For more than three weeks, California has been pummeled by a series of storms arriving one after another from the Pacific Ocean. Torrential rains, mudslides, power outages, and floods plague state residents from north of San Francisco down to Los Angeles, 400 miles to the south. Scientists attribute this event to an “atmospheric river” condition in the Pacific Ocean. Many also claim that this phenomenon is due to human-caused climate change.

According to the National Oceanic and Atmospheric Administration, an atmospheric river is a long, narrow region in the atmosphere that can transport large amounts of water vapor, roughly equivalent to the average flow of water at the mouth of the Mississippi River. When atmospheric rivers make landfall, they release this water vapor in the form of rain or snow. An atmospheric river that forms in the tropics near Hawaii, sometimes called the Pineapple Express, can deliver large amounts of water to the U.S. West Coast.