What is an atmospheric river?

Atmospheric rivers are hugely influential for California’s weather and water supplies. They cause the state’s heaviest rains and feed the biggest floods. They drive its cycles of dry and wet, famine and feast. But they also cause a large share of the state’s levee breaches and debris flows.

One atmospheric river can be enough to flood homes, down power lines and wash away hillsides and highways. And when several sweep ashore in a matter of days or weeks, as has been happening this season, the potential damage is multiplied.

Atmospheric river storms get their name from their long, narrow shape and the prodigious amount of water they carry.

They form when winds over the Pacific draw a filament of moisture from the band of warm, moist air over the tropics and channel it toward the West Coast. When this ribbon of moisture hits the Sierra Nevada and other mountains, it is forced upward, cooling it and turning its water into immense quantities of rain and snow.

Climate scientists also distinguish atmospheric rivers from other kinds of storms by the amount of water vapor they carry. These amounts form the basis for a five-point scale used to rank atmospheric rivers from “weak” to “exceptional.”

As humans continue burning fossil fuels and heating the atmosphere, the warmer air can hold more moisture. This means storms in many places, California included, are more likely to be extremely wet and intense.

Scientists are also studying whether global warming might be shifting the way winds carry moisture around the atmosphere, potentially influencing the number of atmospheric rivers that sweep through California each year and how long they last. They have not yet come to firm conclusions on these questions, though.