3 min read

What’s Made in a Thunderstorm and Faster Than Lightning? Gamma Rays!

A flash of lightning. A roll of thunder. These are normal stormy sights and sounds. But sometimes, up above the clouds, stranger things happen. Our Fermi Gamma-ray Space Telescope has spotted bursts of gamma rays – some of the highest-energy forms of light in the universe – coming from thunderstorms. Gamma rays are usually found coming from objects with crazy extreme physics like neutron stars and black holes. So why is Fermi seeing them come from thunderstorms?

This animated GIF shows clouds moving across a dusky sky. The clouds on the right side have a gray haze extending down to the bottom of the image, where there is rain. Flashes of lightning, stretching from the cloud to the ground, light up the screen periodically.

About a thousand times a day, thunderstorms fire off fleeting bursts of some of the highest-energy light naturally found on Earth. These events, called terrestrial gamma-ray flashes, last less than a millisecond and produce gamma rays with tens of millions of times the energy of visible light.

NASA's Goddard Space Flight Center

Thunderstorms form when warm, damp air near the ground starts to rise and encounters colder air. As the warm air rises, moisture condenses into water droplets. The upward-moving water droplets bump into downward-moving ice crystals, stripping off electrons and creating a static charge in the cloud.

This animated GIF shows charge accumulating in a cloud. The cloud looms over a landscape. The bottom part of the cloud stretches nearly all the way across the image. On the left edge of the cloud, a thin portion juts upward and spreads out, looking almost like the neck of a bird with a stumpy beak on one side and a long plume on the other. During the animation, blue dots appear in the bottom part of the cloud, representing negative charges. Red dots appear in the upper part, representing positive charges.

Updrafts and downdrafts within thunderstorms force rain, snow and ice to collide and acquire an electrical charge, which can cause lightning. Under just the right conditions, the fast-moving electrons can create a terrestrial gamma-ray flash.

NASA's Goddard Space Flight Center

The top of the storm becomes positively charged, and the bottom becomes negatively charged, like two ends of a battery. Eventually the opposite charges build enough to overcome the insulating properties of the surrounding air – and zap! You get lightning.

An oval cloud dominates the center of this animation, with smaller, puffier clouds below and around it. A flash of light, signaling a lightning strike, appears below the right side of the cloud, and a cone of particles erupts from the top of the cloud. The cone starts small with just yellow particle, but as it expands upward, the yellow gives way to magenta. Then a flash occurs on the left side of the cloud, and another cone with similar colors lifts away from that site.

This illustration shows electrons accelerating upwards from a thunderhead.

NASA's Goddard Space Flight Center

Scientists suspect that lightning reconfigures the cloud’s electrical field. In some cases, this allows electrons to rush toward the upper part of the storm at nearly the speed of light. That makes thunderstorms the most powerful natural particle accelerators on Earth!

This animation shows processes where gamma rays can be created and destroyed in interactions with matter. The background is a cloudy haze, representing a sub-atomic scene. The animation opens with a large blue circle representing an atom moving across the screen. A small electron, shown as a yellow dot, arcs across the screen, just grazing the atom. In the process a gamma ray is produced, which is represented by a magenta squiggle. The gamma ray moves across the screen and runs into another blue circle, representing another atom. The gamma ray disappears, and two particles appear, a yellow dot representing an electron, and a green dot representing a positron.

Interactions with matter can produce gamma rays and vice versa, as shown here in this illustration. High-energy electrons traveling close to the speed of light can be deflected by passing near an atom or molecule, producing a gamma ray. And a gamma ray passing through the electron shell of an atom transforms into two particles: an electron and a positron.

NASA's Goddard Space Flight Center

When those electrons run into air molecules, they emit a terrestrial gamma-ray flash, which means that thunderstorms are creating some of the highest energy forms of light in the universe. But that’s not all – thunderstorms can also produce antimatter! Yep, you read that correctly! Sometimes, a gamma ray will run into an atom and produce an electron and a positron, which is an electron’s antimatter opposite!

Animation of the Fermi Gamma-ray Space Telescope. The satellite features a large black box structure with white instruments underneath. Two long solar arrays extend from opposite sides, just under the black box.

NASA’s Fermi Gamma-ray Space Telescope, illustrated here, scans the entire sky every three hours as it orbits Earth.

NASA’s Goddard Space Flight Center Conceptual Image Lab

Fermi can spot terrestrial gamma-ray flashes within 500 miles (800 kilometers) of the location directly below the spacecraft. It does this using an instrument called the Gamma-ray Burst Monitor which is primarily used to watch for spectacular flashes of gamma rays coming from the universe.

This animated GIF shows a map of the world stretched out to show all the continents in a rectangular layout. Magenta spots show up, indicating where Fermi has detected terrestrial gamma-ray flashes. The spots are concentrated on either side of the equator, which is where Fermi can detect them.

Visualization of ten years of Fermi observations of terrestrial gamma-ray flashes.

NASA's Goddard Space Flight Center

There are an estimated 1,800 thunderstorms occurring on Earth at any given moment. Over its first 10 years in space, Fermi spotted about 5,000 terrestrial gamma-ray flashes. But scientists estimate that there are 1,000 of these flashes every day – we’re just seeing the ones that are within 500 miles of Fermi’s regular orbits, which don’t cover the U.S. or Europe.

The map above shows all the flashes Fermi saw between 2008 and 2018. (Notice there’s a blob missing over the lower part of South America. That’s the South Atlantic Anomaly, a portion of the sky where radiation affects spacecraft and causes data glitches.)

This animation pans in on satellite imagery of the swirl of clouds that was forming into Hurricane Julio. The image is a grayish-blue color with wisps of clouds. In the center is an oval with a faint cloud ring surrounding it and heavy cloud cover on the left side of the oval. An inset square pops up showing one region and marking two spots in purple to show where terrestrial gamma-ray flashes were observed. The inset is dated Aug 2, 2014.

Storm clouds produce some of the highest-energy light naturally made on Earth: terrestrial gamma-ray flashes. The tropical disturbance that would later become Hurricane Julio in 2014 produced four flashes within 100 minutes, with a fifth the next day.

NASA's Goddard Space Flight Center

Fermi has also spotted terrestrial gamma-ray flashes coming from individual tropical weather systems. In 2014 Tropical Storm Julio produced four flashes in just 100 minutes!