In just two hours, in 1894, a wall of flame took out six Minnesota towns. A lot of factors came together to create the firestorm, but the main one was a nasty, but innocuously-named phenomenon called a “temperature inversion.”

Hinckley was a logging town in Minnesota. It wasn’t a city, but it was a large town and had satellite towns around it. All the local communities were dependent on timber, and the people there had cleared much of the older forests in the area. The optimistic among them must have hoped that these areas would be further cleared and built up, turning the small towns into larger and more powerful communities. At the end of the 1800s, though, the houses hadn’t come yet and the open areas were full of dead wood and scraggly second growth.

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The first of September came at the end of an extremely dry summer. The world around Hinckley was tinder. Lumber trains regularly set small fires, as they threw off sparks, but they rarely spread. Instead they generally smoldered and went out. The smoke was a nuisance, but it dissipated quickly.

On September 1st, it didn’t dissipate. The area had been surrounded by an odd high-pressure system—what some people call an anticyclone. This high pressure created a sort of invisible shield around Hinckley, preventing any air from rushing into or out of the area. It also lead to a temperature inversion. Hot air, whether warmed by the Earth or by fires, rises up into the high atmosphere and cool air rushes in to fill its place. But if a large amount of cool air at high pressure forms a cap over the top of an area, the hot air stays right where it is. The air around Hinckley was thick with trapped smoke. At around 10:00, a tiny breeze broke through. To people in the area, it would have been refreshing, but it also fanned smoldering fires, combining small fires and pushing the flames higher.

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The real horror started when, according to the Hinckley Museum, “Two fires managed to join together to make one large fire with flames that licked through the inversion finding the cool air above. That air came rushing down into the fires to create a vortex or tornado of flames which then began to move quickly and grew larger and larger.” What had been a group of smoldering tiny fires turned into 200 foot high flames. The “firestorm” started at 2:00 PM and swept through Hinckley and five other towns in two hours. Four-hundred-and-eighteen people died. Some managed to get to pools or rivers and save themselves by throwing themselves inside. Many were carried away by special trains sent through to get people out of the area.

Hinckley was wiped off the map that day, but people re-built. Today, one of the sights of Hinckley is the monument to the people who died in the fire.

The Great Hinckley Firestorm remains the most dramatic and frightening examples of what can happen during a temperature inversion. It is, however, far from the deadliest. Another temperature inversion, in London in the 1950s, killed between 4,000 and 10,000 people—not through fire, but through trapped pollution.

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Top Image: Minnesota Historical Society Firestorm, 1988, in Grant Village Image: National Park Service