An atmospheric river is exactly what it sounds like: a ribbon of concentrated moisture that can stretch for 1,000 miles through the sky. The one that brought all manner of chaos to Los Angeles this week formed when water vapor rose from the sea’s surface somewhere east of Hawaii. As the planet turned, it got caught in a narrow channel between pinwheeling pressure systems. Strong winds pushed it east, until it came to hover like a snake over Southern California. Think of its tail as having sections. Mountains and pressure systems popped some up into the colder parts of the atmosphere, and the droplets in them cooled until they fell to Earth as unusually intense rain. In one 12-hour period, Bel-Air received more than eight inches of rain—a deluge that may not repeat for 500 years. Joan Didion—who must always be consulted on this subject—wrote, “Los Angeles weather is the weather of catastrophe, of apocalypse.” Having grown up in the region, I have some sense of what she meant. Whole months of relatively undifferentiated sunshine …