By the time you’ve read this sentence, lightning could have struck Earth as many as 270 times. That may sound like a lot, but according to the UK’s Met Office, there are about 44 lightning strikes every second. Put another way, that’s about 3,000,000 strikes a day, globally.
Given that amazing frequency and the fact that scientists have been observing this natural phenomenon for centuries, we must have masses of data on lightning and a pretty good idea of what causes it, right?
Well, not quite.
We’ve certainly tried to get to the bottom of it and, over the years, the quest to understand lightning has inspired many strange, and in some cases productive, endeavours. In 1752, for example, Benjamin Franklin’s famous kite experiment proved that lightning was an electrical phenomenon.
Since then, we’ve sent balloons and rockets into storm clouds in a bid to get a closer look at the conditions that produce it, and have gotten an entirely new perspective on lightning by observing it from space.
In 2023, scientists from Norway reported new data, collected with the help of a Cold War-era spy plane, that offered exciting new insights. But despite all this, exactly what sparks off a lightning strike remains a mystery.
That’s not to say we’re still entirely in the dark about it, though. We do know that lightning is a large-scale discharge of electrical energy. It can discharge from clouds towards the ground, within clouds or even from clouds upwards into space.
That charge is built up as tiny particles of hail, ice and water move through the turbulent environment within a thundercloud. When they bump into each other, they knock electrons free and transfer charge from one particle to another – much like rubbing a balloon against your head can create a static charge that makes your hair stand on end.
Typically, small, positively charged ice crystals tend to get carried upwards to the top of the cloud, while heavier, negatively charged lumps of wet hail drift down towards the bottom. The resulting polarisation generates an electric field.
At the same time, a pool of positive charge builds up on Earth’s surface below the storm cloud. It’s now essentially a giant battery in the sky, but instead of the batteries containing a few volts that we’re accustomed to using in our homes, this cloud battery consists of a hundred million volts.
Once the electric field becomes great enough, a lightning channel forms and dissipates the electric field. The lightning channel is “very narrow, very hot and extremely conductive of electricity. It almost behaves like a wire in the cloud,” says Prof David Smith, who specialises in atmospheric science and astrophysics at the University of California, Santa Cruz, in the US.
We’re most familiar with lightning that travels from clouds to the ground, but around 75 per cent of lightning actually happens within the storm cloud. That’s because the electric field within the clouds is much stronger than the one between the clouds and Earth’s surface.
But wherever it comes from, there’s still a fundamental question we can’t answer: how does lightning start?
Hidden in the clouds
Part of the reason we still don’t fully understand how lightning forms is that it’s extremely difficult to study. “It’s happening up in a cloud, which is a very hostile place,” says Smith. “And in the moments before a strike, when lightning is forming, it’s shrouded from sight by clouds.”
It’s very dangerous to fly directly through a storm cloud and most planes can’t reach altitudes high enough to fly above them, so researchers have largely been restricted to studying lightning from the ground below or from above, in space.
And that’s not all. The very act of trying to study lightning risks disrupting the conditions needed to create it. “An aeroplane is a big piece of metal and when you put that in the cloud, you’re disturbing the electrical environment that you want to explore,” Smith says.
Because we can’t easily get inside a storm cloud to study lightning directly, much of the research into it relies on measuring lightning indirectly. Light and radio waves, for example, are created when electrical charges move – giving scientists clues as to where lightning is forming.
A common method to study lightning as it forms uses a technique called radio interferometry, which detects the radio waves using antennae at ground level.
Scientists can use this approach to triangulate the location of electrical discharges inside thunderclouds to within a few tens of meters. But it doesn’t tell them exactly what’s triggering the lightning.
Another method in the lightning scientists’ toolkit is creating artificial lightning in a specialised laboratory.
“The advantage of using a laboratory is that it’s very controllable and repeatable,” says Dr Daniel Mitchard, a particle physicist at Cardiff University’s Lightning Laboratory. In other words, researchers can collect measurements that would be nearly impossible in real clouds due to the unpredictable nature of lightning.
