The Western US is an empire built on snow. And that snow is vanishing.

Since most of the region gets little rain in the summer, even in good years, its bustling cities and bountiful farms all hinge on fall and winter snow settling in the mountains before slowly melting into rivers and reservoirs. That snowmelt, often traveling hundreds of miles from mountain top to tap, sustains the booming desert communities of Las Vegas, Phoenix and Salt Lake City — even coastal Los Angeles and San Francisco. A civilization of more than 76 million people, home to Silicon Valley and Hollywood alike, relies on snow.