What Are Atmospheric Rivers? Why Do They Hit California Especially?

Winter storms that strike the US West Coast can dump rain non-stop for days, in the worst cases triggering floods and mudslides. The strongest of these are usually a phenomenon known as atmospheric rivers, weather patterns notable for their shape and the sheer volume of water they can deliver. While they can strike anywhere, California typically gets the planet’s most punishing atmospheric rivers and is currently being doused by a version known as the Pineapple Express. The storms aren’t new, but they’ve been getting more attention in recent years as researchers learn more about how to track them and climate change potentially makes them stronger.

These weather patterns get their name primarily from their shape. They are long, narrow bands of water vapor traversing the sky, a few hundred miles wide and stretching for thousands of miles. They sometimes span an entire ocean, and can carry an enormous amount of water. One study recorded atmospheric rivers delivering as much as 26 cubic kilometers (6.2 cubic miles) per day, about 15 times the amount of water the Mississippi River dumps into the Gulf of Mexico every day.