“You can put a lot of instrumentation very close to that lightning arc to measure things like energy and temperatures,” Mitchard explains.
However, artificial lightning strikes are “as destructive as they are in real life,” he says. “So, we can’t be anywhere near [the generated lightning] when it goes off. You can’t look at it because it’ll blind you. You have to wear ear protection, because we also produce the thunder – and it’s all within a shielded room.”
Only in the last 30 years or so have we had the technology to study lightning clouds from above – using space shuttles and satellites – and this has opened new avenues of research and reinvigorated the quest to understand how lightning forms.
Read more:
- What is ball lightning?
- This is what it's like to be struck by lightning
- Are giraffes more at risk of being struck by lightning?
Electron Avalanche
A key player in the mystery of how lightning starts is the fact that air is an extremely good insulator – in other words, it doesn’t readily conduct electricity. So, an enormous electric field is needed to overcome that insulating power and create lightning that can jump gaps spanning several kilometres.
Scientists have calculated the size of the electric field needed (known as the ‘breakdown threshold’), but when they’ve measured the electric fields inside thunderclouds, they don’t come anywhere close to that threshold.
“You’ve got millions of lightning flashes a day and yet you never seem to see an electric field big enough to actually make a spark,” says Joseph Dwyer, professor of physics and astronomy at the University of New Hampshire, in the US. “It’s one of the biggest mysteries in the atmospheric sciences.”
Scientists have proposed a number of explanations for this apparent contradiction. The first is that the huge electric fields needed to overcome the insulating power of air really are present in storm clouds, but we’ve just failed to measure them.
For example, the electric field might only occur in small pockets within the thundercloud, or the equipment we’re sending up might be discharging the field before we can measure it.
Another possibility is that ice crystals increase the strength of the electric field around their tips, creating small bursts of electrical discharge that can merge to create a lightning channel.
But many lightning scientists now believe there’s another piece to the puzzle of lightning initiation, involving high-energy particles. According to this hypothesis, the electric field within a thundercloud launches free electrons at almost the speed of light.
These high-energy electrons collide with atoms in the cloud, knocking loose more electrons and creating “an avalanche of high-energy particles,” Smith says.
This process might even be jump-started by cosmic radiation: high-energy particles originating from space. Experts believe that the electron avalanche could generate electric fields large enough to exceed the breakdown threshold (that massive electric field needed to counteract the insulating power of air) and generate lightning.
All these colliding electrons also release gamma rays – a type of high-energy radiation more commonly associated with cosmic events like supernovas than everyday atmospheric events here on Earth.
Some experts believe that measuring the gamma rays emitted by thunderclouds is a way to probe the electrical fields within the cloud without disturbing them, bringing us closer to understanding how lightning strikes begin. “You can sort of X-ray a thunderstorm with this method and see what’s going inside,” says Dwyer.
Glowing clouds
In the 1990s, NASA spacecraft detected short, extremely bright flashes of radiation coming from Earth’s atmosphere, which they termed ‘terrestrial gamma-ray flashes’ or TGFs. These intense bursts of gamma rays were later shown to come from thunderclouds.
“They’re so bright that satellites in low Earth orbit are temporarily blinded by them,” Dwyer says.
Perhaps most excitingly: the TGFs were almost always associated with lightning activity.
This supports the high-energy hypothesis and suggests that at least some lightning strikes may be kick-started by an electron avalanche. “The fact that you see gamma rays means the electric fields are already getting pretty strong over large parts of the thunderstorm,” says Dwyer.
Before you ask whether so much radiation happening in the clouds could seriously hurt us, it’s worth noting that experts emphasise that it’s very unlikely to pose a risk to people, even when lightning strikes a commercial plane.
