The report from Pacific Northwest National Laboratory said this dirty snow affects water availability in the region, which gets almost 75 percent of its water from mountain snow melt.

Dirty snow absorbs more solar radiation and heats the ground and surrounding air. "This means less snow accumulation in winter, less snow melt in spring, and earlier spring melt dates," the report said. "In short, soot from fossil fuel production is causing earlier melt dates in the western United States, which could affect water availability in warmer months."