Mountains challenge skiers, climbers, and road builders. But when it comes to clouds, they offer an assist. Researchers from Pacific Northwest National Laboratory and Colorado State University found that mountainous, water-ice (a.k.a. mixed-phase) clouds have a dual response when injected with numerous tiny pollution particles. Their study showed that pollution particles near the Sierra Nevada Mountains in the western U.S.A. ripen conditions for forming droplets and ice particles. Initially, mountain-side precipitation decreases. But when the particles reach a certain amount, snowfall dramatically increases over the mountain.