Two-Minute Takeaway: What is an Atmospheric River?

Photo © NASA

By Emily Howe, Aquatic Ecologist

On Oct. 15, NASA released an image of a 5,000 mile swath of moisture stretching across the Pacific Ocean, heading toward the Pacific Northwest. Western Washington experienced a very wet 72 hours as a result of this cloud band, with more than 10 inches of rain falling in parts of the Olympic Peninsula. Rivers hit flood stage throughout the state.

These weather events are called "atmospheric rivers." We also refer to them as the Pineapple Express, as they often originate in the tropics near Hawaii, sending warm, moist air into the Northern Hemisphere.


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Atmospheric rivers are characterized by moisture from the tropics confined to a narrow corridor, warm temperatures and high freezing levels. They are neutrally buoyant, so when they encounter high terrain like mountains, they lift easily and cause heavy precipitation.

In the Pacific Northwest, every major flood has been associated with an atmospheric river event. Particularly damaging events occur when heavy rains fall atop snow, sending deluges down from the mountains, through the foothills and spreading across floodplains.

As our climate changes, the intensity and frequency of atmospheric rivers are projected to increase. Green infrastructure that encourages the land to hold more water will be an essential solution, as will floodplain restoration that gives rivers to roam and allows wetlands to soak up storms.

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