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Public works employee cleaning clogged storm drain in San Francisco. A San Francisco Public Works employee works to unclog a drain in San Francisco, Calif. on Dec. 11, 2014. San Francisco was reeling under a Pacific storm that flooded transit stations, grounded flights and left more than 90,000 people without power. (Photo: David Paul Morris/Bloomberg)

Meteorologists nationwide — the National Weather Service, California Department of Water Resources, U.S. Geological Survey and U.S. Army Corps of Engineers — have decided to begin rating atmospheric rivers (AR), storms that have the potential for millions of dollars in losses and severe flooding. But what exactly is an atmospheric river? It’s a long, narrow river of condensed water vapor in the atmosphere that moves with the weather.

A strong river can carry between 7.5 to 25 times the flow of water that passes through the mouth of the Mississippi River. These rivers in the sky can be 300 miles wide, a mile deep and more than 1,000 miles long. Most are small, but they can be massive and fast-moving, and when the storms make landfall, they often release rain or snow, sometimes in large amounts. They account for 50% of the annual precipitation in California and are the West Coast’s big storms.

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