Over the summer of 2018, the worst wildfire season the western United States had ever seen sent thick plumes of smoke high into the sky. Atmospheric scientist Cynthia Twohy and her colleagues spent weeks flying a giant C-130 research airplane through the smoke, which was so dense that the light inside the plane sometimes went dusk-dim.

“You couldn’t even see the clouds outside, the smoke particles were so thick,” says Twohy, who works for NorthWest Research Associates, a scientist-owned research organization. All that smoke, she thought, must have an impact on the clouds forming above the West’s scorched surface.

It does indeed, according to a study she and her colleagues published recently in Geophysical Research Letters: Smoke particles make some