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Tom Corringham, University of California San Diego

(THE CONVERSATION) Ask people to name the world's largest river, and most will probably guess that it's the Amazon, the Nile or the Mississippi. In fact, some of Earth's largest rivers are in the sky – and they can produce powerful storms, like those currently soaking the Pacific Northwest.

Atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the s...