Powerful rainstorms have battered Northern California this week. The culprit? Atmospheric rivers. The rains were born far away, deep in the tropical Pacific, where water evaporated from the warm ocean surface and fizzed into the atmosphere. The drenched air parcel flow then moved sinuously along, an “atmospheric river” winding its way toward land. When that wet air hit a coast—in this case the West Coast of the U.S.—it dumped that water out as the rain and snow that has overwhelmed Northern California this week.