Atmospheric rivers, explained
An atmospheric river is a long, narrow band of water vapor flowing through the sky, often from the tropics near Hawaii to the West Coast. When it runs into the coastal mountains the air is forced up, cools, and wrings out its water as heavy rain and mountain snow.
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A river you can't see
Most of the water vapor moving through the atmosphere outside the tropics travels in a few narrow corridors, typically a few hundred miles wide and well over a thousand miles long, a mile or so above the ground. A strong one carries an amount of water vapor that NOAA compares to 7.5 to 15 times the average flow of the Mississippi River at its mouth. The ones that start near Hawaii are nicknamed the Pineapple Express.
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Why the mountains matter
Over the ocean the moisture mostly just passes through. When the river hits the Coast Ranges, the Transverse Ranges or the Sierra Nevada, the air has to climb. Rising air cools, cool air can't hold as much water, and the excess falls as rain and snow. Windward slopes get drenched while the far side, in the mountains' rain shadow, stays much drier.
Helpful and dangerous
Atmospheric rivers provide a large share of California's yearly rain and snowpack, so a few good ones can end a drought. Too many in a row cause floods, mudslides and debris flows, especially on slopes stripped bare by recent fires. Scientists rate them from AR1 (weak, mostly beneficial) to AR5 (exceptional, mostly hazardous) based on how much water they carry and how long they last.
Questions people ask
- Why are atmospheric rivers warm?
- They carry tropical or subtropical air, so the rain can fall unusually high into the mountains and melt existing snow, which adds to flooding.
- When do atmospheric rivers hit California?
- Mostly from November through March, during the West Coast's wet season.