Humans can’t control the weather. But we certainly make a mess of it, often in ways we’re just beginning to understand.
Four years ago, Velle Toll, a climate researcher at Estonia’s University of Tartu, was combing through archived satellite data from NASA when he noticed an odd, plume-shaped hole in an otherwise pristine deck of clouds. Toll assumed it was a rare occurrence that didn’t merit much more of an investigation until, the following year, one of his graduate students found even more of them.
They began to study these rifts and noticed that what looked like plume shaped holes in the clouds were actually spots of thinning in the cloud layer that were directly in line with streams of dirty air spreading downwind of smelting facilities and coal plants. The dirty air, they found, was causing the clouds to spontaneously freeze, which led to thinning. These supercooled clouds could even trigger bursts of snowfall over hundreds of miles.
In cold weather, clouds don’t always turn to ice when the thermometer says they should. When temperatures fall between 32 and -33 degrees Fahrenheit, unless the airborne droplets of water have something to freeze around, they’ll persist as a supercooled liquid due to a weird quirk of physics. In the past, lab experiments have shown that the microscopic pollutants released by certain industrial facilities are prone to cause supercooled water to form ice particles. This phenomenon had, however, never been documented at work in the real world until now.
A telltale marker in the clouds pointed straight back toward the offending facilities.
Getting the results was no simple feat, however. It took months of exhaustive review, Toll said. “We have visually gone through hundreds and thousands of satellite images to identify these features.” Toll and his team recently published their results in Science.
When they’d finished their hand tally, they’d spotted 67 different locations where this phenomenon occurred, mostly in Canada and Russia during the coldest months of the year, between late fall and early spring, when the conditions were favorable.
In each case, as the ice particles grew—darkening and thinning the clouds as a result—they left behind a telltale triangular marker in the cloud deck that pointed straight back toward the offending facilities. Not all factories were equally culpable though. The worst culprits were metallurgical plants, coal stations, and oil refineries. Just five sites caused caused the clouds to freeze nearly 300 times from 2000 to 2021. Roughly a third of all the flash-freezes triggered detectable snowfall downwind, with a half-inch of powder dusting the affected area in just a day.
“The data has been there already for 20 years and publicly available,” Toll said. “People just haven’t been looking for it.”
Toll and his team hope to scale up their dataset with further manual tallies until they have enough material that they can train a machine learning model to help automate and accelerate the process of identifying cases of smog-induced cloud-freezing and snow, so they can focus on better characterizing the processes at play. While the particles emitted by smelting plants and facilities cause wintery clouds to congeal, they also found rare instances of the same phenomena occurring downwind of nuclear power plants, which they can’t yet fully explain.
Nuclear cooling towers don’t typically emit any kind of aerosol—the term used to describe microscopic particles of solids and liquids suspended in the atmosphere—yet it is the metallic and mineral aerosols from other facilities that seem to serve as triggers.
Toll hopes the work he and his team are doing can shed light on some of the ways human industry reshapes the natural processes of our planet, and help to improve climate predictions. 
Lead image: vladimir salman / Shutterstock
