Essay: ‘Lightning – The Dreaded Bolt Partially Explained’ by Gavin Pretor-Pinney

To end this week, we’ve got another lovely literary extract for you to ponder across the weekend. Vauxhall are collaborating with West London publisher The Idler, to release ‘We’re Electric‘ – a book which sees the likes of Will Self and Gavin Pretor-Pinney examine the part electricity has played in industry, entertainment, art and fashion. The book collates essays, illustrations and reflections on electricity, to mark the launch of Vauxhall’s first electric car. We’ve got an exclusive extract from celebrated Cloudwatching author Gavin Pretor-Pinney here. It’s perfect weekend reading, so sit back, relax and read on…

“We call that fire of the black thunder-cloud ‘electricity,’” wrote Thomas Carlyle in 1876, “…but what is it? What made it? Whence comes it? Whither goes it?” You’d think that by now we’d have worked out the answers, but lightning is no easy thing to study. The ferocious, turbulent heart of a storm cloud isn’t an environment conducive to careful scientific observation, not least because it is very hard to predict exactly when and where a bolt is going to strike. To this day, the finer points of lightning remain obscure.

We’ve got the basics worked out, of course. The cloud type that produces thunder and lightning is the Cumulonimbus. Often described as the King of Clouds, this mighty meteorological beast can tower 8–10 miles up into the atmosphere, spreading out over hundreds of miles at its top to resemble an enormous mushroom, or a blacksmith’s anvil. Within its tumultuous core, the cloud generates powerful currents of rising air, which churn up a furious confusion of raindrops, hailstones and ice crystals. These clouds produce lightning on account of the violent collisions that occur between their hail and the ice crystals.

The secret lies in the differences between the surfaces of the hail and ice crystals. The hailstones are irregularly shaped lumps of ice, growing as rainwater freezes onto them in layers while they are swept around within the cloud. The particles of hail that are yet to freeze completely solid are known as soft hail or ‘grauple’. They have fluffy, snowy surfaces, quite different from the cloud’s ice crystals, which as well as being far smaller than the hail, have smooth surfaces and orderly hexagonal forms. The differences may be microscopic but as they jostle and barge within the celestial mosh pit they lead to an enormous separation of electric charge within the cloud – the essential prerequisite for lightning to form.

It turns out that the fluffy surface of grauple makes it rather adept at picking up electrons. Grauple is better at holding onto electrons than the orderly little ice crystals. So when the two collide within the turbulent heart of a Cumulonimbus, the soft hail robs the ice crystals of electrons.

Since electrons are negatively charged, the grauple, having a surfeit of them, develops a slight negative electric charge. The electron-depleted ice crystals, by contrast, are left with slightly positive charges. The fierce storm winds that sweep up the centre of the cloud do the critical job of separating these charges. They waft the lighter ice crystals upwards more easily than the heavier hail. Positive charge therefore builds up amongst all the ice crystals that make up the anvil in the cloud’s upper reaches, while negative charge develops amid the hail that predominates in the middle and lower parts. At least, that is the way the charge normally separates out. On occasions, for particularly large storms, the negative charge can develop above the positive charge. Don’t ask me why. Whatever the orientation, the separation of charge within the storm grows and grows until it becomes so great that the air in between no longer works as insulation and an almighty redistribution takes place. A massive spark of electrical discharge, the lightning bolt, leaps across the divide to even out the difference between positive and negative regions of the cloud.

‘Within-cloud’ lightning like this tends to be hidden from view by the cloud itself. Diffused by the mass of water particles, the bolts illuminate large areas of the cloud all at once. This is an effect that people sometimes describe as sheet lightning, though the bolts themselves have the same forking paths as the familiar ‘cloud-to-ground’ lightning.

It is the extreme heat of the lightning bolt that produces the crashing rumble of thunder. The bolt instantaneously heats the air to around 27,700°C, more than four times as hot as the surface of the Sun. Since the heating takes place within a few millionths of a second, it causes an explosive expansion of the air around the channel of the bolt. The resulting shock waves are what cause the sound we hear as the tearing clap of thunder.

