科学美国人60秒:Biggest Rivers Are Overhead

Back in January, one of California's oldest and most iconic residents keeled over: the Pioneer Cabin tree, a giant sequoia in Calaveras Big Trees State Park…so big you used to be able to drive through it.

The giant was blown over by high winds—delivered by what’s called an atmospheric river, a long stream of water vapor in the atmosphere, 100 miles wide. These systems might be thought of as some of the biggest rivers on Earth. 

"You could kind of pose it that way, yeah." Duane Waliser (WALL-i-ser), an atmospheric scientist at the Jet Propulsion Lab. "An atmospheric river will carry the same amount of water vapor as, say, 15 to 20 Mississippi Rivers." 

Waliser and his JPL colleague Bin Guan developed an algorithm to detect atmospheric rivers in historical data, so they could connect the sky flow to extreme events on land. And they found that, if you look at just the top 2 percent most extreme wind- and rain- and snow-storms in the world's midlatitude regions—atmospheric rivers are linked to up to half of them.

And of the 19 windstorms in Europe that cost insurance companies the most money—billions of dollars in damage—atmospheric rivers were behind three-quarters of those events. The study is in the journal Nature Geoscience. [Duane Waliser and Bin Guan, Extreme winds and precipitation during landfall of atmospheric rivers]

Looking ahead, as global temperatures rise, that warmer air holds more water vapor. "And so the tendency looks like, if the climate does warm, you would tend to have stronger or more frequent atmospheric rivers." And as this study shows: it won't just be that a hard rain's gonna fall. We'll be blowing in the wind, too.

—Christopher Intagliata