Around the world, huge streams of water vapor known as atmospheric rivers can generate heavy rain or snow as they flow over land. Just a few atmospheric rivers, each typically more than 2,000 kilometers long and less than 1,000 kilometers wide, can deliver up to half of the annual precipitation on the U.S. West Coast, on average. Atmospheric rivers are common over the northeastern Pacific Ocean and California, where their saturated air rises and cools over mountain ranges.