Industrial plumes seed snow

The effects of industrial pollution on cloud formation have long interested scientists. Velle Toll, a climate physicist at the University of Tartu, uses satellite data to study how microscopic solid and liquid particles emitted by human activities—anthropogenic aerosols—affect clouds. While parsing thousands of satellite images, Toll started noticing something strange: holes in the wintertime cloud cover near industrial factories (Science 2024, DOI: 10.1126/science.adl0303).

Once Toll recognized the pattern, it took some time to determine what caused the holes in the cloud coverage. “We detected snowfall in the same areas where we saw these holes,” he explains, and realized that the water in the cloud was condensing into snow and falling to the earth, leaving behind reduced cloud cover. Interestingly, the pattern of snowfall appeared to follow the path of each industrial site’s pollution plume while nearby areas remained bare.

With temperatures at the sites between 0 and –36 °C, water droplets required some kind of ice-nucleating particle before freezing could occur, Toll says. Of the 67 industrial sites whose plumes were associated with increased snowfall, the team categorized 32 as metallurgy sites and 20 as cement industries. These plants generate aerosols containing metals or minerals, which have previously been shown to initiate ice formation. Ultimately, this finding suggests that the pollution from the factories is providing ice-nucleating particles and causing snowfall, Toll says.

But Edward Gryspeerdt, a climate scientist at Imperial College London unaffiliated with the work, finds it more interesting that four sites are nuclear plants. “We don’t think nuclear power stations emit the particles needed to glaciate clouds,” he writes in an email. “This is something really new and not noted in previous studies of industrial snow.”

Johannes Mülmenstädt, an earth scientist at the Pacific Northwest National Lab who is unaffiliated with the work, agrees that the observed snowfall at nuclear power plants is curious. “Like with all groundbreaking studies,” he says, “there are plenty of questions still to be answered.” But there is so little research on how aerosols affect cold clouds, he says, that “this paper fills a really big gap.” People have hypothesized that anthropogenic aerosols generate snowfall, he adds, “but here we have actual smoking gun observations of it happening in real time.”