The Electrifying Debate Around Where Lightning Comes From

Along with many other natural phenomena, lightning is probably familiar to most. Between its intense noise and visuals, there is also very little disagreement that getting hit by a lightning strike is a bad thing, regardless of whether you’re a fleshy human, moisture-filled plant, or conductive machine. So it’s more than a little bit strange that the underlying cause of lightning, and what makes certain clouds produce these intense voltages along ionized air molecules, is still an open scientific question.

Many of us have probably learned at some point the most popular theory about how lightning forms, namely that lightning is caused by ice particles in clouds. These ice particles interact to build up a charge, much like in a capacitor. The only issue with this theory is that this process alone will not build up a potential large enough to ionize the air between said clouds and the ground and cause the lightning strike, leaving this theory in tatters.

A recent study, using data from Earth-based radio telescopes, may now have provided fascinating details on lightning formation, and how the charge may build up sufficiently to make us Earth-based critters scurry away to safety when dark clouds draw near.

Follow The Streamers

The triboelectric effect, demonstrated by a cat and styrofoam peanuts. (Credit: Sean McGrath)

The most succinct way to summarize lightning is probably as a ‘really big spark’. Like any spark, this requires the build up of a potential to the point where the dielectric medium between the potential and a target breaks down. In the case of air as the dielectric medium, this involves the breaking down of air molecules, at roughly 2 MV/m, into electrically conductive plasma by stripping electrons from atoms. Once there is a plasma track, the rest of the charge is free to dump; during the subsequent discharge in a lightning strike, in the order of a gigajoule of energy is transferred.

To build up this charge, the triboelectric effect is deemed to be primarily involved. While heavier graupel (soft hail) remains mostly stationary inside the cloud, updrafts carry lighter ice crystals and super-cooled droplets between the graupel, resulting in the physical interaction that underlies the triboelectric charging effect.

As noted earlier, the resulting charge that builds up inside a thundercloud in this manner is not sufficient by itself to cause a lightning stream with the ground. This has for ages left open the question of what initiates the build-up of such a breakdown. One theory here is that of relativistic runaway electron avalanche (RREA), with an extensive explanation provided by Gurevich et al. (2005) in Physics Today (PDF).

Interaction between rising ice crystals and graupel (soft hail) in thunderclouds. (Credit: NOAA)

The short version of this theory is that relativistic electrons from outside the atmosphere are the trigger for the sudden initiation charge of a lightning strike event. As the electrons that exist inside the cloud at that point are thermal (‘slow’) electrons, this would provide the impulse that would set off the formation of self-replicating streamers.

Streamers are the clearest observable sign of a lightning strike in progress: they are ionization events caused by the breakdown of the air molecules into a plasma. This does not only provide an electrical path for the stored charge, but also causes the release of electromagnetic radiation. If there are enough free electrons to continue the further ionization of the surrounding air, the streamer will propagate, with ultimately a leader surge forming.

It is this leader that forms the clear bright line in a lightning strike, preceded and surrounded by streamers.

Lit Up Like A Christmas Tree

Thunderstorm in the Mojave Desert. (Credit: Jessie Eastland)

If there’s one property of clouds that makes them a bother when it comes to observing the formation of lightning, it is that they are opaque to most types of observations. Whether we send up balloons with cameras, fly airplanes around or through thunderstorms, the problem remains that we cannot see a lot of what’s happening inside the clouds. The main exception to this is in the RF spectrum, which is where thunderstorms are exceptionally visible.

As mentioned by Gurevich et al., during thunderstorms RF events called narrow bipolar pulses of about 5 μs are common, along with X-ray bursts that can last up to a minute. These events are in addition to the release of gamma ray bursts (0.05 MeV – 10 MeV) that occur at an altitude of between 500 and 600 km. For Gurevich et al. these highly energetic events were highly indicative of relativistic electrons playing a role in lightning ignition.

Due to the aforementioned difficulties with actually seeing inside clouds, all of this was based on observations in mostly the radio spectrum, until a 2018 attempt by the Dutch Low-Frequency Array (LOFAR) radio telescope was it possible to image the formation of a lightning strike in 3D space. As implied by the name, LOFAR consists out of an array of distributed radio telescopes, with a core in the Netherlands and further distributed throughout Europe.

Using the Dutch part of the array, it was possible to use the time-delay from the observed RF events during a lightning storm at the different parts of the array to form a picture of the events as they developed. This enabled Sterpka et al. to create the following hypothesis based on these observations.

Sketch of proposed initiation process based on observations. (Credit: Sterpka et al.)

What they found is that, rather than an external trigger, the process starts (a) with a single positive streamer (b), which creates the initial VHF field (c). The middle panel describes an avalanche cascade of streamers, with each streamer producing more streamers, creating the ever stronger VHF field, which the LOFAR equipment captured. Finally, the right panel shows the formation of a leader (h).

Sterpke et al. propose that the narrow bipolar pulses that are observed at the initialization event are due to the breakdown of virgin air or similar, which forms a precondition for the lightning initiation. Perhaps the most interesting finding is that a system of streamers appear to have very different properties from an individual streamer, which might play a significant role in the formation of lightning.

Flashy Science

A fulgurite from Okechoobee in Florida. (Credit: Mario Hendriks)

Interestingly, lightning is a phenomenon that doesn’t just affect the present, but also the past. When lightning strikes the ground, it can cause the formation of fulgurites (from Latin fulgur, meaning ‘lightning’). This is a type of mineraloid formed by the fusion of mineral grains, which follows the shape of the lightning strike which was responsible for its formation. Within a lightning channel, temperatures can exceed 30,000 Kelvin.

More than just a curiosity, fulgurites are an invaluable paleoenvironmental indicator, giving us an idea of what the climate including lightning frequency was like in a region. This is part of the field of paleolightning. Along with evidence of the role lightning may have played during the formation of life on Earth (e.g. the Miller-Urey experiment), lightning plays a role in nitrogen fixation.

Another interesting, more recent finding came from during the SARS-CoV-2 pandemic and the shutdowns of 2020. Namely that of the correlation between pollution and the frequency of lightning. This would presumably be due to anthropogenic pollution providing e.g. fine dust particles which form a nucleus for ice formation inside clouds.

Hopefully over the coming years we’ll find out more details about what makes lightning work, as well as what affects the formation of the cumulonimbus clouds. With more refined radio telescope arrays like the Square Kilometre Array (SKA) coming online, it might be that they could give us more glimpses at this phenomenon that has mesmerized human cultures since the dawn of time.

While being an obvious risk to life and property, lightning remains one of the primal forces that shaped the biosphere of the Earth, and continues to do so to this day. Even as we admire its beauty from a (hopefully) safe distance, it might behoove us to acknowledge how little we truly know about it and the high-energy physics that underlie it.