Researchers have also detected another type of gamma-ray emission coming from thunderclouds. These ‘gamma-ray glows’ are much dimmer and last much longer, continuing for seconds or even minutes, rather than microseconds.
It’s not clear how TGFs and gamma-ray glows are related to each other, however, or how they’re connected to lightning… if at all.
High-altitude mission
Hoping to get to the bottom of all this, researchers at the University of Bergen, in Norway, set out on a new mission to study gamma-ray-emitting thunderclouds at close range in 2023.
They used a specialised research plane to fly over an active thundercloud in the Caribbean. The plane, a Cold War-era spy plane retrofitted by NASA to act as a flying laboratory, is known as the ER-2.
Using the ER-2, the researchers were able to reach an altitude of 20km (65,000ft) – around 6km (20,000ft) higher than any previous lightning research mission had been able to get.
From this new viewpoint, they managed to detect much weaker gamma-ray flashes than those detected by satellites and spacecraft, and measure TGFs and gamma-ray glows in unprecedented detail.
Research flights like this carry a wide array of sensors that generate huge amounts of data – far too much to download in real time. That means the data is usually only analysed after the flight is complete. But the researchers had a trick up their sleeves: they downloaded a low-resolution version of the gamma-ray data they were collecting every second.
This allowed them to direct the pilot to track the gamma rays more closely for as long as the ER-2’s fuel reserves would allow.
They found a gamma-ray-emitting thundercloud thousands of square kilometres across that glowed for hours on end. And, far from a static, continuous emission, the gamma-ray glows pulsated through the cloud.
“It’s not uniformly glowing. It’s like a boiling pot,” says Prof Nikolai Østgaard, a space physicist at the University of Bergen and one of the scientists behind the research.
They also discovered an entirely new phenomenon, which they called ‘flickering gamma-ray flashes’. Longer than TGFs, but shorter than gamma-ray glows, these pulsing emissions seem to be the missing link between the two. They also seem to be linked to lightning.
“After these flickering gamma-ray flashes, there was intense lightning activity,” says Østgaard.
But, as Østgaard points out, it’s not the gamma rays that are initiating the lightning. The gamma rays are merely a by-product of the avalanche of high-energy electrons, which may play the initiating role.
Computer modelling suggests that the flickering effect is caused by the generation of positrons (the anti-matter equivalent of electrons) during the high-energy electron avalanche.
The positrons in turn generate more electron avalanches, creating a positive feedback loop that experts believe might be the crucial clue in the mystery of lightning initiation.
These models also show that the flickering gamma-ray flashes discharge the electric field in one part of the thundercloud, while increasing it in another – which could generate electric fields large enough to initiate lightning.
“It’s kind of like stepping on a bump in the carpet. You can push it down in one place, but then it pops up someplace else,” says Dwyer.
Spurred on by their recent discovery, Østgaard and his colleagues are already planning their next mission. They want to kit the ER-2 out with even more sensitive instruments to gain a better understanding of the link between flickering gamma-ray flashes and lightning.
They hope this will bring us one step closer to a full scientific understanding of how lightning forms. But, if past discoveries are anything to go by, what they find may also generate more questions.
About our experts
Prof David Smith is a professor of atmospheric science and astrophysics at the University of California in the US. He has been published in a variety of scientific journals including The Astrophysical Journal, Solar Physics and Nature.
Dr Daniel Mitchard is a particle physicist at Cardiff University who researches lightning at the Lightning Laboratory. His work is published in the likes of Applied Composite Materials, Scientific Reports and Applied Sciences.
Joseph Dwyer is a professor of physics and astronomy at the University of New Hampshire in the US. He has been published in the Journal of Geophysical Research - Atmospheres, Geophysical Research Letters and Physics Reports - Review Section of Physics Letters to name a few journals.
Prof Nikolai Østgaard is a space physicist at the University of Bergen in Norway. His work has been published in journals such as Scientific Reports, Earth and Planetary Physics and Journal of Geophysical Research.
Read more:
