to the understanding of lightning is water, in its various forms. Firstly, let me briefly describe the world’s water cycle. Surface water gets warmed by the sun and the surrounding sun-baked earth, which causes its molecules to become active or excited, and to move about. Eventually they move about fast enough to escape the surface and rise into the atmosphere as vapour or steam, a process called evaporation. The water molecules rise and rise, but as they do so, they begin to lose their heat to the upper, cooler atmosphere, and return to their less excited, watery state. This is called condensation. The condensing molecules collect together to form clouds in the sky, which eventually become heavy enough to fall back to the ground as rain.

Sometimes an electric field develops within these clouds, with a positively charged upper portion and a negatively charged lower portion. This charge separation appears to be the result of complex interactions and collisions between the millions of condensing, and sometimes freezing, water droplets in the cloud, and vapour rising from below to join the cloud. As a result of these collisions, electrons are removed from the rising vapour, and they collect near the base of the cloud, forming a negative charge, while the now positively charged vapour continues to rise up through the cloud. The freezing of water molecules contributes to, and complicates, this process. The ice becomes negatively charged while the positively charged unfrozen droplets are carried by air currents to the top of the cloud. The charge separation and its associated electric field thus result from a complex combination of collisions and freezing. Over time, the electric field tends to become more intense, and the negatively charged base of the cloud repels electrons on the ground, driving them deeper underground, and leaving a strong positive charge over the earth’s surface. The situation is now set for a conductive pathway to neutralise the charge separation between cloud and ground.

The conductive pathway is created when the electric field is strong enough to ionise the air around it. That means the air becomes separated into electrons and positive ions. Its original molecular and atomic structures have broken down, allowing electrons to move much more freely – which is essential to electrical conductivity. The ionised air or plasma has properties similar to those of a highly conductive metal. It essentially burns a pathway from the cloud base to the ground, which the lightning follows. Lightning is in fact a discharge, because it’s a release or neutralization of accumulated electric charge.

The pathway between cloud and ground isn’t instantaneously created, but occurs in steps. Often a number of first steps are taken out from the cloud base, and these are called step leaders. They seek a pathway to the ground along the line of least resistance, which isn’t necessarily a straight line. Variations in air temperature, pressure, wind, dust particles, all of these can affect the jagged trajectory of the lightning path. This also explains why, contrary to prevailing myths, lightning doesn’t always strike the highest object in the target landscape.

The field of attraction between positive ground and negative cloud base also results in phenomona called positive streamers, that’s to say, streams of positive charge reaching up from the ground, or from objects on the ground, towards the step leaders. These purplish-white streamers, only recently discovered, have been caught on camera many times. They only rise to a limited height, and the ‘successful’ step leader connects with a streamer close to the ground. Once this pathway is created, the surge of electrical discharge comes.

We see the effects of this discharge in a flash of blue-white light, and we also hear it. The extreme heat from the discharge causes the air to explode outward, creating a compression wave or shock-wave we know as thunder. While not as dangerous as lightning, thunder can also do damage to humans and buildings if they’re close enough to the action. Of course, thunder propagates through the air much more slowly than light, so it’s true that you can roughly judge the distance of the strike by counting the seconds it takes for the thunder to arrive afterwards. Often, though, you get a sound of rolling thunder, because the jagged trajectory of the lightning means that the thunder starts out from different distances.

Lightning occurs most often in tropical regions, where warm, moist air circulates through an electric field, though it can also occur during snow-storms, dust-storms, bushfires and volcanic eruptions. In fact, though, tropical lightning is less likely to hit the ground, because the cloud ‘freeze zone’ is too high, due to the generally warmer lower atmosphere. In Scandinavian countries, where the freeze zone is much lower, as much as half of all lightning is CG, or cloud to ground lightning, though in those parts of the world, lighting in general is much rarer. In fact, the most common form of lightning occurs within individual clouds, with only about a quarter of all lightning globally being cloud to ground lightning.

Now, what about lightning and safety?

It’s estimated that about 10% of lightning strikes to humans are fatal, and those are not good odds. The safest place to be during a lightning storm is inside your house, or anyone else’s for that matter. Don’t stand under trees, they’re very likely to be hit in a lightning strike. You’re also safer in a car, because of the shielding effect of the metallic car body. The electrical current travels around the body of the car, to the ground. This was an effect first described by Michael Faraday. A metal cage, which in a sense is what a car is, protects its contents from damage because the highly conductive metal diverts the high electrical potential around the contents, to the ground.

Also, if you’re outside, crouching down is best – but don’t lie on the ground, as an electric potential radiates out from the point of ground contact. If you’re within this field you could become electrically charged. Maintain as little contact as possible with the ground, and make yourself small and compact. That’s the safest option for avoiding being electrically charged.

There have been quite a few recent developments in the science of lightning. For example, in the difference between positive and negative lightning strikes – what is called strike polarity. Positive strikes are much rarer, and they’re generally considered stronger, though this isn’t necessarily the case. There’s also the intriguing phenomenon of upward lightning, which you can see in online videos.

Also, theoretical models indicate that lightning quantity is very sensitive to temperature and cloud-top altitude. So, with global warming, it’s likely that lightning will be an even more regular occurrence than it is today. Just another thing for our burgeoning population to worry about.

As always, I rely heavily on other sources for these podcasts. I recommend the extensive article on lightning in ‘How stuff works‘, and there are plenty of cool videos of lightning of all kinds to be found online.

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Written by stewart henderson

January 5, 2013 at 5:38 pm

Posted in science

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