What Is an Atmospheric River?

In addition to rainfall, atmospheric rivers can include strong winds and thick fog, like this fog descending on High Desert, California. (Photo: Jessie Eastland/Wikimedia Commons)

Normally, as I sit at my desk in front of a window that overlooks the San Francisco Bay, I can see much of Berkeley, all of Oakland and the Bay Bridge. But for a week in January, I couldn't see any of it. That's because back-to-back storms dumped rain throughout much of the West Coast, from Oregon down to the Sierra Nevada Mountains — where they got massive snowfalls — and down to Los Angeles.

An atmospheric river or AR caused the intense storms, the likes of which the state hadn't seen in a decade. "This has been a very active winter, atmospheric river-wise," Jeff Zimmerman of the National Weather Service told NPR. "We've probably had 10 or more ... this winter." (A normal winter would bring just a few.)

The Earth Systems Research Laboratory at NOAA describes ARs as "relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. While ARs come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods."

While most of these rivers are weak, when they're not, they can wreak havoc — and that's exactly what happened in California recently.

Wet and windy

This satellite image shows a large atmospheric river over the eastern Pacific Ocean and headed for California.
This satellite image shows a large atmospheric river over the eastern Pacific Ocean and headed for California. (Photo: U.S. Naval Research Laboratory, Monterey/Wikimedia Commons)

A new study by atmospheric scientist Duane Waliser, Ph.D., shows these storms not only cause heavy amounts of precipitation, but they're remarkably windy, too. As NPR reports:

Waliser studied two decades of storms around the globe at mid-latitudes — that is, outside the tropics. When he focused on the very windiest — the top 2 percent — he found that "atmospheric rivers are typically associated with 30 and even up to 50 percent of those very extreme cases." ... Waliser found that winds during an atmospheric river are typically twice the speed of the average storm. He says emergency responders need to know that.

These rivers can act like their terrestrial cousins, flowing at a regular (or reduced) rate, or getting into extreme events. A strong AR can move 7.5 to 15 times as much water as the average flow on the Mississippi.

Considering the years of drought the West Coast has experienced, these events can be useful: "On average, about 30 to 50 percent of annual precipitation in the West Coast states occurs in just a few AR events, thus contributing to water supply," according to NOAA. But too much water all at once means that some of it comes down quickly and washes off into the ocean, meaning it doesn't make its way into aquifers and other natural water storage systems. So a large AR may not alleviate drought as much as you might expect just from looking at rainfall totals.

But where the precipitation falls as snow, that contributes to the snowpack, which has been light for the last several years — an extra-thick snowpack in the Sierra Nevada mountains means more water come spring and summer as those snows melt slowly. (But if melting happens fast, that can cause flooding too.)

The recent storm system brought flooding in areas where the precipitation came down as rain, some of it extreme, as in the Truckee River in Nevada, which overflowed into Reno. And trees were down everywhere (including a 1,000-year-old redwood).

The term atmospheric river has only been around since 1998, when MIT researchers coined the term to help people understand the concept of vast amounts of water vapor travels on four-to-five narrow bands of the atmosphere. It's a sort of magical and terrifying vision, of rivers of water just above all our heads.