Large factory emissions may contribute to snowfall.

Satellite images show that plumes of pollutants from large factories can cause snowfall and leave holes in widespread clouds.

It has long been known that fine particles of soot-like pollutants, known as aerosol pollution, can affect clouds in a variety of ways. Water vapor can condense on pollutant particles and cause cloud formation, and pollutants can also change the properties of existing clouds.

While researching these effects, Vere Thor Researchers at the University of Tartu in Estonia noticed that holes sometimes appear in clouds downwind of major pollution sources. He and his colleagues have now analyzed thousands of satellite images of North America and Eurasia and found 67 locations where this effect can be seen under the right atmospheric conditions.

Weather radar confirmed that these events were causing snowfall. In the largest example the researchers found, up to 15 millimeters of snow fell over an area of ​​2,200 square kilometers (850 square miles).

This happens because pollutant particles freeze around supercooled water droplets in the cloud, creating ice crystals that grow into snowflakes, Toll said. “And as the water comes out of the clouds as snow, you end up with fewer clouds.”

In the absence of particles, water droplets in clouds remain liquid even when the air is as cold as -40°C (-40°F).

Most of the 67 sources of pollution found by the research team were oil refineries and factories producing metals, cement, and fertilizers. But surprisingly, the researchers occasionally observed similar effects near four nuclear power plants that do not produce any aerosol emissions.

This could be because the warm air rising from these power plants is picking up aerosol pollution from elsewhere, but the researchers have not confirmed this. “There's no clear explanation for that,” Toll said.

In theory, it's possible to intentionally induce snowfall using aerosol effects, but that would only work if a cloud of supercooled liquid water droplets was already present, Tolle said. say.