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Early one morning in late August 2019, Colorado photographer Glenn Randall hiked several miles to a stream flowing into Lake Isabelle in the Indian Peaks Wilderness. He set up his camera near the stream and began photographing about 20 minutes before sunrise when a golden glow developed at the horizon.听

It wasn鈥檛 until Randall was back at home, however, that he noticed something odd: The sky above the golden glow and its reflection in the water were both a deep violet.

鈥淚 was editing the photos and thought, 鈥榃ow, that water is purple,鈥欌�� Randall said.

He鈥檚 not alone. Photographers across the country have noticed that sunrises and sunsets have become unusually purple this summer and early fall.

Now, researchers at CU Boulder have collected new measurements that help to reveal the cause of those colorful displays: an on a Russian volcano called Raikoke.

Around the same time that Randall was taking his photos, the team of atmospheric scientists used a high-altitude balloon to detect particles created by that eruption in Earth鈥檚 stratosphere. These particles, or aerosols, scatter sunlight as it passes through the air, which in combination with the absorption of light by the ozone layer, gives sunrises and sunsets that purple tint.听

There鈥檚 no cause for concern, said Lars Kalnajs, who led the balloon team. But the purple skies show that even seemingly-isolated volcanic eruptions can have global consequences.听

听鈥淎 really big eruption would have a major impact on humanity, so it鈥檚 something we need to be ready for,鈥� said Kalnajs, a research associate in the (LASP).

The year without summer

Such big eruptions have happened before. In 1815, for example, Mount Tambora in modern-day Indonesia ejected a historic volume of ash and sulfur dioxide high into the air. The plume lingered in the atmosphere, shading the planet and disrupting weather around the world.

鈥淭he year after in history is known as the year without summer,鈥� said Kalnajs, also of the Department of Atmospheric and Oceanic Sciences. 鈥淭here were crop failures all over the world, and there was ice in rivers in Pennsylvania that didn鈥檛 melt until June.鈥�

It鈥檚 the sort of disaster that he and his colleagues from NASA, the U.S. National Oceanic and Atmospheric Administration听and several European institutes are ready for.

The team relies on high-altitude balloons, which can soar to more than 20 miles above the ground, to measure natural aerosols and occasional biproducts of volcanic eruptions in the atmosphere.

鈥淚t鈥檚 really important when major eruptions happen that we get data quickly,鈥� Kalnajs said. 鈥淲e need to figure out if this is going to be the kind of thing that impacts hundreds of thousands of people around the world, or is going to be to be more minor?鈥�

Raikoke Volcano

Raikoke gave them a good test for their skills. The volcano, which sits in the Kuril Islands off the eastern coast of Russia, erupted on June 22, 2019. Early data from NASA suggested that the explosion injected sulfur dioxide gas, which can lead to the formation of aerosols, into Earth鈥檚 stratosphere鈥攁 layer of the atmosphere that begins about 7.5 miles above the surface.听

Kalnajs and his colleagues wanted to see if they could find hints of that eruption in the stratosphere. In August, the team released a balloon near Laramie, Wyoming, and hit paydirt.

Based on the group鈥檚 preliminary data, which it will publish later this year, some aerosol layers in the stratosphere were 20 times thicker than normal in the wake of the eruption.

鈥淚t makes you realize that you don鈥檛 have to put a whole lot of aerosols into the stratosphere to change its composition,鈥� Kalnajs said. 鈥淭his was a relatively small volcanic eruption, but it was enough to impact most of the northern hemisphere.鈥�

It could also explain why, at the same time, observers with keen eyes like Randall鈥攚hose wife, Cora, is a professor in atmospheric and oceanic sciences at CU Boulder鈥攚ere noticing purple skies across the U.S. Such light displays likely haven鈥檛 been seen since Mount Pinatubo in the Philippines erupted in 1991.听

Kalnajs noted that the effect won鈥檛 last for long and is subtle. You鈥檒l need the right weather conditions and a bit of luck to spot the phenomenon. But, he said, it鈥檚 a reminder that scientists will need to keep an eye out for the next Mount Tambora.听

鈥淚t will happen at some point, whether that鈥檚 in one year, five years or 100 years,鈥� Kalnajs said.