Senior Editor
Hydrologist talking about atmospheric river

A storm darkens the sky at the mouth of the Russian River, north of Bodega Bay, Calif. The storm was driven largely by an “atmospheric river” over California. Photo: NOAA

The Inertia

On February fifth, millions of residents of California were under flash flood alerts. The rain poured down, flooding the streets and muddying the waters, as a result of a powerful atmospheric river that sat on top of Southern California. Los Angeles in particular saw one of the wettest days since record keeping began, with over four inches of rain falling. Mudslides have shut down sections of the iconic highway through Malibu and spreading is relatively widespread.

Chances are that you’ve heard the term “atmospheric river” before — they’re getting a little more frequent in recent years — but what, exactly, are they? Qian Cao, a hydrologist at the University of California, San Diego, broke it down for The Conversation.

In short, an atmospheric river can be described as a river in the sky. They are narrow bands of concentrated water vapor that have, on average, about twice the regular flow of the Amazon River.

When they hit something like a mountain or anything else that forces them to rise, the moisture hidden within cools down and condenses, which makes it rain. Because of the incredible amount of water they carry, when it rains from an atmospheric river… well, it pours.

Atmospheric River graphic

Not all atmospheric rivers cause damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply. Image: NOAA

They generally are born over warmer waters like tropical oceans, then make their way to coastlines as the various jet streams push them around. “Along the U.S. West Coast,” Cao explained, “the Pacific Ocean serves as the reservoir of moisture for the storm, and the mountain ranges act as barriers, which is why the western sides of the coastal ranges and Sierra Nevada see so much rain and snow.”

While one atmospheric river can be pretty devastating, the real issues arise when we see them back-to-back. The first one saturates the area, and when another one hits before all that moisture has had a chance to dissipate, the new rainfall has no where to go. That phenomenon was laid bare in December and January of last year, but those storms also served to add desperately-needed water to California’s dry reservoirs after years of drought.

“Although they can lead to flood hazards, atmospheric rivers are also essential to the Western water supply,” Cao wrote. “Atmospheric rivers have been responsible for ending more than a third of the region’s major droughts, including the severe California drought of 2012-16.”

On the West Coast, atmospheric rivers are responsible for some 30 to 50 percent of annual precipitation in the form of not only rain, but snowpack as well. In the Sierra Nevada especially, atmospheric rivers make up about 30-to-40 percent of an average season’s total snow accumulation.

Climate change is indeed adding fuel to the fire of atmospheric rivers. Since warmer air can hold more moisture, more and more is added to those flowing aerial rivers.

“As global temperatures rise in the future, we can expect more intense atmospheric rivers, leading to an increase in heavy and extreme precipitation events,” Cao said. “My research also shows that more atmospheric rivers are likely to occur concurrently during already wet conditions. So, the chance of extreme flooding also increases.”


Only the best. We promise.


Join our community of contributors.