Last Friday, Norway’s Hardangervidda mountain plateau looked like what would happen if Miguel Sapochnik directed a holiday TV special. More than 300 reindeer corpses were found piled up and strewn across the mountainside, in a natural massacre that Norwegian officials are calling the deadliest lightning strike in their country’s history. Of course, lightning strikes are not uncommon, nor are animals getting killed by them. Sheep, cattle, bison, geese, elephants, and even seals have been struck down by the dozens. So it’s really the scale of the Norway event that is puzzling experts.

At this time, the Norwegian Environmental Agency has not released details of the investigation, but some scientists are formulating a few theories of their own. And they all involve some very basic principles of electromagnetism.

When Glenn Shaw saw the news from Norway over the weekend, he felt deja vu. A now-retired lightning researcher, he remembered being in a helicopter flying over the Alaska Range back in 1972 and coming upon a similarly grisly scene: 53 dead caribou on the side of a mountain. And there was something else: a central burned area about 15 feet wide, radiating out into nine individual branching spokes in an oval shape, getting smaller as they progressed outward. A Lichtenberg pattern. The tell-tale sign of a lightning strike.

What’s more, he noted that the caribou bodies were consistently located on the burn areas. “You could see them lying right on these torturous paths,” he says. “And the fur by the hooves was a bit singed. It was definitely lightning that killed them. No doubt about it.”

He and a guy from the Alaska Department of Fish and Game wrote up the event for a paper, investigating whether the caribou’s antlers made them more susceptible to the strike. No, they concluded: The antlers would have to be much bigger to have any sort of lightning rod effect. Rather, it was the large horizontal extent of the lightning along the ground that really did them in.

When Shaw saw the photos from Norway, he was struck by how the reindeer were found across a similarly large area—hundreds of feet apart. That could only happen with a massive ground current discharge. And it reminded him of something else the two places have in common. Both locations sit at something called a zero degree isotherm, where the average air temperature hovers right around the freezing point of water. That means that permafrost (soil that’s been frozen for two plus years) occurs a bit deeper down—but near the surface it’s constantly in flux between melting and freezing, depending on the season.

This is important, because while liquid water is a strong electrical conductor, frozen water very much is not. If you were to attach high voltage cables on either side of a block of ice, nothing would happen—electricity just can’t pass through it. In physics, we would say that ice has very high resistance.

When lightning strikes, the current flows into the ground and outward, following the path of least resistance. In a warmer place, the electricity would penetrate deep into the soil and disperse quickly (this is called grounding). But in a place like the Hardangervidda, as the current runs into the soil and hits the permafrost layer, it instead spreads out along the surface of the soil, which is saturated with water from annual cycles of melting—and in this case, the massive rainstorms that generated the lightning strike. So the area that gets zapped is way bigger.

The zap is also way stronger. Voltage, which is equal to current multiplied by resistance, goes up as resistance goes up. So as current from a lightning strike encounters the high resistance of permafrost, it magnifies the voltage experienced by any object that happens to be unlucky enough to be on the surface at the time. Like a herd of reindeer.

And it’s about to get even worse for our four-legged friend.

Reindeer are big animals—the space between their front and back legs is separated by a few feet. That creates what is called a large step potential, basically another voltage increase, within the animal itself. If you’ve got supercharged current running along the ground, it eventually encounters the front legs of a reindeer. The electricity takes the path of least resistance, flows up the front legs, through the body cavity (where we find such vital organs as the heart and lungs), down the back legs and back into the ground. In a human, where the distance between the legs is closer and the path to go from one foot to the other bypasses the heart, you might just get paralyzed temporarily. But for a reindeer, having four legs makes for a lethal dose of electrons. Even if the animal is standing perpendicular to the discharge, it will still go through the heart while traveling from front leg to front leg.

Richard Sonnenfeld, who studies lightning propagation at Langmuir Laboratory, said that when he first saw the news article from Norway he didn’t think it had anything to do with permafrost. “But after digging around a little, I think it might and that’s really interesting,” he says. “There’s no question that resistance increases in frozen soil. And it may have been enough to make just one flash count for a whole lot of carnage.”

Norway has very little lightning activity compared to other parts of the world, 100 times less than Florida, which has the most flashes per square mile in the US each year. So it’s unlikely that anything like this will happen again anytime soon. Except for the fact that climate change is expected to increase lightning strikes by 50 percent over the rest of the 21st century. More extreme weather means more potential energy in the atmosphere—and that’s a recipe for more spectacularly deadly lightning.

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How Lightning Can Kill 300 Reindeer With One Strike