A peculiarly large hole appeared in the clouds near Russia's Norilsk Nickel factory, one of the world's largest producers of nickel and a heavy air polluter. Scientist Velle Toll was intrigued by the satellite imagery but didn't think much of it. But as he and his graduate student analyzed more data, they noticed other odd holes in clouds -- all surrounding major polluting industrial plants.

More than three years later, he and his colleagues have uncovered the mechanism leading to these gaps: snow.

Specifically, pollution particles from metal factories and power plants act as seeds that help ice form within clouds, initiating snow that falls from and leaves holes in them. These pollutant-induced ice clouds are also thinner, cover less area and reflect less sunlight to space -- potentially affecting at least local environments, a study released Thursday in the journal Science found.

The team found unique plumes of ice clouds near 67 metal and cement factories, paper mills and power plants in North America, Europe and Asia, notably in Canada and Russia.

"It was surprising that nobody had discovered this phenomenon. It's not that rare," said Toll, lead author of the study. Although even he admits that it was a "lucky accident" that he noticed the odd sighting.

Initially, he was scanning satellite imagery to study how pollution from factories and ships affects clouds, which are typically made of mostly liquid droplets. Previous research has shown that industrial pollutants tend to increase the number of liquid cloud droplets, which makes the clouds brighter and reflect more sunlight back to space. Some have even tossed this cloud-brightening idea as a way to combat climate change and cool the Earth.

But research on how these pollutants promote ice in clouds is less robust.

Scientific theory and lab measurements indicated that they could initiate ice formation, but no one uncovered direct observational evidence of this process -- until now.

When Toll decided that these holes warranted further study, he and his team turned to 21 years of satellite imagery and ground-based weather radar data. They searched through hundreds of thousands of images to identify cloud holes near industrial hot spots. The satellite data was able to identify whether the clouds had liquid or ice, whereas the ground data could indicate snowfall.

Toll said they had to use an "old-school" approach to manually inspect all the data instead of a machine learning algorithm; they had no data to train the software since it was such a new discovery.

Across industrial hot spots, the team found clouds increased daily snow accumulations up to 15 millimeters (0.59 inches) -- enough to cover a small town, Toll said.

The snowfall, Toll explained, is tied to the industrial pollutants. In an environment without any industries or human pollution, liquid cloud droplets are cooled to temperatures below zero degrees Celsius. Add pollution from metal or mineral industries, and that can initiate the formation of ice crystals. As the ice crystals grow, they consume the liquid droplets in the cloud. If they grow large enough, it snows. The pollutants that helped form the snow also fall with the flakes.

Not every pollutant is a good ice-forming particle, Toll said, but more research is needed to find out which ones play the biggest role.

The ice particles also changed the properties of the clouds. Whereas liquid droplets made clouds brighter, the study showed that the ice clouds reflected 13.7 percent less sunlight and absorbed more infrared radiation.

Toll explained that the ice crystals are bigger than liquid droplets and don't reflect as much sunlight, so the cloud appears "darker." This could diminish the cooling observed by liquid clouds, he said.

After the snow fell, the clouds also became smaller -- increasing the amount of sunlight reaching the surface. The team found that cloud cover near these industrial hot spots was about 8.3 percent less and 18 percent thinner. The study showed only local effects on incoming sunlight, but larger-scale changes would need to be seen for a wider climate impact.

"While this study shows that plumes will be impacted, it is not at all clear that the effects seen in these plumes will carry over to larger spatial scales where climate might be impacted," said Joyce Penner, an atmospheric scientist at the University of Michigan who was not involved in the study.

Atmospheric scientist Yuan Wang, who was not involved in the study, said he was amazed that the authors can accurately pinpoint the industrial aerosol hot spots and link them to the formation of ice clouds.

"This study advances our understanding by illustrating what types of man-made aerosols can serve as ice nucleating particles and where to find them," said Wang, a professor at Stanford University.

He said the study also uncovers another process to be considered in weather and climate models.

The models need to better account "for those effects of human activities on precipitation, radiation, and climate predictions."