When you’re hurtling through a thunderstorm at thirty-seven thousand feet, a lot of wacky stuff can happen. Of all the crazy things to see though, one of them is rarer and more spectacular than just about everything else: the incredibly-elusive St. Elmo’s Fire.

The phenomenon, a fork-shaped discharge of atmospheric electricity that’s not lightning but has that look, was caught on camera earlier this week on a flight from Rome to Barcelona. As the plane flew over the French island of Corsica, it encountered a strong thunderstorm. The plane entered a region of an exceptionally high electric field, which touched off the amazing spectacle.

Maria Ines Rubio was the lead flight attendant on this supercharged flight. She’s worked in the industry for over a decade, and was “hoping to someday see this.”

“I was in the cockpit, bringing dinner to the pilots,” she said. “It lasted about three minutes. I wasn’t nervous, because it a rather normal occurrence when you get into a strong enough storm.” The video she shot shows dozens of flashes dancing across the plane’s windshield, arcing like small lightning bolts along the skin of the aircraft.

But how can such a freaky scene happen?

Electricity is all around us, whether or not we can see it. Just like your hair may stand on end when you walk across a wool carpet in the wintertime, you know that electrons are always leaping around from one surface to another. When there is a difference in the concentration of electrons between two objects, the potential difference induces an electric field. This means that charged objects will feel a force in the direction of the field. But when the field becomes strong enough to overwhelm air’s ability to suppress an avalanche of moving electrons– ZAP! A spark jumps across the field, and if you’re anywhere nearby, you’ll know it!

That’s what gives us a tiny shock when we touch a doorknob. It’s like a miniature lightning flash. Thunderstorms occur on a much larger scale, which is why the lightning that forms within them can stretch for miles. But to generate a visible spark of any size, the requirement is always the same. The electric field has to be high enough to promote the jumping of electrons through clear air. In the atmosphere, this is equal to three million volts per meter.

How can that be? With numbers that high, it’s a wonder we’re not fried any time we touch that doorknob! It’s all thanks to the shape of the object. Pointy objects can allow large amounts of charge to gather over a relatively small area, reaching that three megavolt per meter criterion on a very local scale. These “coronal discharges” can occur even when the ambient electric field is considerably lower than this threshold. On rare occasions, congregating electrons can glow blue or purple when the field becomes high enough and a sufficiently large charge builds up. This can be an early indicator of danger, since a damaging lightning strike might be soon to follow.

On an airplane, this is nothing to worry about. The metal shell of the fuselage is designed to conduct any possible electric discharges harmlessly away from the interior, even if lightning strikes the aircraft. Only when the field is exceptionally strong does St. Elmo’s Fire begin to flicker along the windshield– where the shape enhances the field locally.

On the ground, though, St. Elmo’s Fire is Mother Nature’s way of warning you that you’re in a bad place. It was oftentimes sighted on the masts of ships caught at sea during violent tempests. There have even been a few reports of it being seen on people!

The remarkable video below was captured on Lake Monona in Dane County, Wisconsin in January of 2016 during an intense snowstorm. In it, a stream of electricity is seen emanating off the outstretched fingertip of an awestruck viewer. As neat-looking as that is, if you find yourself in the same situation, you need to get out– immediately.

Fortunately, Rubio knew she was safe on her flight and wasn’t overly concerned. “I had always been eager to see it,” she said. Next time you’re on an airplane during heavy weather, look out the window – you may just see something truly “shocking.”