Most of California's water comes from the snow stored in the Sierra Nevada each winter. In the spring, melting snow helps fill the state's reservoirs for the dry summer. (Read "When the Snows Fail" in National Geographic magazine.)

As the state's historic drought drags on, scientists are watching the Sierra snow with intense interest—and they're worrying that even tiny airborne particles of dust may have a big effect on water supplies.

Here's how: As California gets drier, it's getting dustier, and at least some of that dust is landing in the Sierra. Dusty snow, with its darker surface, absorbs more solar radiation than clean snow does, meaning it heats up faster and melts more quickly.

That earlier spring snowmelt could mean that