There are actually two distinct types of cloud-to-ground lightning: negative and positive, depending on whether the charge released to the ground comes from a negatively or positively charged part of the cloud. Since the negative parts are more often found at the bottom of the cloud, and so nearer the ground, negative lightning is more common. A huge amount of positive charge needs to build up in the icy upper reaches of the Cumulonimbus for a bolt of positive lightning to leap the eight or ten miles from anvil to ground and only very powerful storms tend to have their positively charged regions down nearer the ground. This is why, on the occasions that it does occur, positive lightning tends to be far more powerful than negative.

It might also help to explain the appearance of ‘sprites’ in the atmosphere above particularly powerful thunderstorms: not mischievous little fairies, but enormous, fleeting and rather mysterious electrical phenomena induced by the massive redistribution of electric charge in particularly powerful lightning flashes below. As is often the case with scientific discoveries, the first observation of sprites came about completely by chance.

In 1989, John R. Winckler, a professor at the University of Minnesota, was testing a very sensitive low-light video camera for a rocket launch. On reviewing the tape, Winckler noticed a frame that appeared to capture a giant column of light rising above a thunderstorm near the US–Canadian border. He showed it to a colleague, Dr. Walter Lyons, who was developing a lightning detection network at the university, and the two decided that this was no technical fault. It appeared to be some hitherto unidentified electrical discharge.

In the following years, Lyons became a world authority on the tricky task of capturing these electrical phenomena on film, which he did from an observation platform at his home looking over the Great Plains of Colorado. This region of the US is the perfect breeding ground for the enormous storms that can produce sprites. “The lightning needs to be quite exceptional,” explains Lyons. “Not every thunderstorm makes sprites. You have to have a very special class of thunderstorm for lightning to become that big and that powerful.” These are called ‘mesoscale convective systems’. They’re what you and I would call whopping great thunderstorms.

A sprite will appear for just a split second in the upper atmosphere some 30–50 miles above the thunderstorm. They are often in the shape of gigantic red jellyfish, with bluish tinges in their descending tendrils, but can also form in a whole range of joyful shapes, which have earned them names like ‘broccoli sprites’, ‘octopus sprites’ and ‘Carmen Miranda sprites’.

As low-light camera technology has become more sophisticated, other strange electrical phenomena have been spotted in the atmosphere above thunderstorms. ‘Elves’, and ‘halos’, for instance. Lasting for less than a thousandth of a second, elves are far too fleeting to be seen with the naked eye. But caught on special cameras, they look like red expanding doughnuts. Rather larger than doughnuts, though. At heights of around 60-65 miles, they expand outward to several hundred miles in diameter. Halos are like elves, just a bit smaller and lower. Both seem to be induced from negative almost as much as positive lightning strikes below.

All these strange light phenomena are disconnected from the storm clouds that generate them. They coincide with the lightning bolts below but never actually connect to them. Other types of luminous discharges can appear, however, that are directly connected to the lightning bolts. Not the cloud-to-ground variety, but lightning that travels upwards from the top of the storm. Known as ‘cloud-to-air’ lightning, this sort can generate a whole other family of electrical phenomena in the atmosphere above.

Most cloud-to-air bolts are just regular streaks that discharge a short distance out of the cloud into the air around it. Very rarely, the bolts head straight upwards and connect to rare electrical events in the atmosphere above. There are ‘blue starters’ that extend just a few miles upwards, ‘blue jets’ that reach over 10 miles and then the very rarely observed ‘gigantic jets’. These start from the flash of lightning and extend right up to the ionosphere, 50 miles above the storm cloud. “The zoo is certainly becoming more crowded,” laughs Walter Lyons. As the sensitivity of modern camera technology increases, more and more of these enormous, fleeting – and still mysterious – light effects are being discovered.

What is it? What made it? Whence comes it? Whither goes it? In many senses, we still don’t know. “When you put very sensitive cameras on electrical storms you start seeing all sort of stuff,” marvels Lyons. “There’s just so much going on inside storm cloud clouds. Benjamin Franklin half explained lightning 250 years ago but we’re still making big discoveries on a yearly basis. There’s always something new.”

-Gavin Pretor-Pinney