What sparks lightning? Scientists have finally solved the mystery

For centuries, scientists have understood that lightning discharges when charge imbalances inside a thundercloud finally break down the surrounding air. Yet the moment-by-moment physics that sets the entire process in motion has been stubbornly elusive.

Now, a research team led by Penn State electrical engineer Victor Pasko may have solved the riddle. Drawing on state-of-the-art mathematical modeling and a growing library of field observations, the group has traced the chain reaction that starts with a few stray electrons and ends with a blinding sky-splitting bolt.

“Our findings provide the first precise, quantitative explanation for how lightning initiates in nature,” Pasko explained. “It connects the dots between X-rays, electric fields, and the physics of electron avalanches.”

Tiny collisions, massive sparks

At the heart of the new explanation lies the junction of two atmospheric realities. First, every thundercloud is threaded by strong electric fields as updrafts separate positive and negative charges. Second, the upper atmosphere is constantly peppered by cosmic-ray electrons that plunge earthward at nearly light speed.

Using a numerical framework dubbed the Photoelectric Feedback Discharge model, the researchers showed that a sufficiently strong cloud field can kick those cosmic-ray electrons into overdrive.

The accelerated particles bang into nitrogen and oxygen molecules, emitting high-energy X-rays and spawning a fresh wave of electrons. That, in turn, triggers still more collisions, creating what physicists call a relativistic runaway electron avalanche.

“By simulating conditions with our model that replicated the conditions observed in the field, we offered a complete explanation for the X-rays and radio emissions that are present within thunderclouds,” Pasko said.

“We demonstrated how electrons, accelerated by strong electric fields in thunderclouds, produce X-rays as they collide with air molecules like nitrogen and oxygen, and create an avalanche of electrons that produce high-energy photons that initiate lightning.”

Tracing lightning back to its roots

Doctoral student Zaid Pervez took the lead in stress-testing the model. He fed it data gathered by satellites, ground receivers, and even high-altitude spy planes that capture the hidden fireworks occurring inside storms.

Pervez also cited earlier work on compact inter-cloud discharges – tiny lightning flashes inside small, dense cloud volumes.

“We explained how photoelectric events occur, what conditions need to be in thunderclouds to initiate the cascade of electrons, and what is causing the wide variety of radio signals that we observe in clouds all prior to a lightning strike,” noted Pervez.

The results aligned. Whenever the model recreated the exact electric-field strengths inferred from observations, it predicted the same bursts of X-rays and radio waves. These are signals researchers have cataloged for decades but never fully understood.

Those invisible signals, the team argues, are the telltale heartbeat of avalanching electrons and the surest sign that a cloud is primed to fire.

The secret storm inside the cloud

Lightning scientists have long puzzled over so-called terrestrial gamma-ray flashes – millisecond blips of X- and gamma-rays detected above thunderstorms. Satellites observe them even when no obvious flash of lightning occurs. The new framework finally explains that mismatch.

“In our modeling, the high-energy X-rays produced by relativistic electron avalanches generate new seed electrons driven by the photoelectric effect in air, rapidly amplifying these avalanches,” Pasko noted.

This runaway reaction varies in strength, producing X-rays even when optical and radio signals remain weak or absent.

“This explains why these gamma-ray flashes can emerge from source regions that appear optically dim and radio silent,” he said.

In other words, a cloud can host a furious sub-visible storm of electrons and photons without creating a full-fledged stroke that humans notice from below. Only when the avalanche grows large enough does the cloud crackle with traditional lightning.

Spotting lightning before it strikes

Because the Photoelectric Feedback Discharge model spells out the exact thresholds required for avalanches to self-sustain, it offers meteorologists and atmospheric physicists a concrete predictive tool.

The published equations invite researchers to use real-time field data or test them in unusual environments.

The study could reshape how scientists interpret satellite detections of gamma-ray flashes and refine lightning-forecast algorithms. It may also illuminate high-altitude electrical phenomena such as blue jets and elves.

The research also has practical implications: lightning remains a leading cause of weather-related fatalities and costs airlines and power companies billions annually.

Storm simulations in 3D

The next challenge is to integrate the model with three-dimensional cloud-scale simulations, capturing how local avalanches merge into kilometer-long channels.

Pervez and colleagues are also eager to test whether dramatically different thunderstorm environments – such as volcanic plumes or Martian dust storms – might follow the same blueprint.

For now, the finding closes one of atmospheric science’s most enduring gaps. The unseen dance of cosmic electrons, roaring electric fields, and cascading photons turns out to be the silent countdown to every crack of lightning we witness.

By capturing that dance in numbers, Pasko’s team has illuminated the hidden spark that electrifies the sky.

The study is published in the Journal of Geophysical Research: Atmospheres.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–