Explainer: Atmospheric Rivers
What exactly is an atmospheric river, and how is it different from a garden variety rainy day? KneeDeep reporter Eliana Perez covers the basics in this short video, also available in Spanish. For the science side, KneeDeep’s water reporter Alastair Bland offers these insights.
More than a rainstorm, an atmospheric river has a basic numeric property: It must transport at least 250 kilograms of water vapor per meter per second; anything less is just a rainy day. These weather systems are also defined by their origin and flow patterns: They tend to occur in the winter, originate over the tropical or subtropical ocean, and stream eastward and poleward. They are a famous weather mechanism along the North American West Coast but occur globally over midlatitude west-facing coastlines, like New Zealand’s and Portugal’s.
Scientists don’t just have standards for defining atmospheric rivers. They also have a scale for ranking them. Running from 1 to 5, it factors in the vapor transport rate with the duration of the storm at a given place. The system expected to soak California this week has been forecasted to be a hefty category 4 storm system.
Atmospheric rivers drop massive volumes of rain, making them major risk factors for destructive winter flooding. In early 2023, California experienced severe flooding as a parade of atmospheric rivers, including eight in January alone, bombed the coast. By the same token, atmospheric rivers are a water supply lifeline for the state, with just several storms capable of dropping all the water that Californians require for the year.
Because warmer air holds more moisture than colder air, atmospheric rivers are growing more severe with global warming. This will contribute to a climate paradox in California, where the future likely holds both more rain and flooding but less snowpack, historically a critical piece of the state’s water supply. In a 2025 paper, a research team led by Alexander Gershunov of the Scripps Institution of Oceanography described how a simultaneous “decrease in the frequency of non-AR storms and an increase in the intensity of strong ARs” will aggravate the volatility of California’s hydrology.
“In a warmer future, California will likely have to contend with more drought—particularly snow drought—as the emergence of low-to-no snow conditions in the Sierra Nevada continues along with increased atmospheric evaporative demands,” Gershunov and his colleagues wrote. “California will also have to contend with more flooding.”
How Rivers in the Sky Travel Across the Ocean
Why California’s Water Extremes are Wilder Than Ever and What We Can Do About It
Video Reporter: Eliana Perez
Video Production: Jocelyn Duffy
Text Explainer: Alastair Bland
Community reporting funded by the CO2 Foundation.
