Atmospheric Rivers Now Have an Intensity Ranking Like Hurricanes

Sometimes beneficial, sometimes catastrophic, these 'rivers in the sky' are becoming more frequent and intense with climate change.

California Hit By Another Winter Storm, Deepening The Already Historic Snowpack In Mountain Regions
Snow falls above snowbanks piled up from previous storms during another atmospheric river storm on March 2023 in Mammoth Lakes, California. Mario Tama / Getty Images

Sometimes weather terms are deceiving. "Wintry mix" brings to mind something cozier and far more charming than the vexing mix of rain, freezing rain, sleet, or snow that it is. Likewise, "atmospheric river" might evoke a misty waterway at sunrise, replete with a dawn chorus of singing birds.

While an atmospheric river is indeed like a river, they exist as long, narrow bands of vapor in the sky, not running water. They generally form when warm temperatures create moist pockets of air that are pushed across the ocean by strong winds. When (and if) they make landfall, they can bring precipitation and much-needed relief to drought-stricken areas.

But when they tend toward the more intense, they can cause devastating floods, landslides, overwhelming snow, and power outages. In the United States, they happen on the West Coast—as winter 2022-2023 has so dramatically illustrated in California.

These weather systems are becoming more intense and frequent with climate change, according to a new study published in the Journal of Geophysical Research Atmospheres. The study shows that a scale for atmospheric river intensity can be used to let people know what to expect when an atmospheric river is in the forecast, both in the U.S. and globally, according to the nonprofit scientific association AGU.

"The scale helps communities know whether an atmospheric river will bring benefit or cause chaos," explains AGU. The scale ranks atmospheric rivers similarly to how hurricanes are ranked. The range runs from AR-1 to AR-5 (with AR-5 being the most intense) based on how long they last and how much moisture they transport.

A chart explains the different levels of intensity rankings for atmospheric rivers
The intensity of an atmospheric river depends on how long it lasts (typically 24 to 72 hours; horizontal axis) and how much moisture it moves over one meter each second (measured in kilograms per meter per second; vertical axis). While weaker atmospheric rivers can deliver much-needed rain, more intense storms are more damaging.

AGU, after Ralph et al. 2019 

The succession of atmospheric rivers that struck California in December 2022 and January 2023, bringing heavy rain and snow, reached AR-4. The atmospheric river that contributed to the disastrous flooding in Pakistan in 2022 was an AR-5.

The study authors note that the term is becoming more broadly known with atmospheric rivers so frequently in the news lately.

"Atmospheric rivers are the hurricanes of the West Coast when it comes to the public's situational awareness," said F. Martin Ralph, an atmospheric scientist at Scripps Institution of Oceanography and a co-author of the study. He said that people need to know when they're coming, have a sense of how extreme the storm will be, and know how to prepare. "This scale is designed to help answer all those questions."

"This study is a first step toward making the atmospheric river scale a globally useful tool for meteorologists and city planners," said Bin Guan, study co-author and an atmospheric scientist at the Joint Institute for Regional Earth System Science and Engineering, a collaboration between UCLA and NASA's JPL. "By mapping out the footprints of each atmospheric river rank globally, we can start to better understand the societal impacts of these events in many different regions."