The year 1816 was a weird one, climatically speaking. Months that would normally be warm and pleasant were cold, rainy and overcast, leading to crop shortages across much of the Northern Hemisphere. It was linked to one of the most powerful volcanic eruptions in recorded history, and now we may know how. A new paper out of Imperial College London explains how electrified ash from the eruption could have “short-circuited” the Earth’s ionosphere and triggered the “Year Without A Summer.”

In April 1815, Mount Tambora, in what is now Indonesia, blew its lid. After a few months of rumbling and smoking away, it finally erupted with a Volcanic Explosivity Index (VEI) of 7, the largest volcanic eruption since the year 180 CE, with the explosion reportedly heard from as far as 2,600 km (1,600 mi) away.

Most importantly, the eruption launched about 10 billion tonnes of material into the atmosphere. Over the next year, this heavy ash cloud blanketed the Earth, reflecting sunlight and significantly reducing temperatures. Rain and snow fell in areas that should have been basking in summery weather, and almost 100,000 people are believed to have died as a result of the food shortage that followed.

Although the link was made between that eruption and the Year Without A Summer long ago, exactly what mechanisms were at play have remained a mystery. The Imperial College London study aimed to help explain how this dramatic climate event played out.

“Previously, geologists thought that volcanic ash gets trapped in the lower atmosphere, because volcanic plumes rise buoyantly,” says Matthew Genge, lead author on the study. “My research, however, shows that ash can be shot into the upper atmosphere by electrical forces.”

As demonstrated by dramatic images of lightning coursing through volcanic plumes, the ash is electrically charged. The interplay of electrostatic forces can lift that ash higher than previously thought, according to Genge.

“Volcanic plumes and ash both can have negative electrical charges and thus the plume repels the ash, propelling it high in the atmosphere,” says Genge. “The effect works very much like the way two magnets are pushed away from each other if their poles match.”

To test the idea, Genge modeled how well charged volcanic ash would electrostatically levitate under these conditions. Their experiments showed that particularly violent eruptions could launch particles smaller than 500 nanometers wide up into the ionosphere.

That’s important because the ionosphere is a very electrically-active region of the Earth’s atmosphere. According to Genge, having charged particles that high up could effectively “short-circuit” the ionosphere, creating climate anomalies such as increased cloud cover that reflects sunlight away from Earth and cools the surface of the planet.

Interestingly, the stars were aligned to make 1816 a bitterly cold year already. It fell towards the end of a natural global cooling period sometimes known as the Little Ice Age, spanning from the 16th to mid-19th centuries. It also fell in the middle of the Dalton Minimum, the decade or so where the Sun’s activity was the quietest it’s ever been on record. The eruption of Mount Tambora, it seems, was just the icing on the cake.

To test the theory, Genge examined the weather records following another massive volcanic eruption that occurred decades later – Krakatau in 1883. Data gathered by the researchers showed that average air temperatures and rainfall dropped almost immediately after the eruption began.

Genge also noted that noctilucent clouds, normally rare clouds that form in the ionosphere, appeared more regularly after the Krakatau eruption. A more recent eruption, that of Mount Pinatubo in 1991, also coincided with reports of ionosphere disturbances.

The research was published in the journal Geology.