A cyclone created by an atmospheric river approaches California in 2023.
Atmospheric rivers are rivers of water vapors low in the Earth’s atmosphere.They can bring heavy rains or snow to parts of the US, which can lead to flooding.Milder versions can alleviate droughts, but stronger ones can be hazardous.
In late January and early February, two atmospheric rivers hit the West Coast. They dumped large amounts of rain over Southern California, causing flooding and mudslides and killing at least nine people, the Los Angeles Times reported.
“An atmospheric river is literally a river of water vapor in the sky,” Jason Cordeira, an atmospheric scientist with the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes, told Business Insider.
Like rivers on land, they come in different sizes and intensities, according to the National Oceanic and Atmospheric Administration. They can stretch over 1,245 miles long and 620 miles wide, carrying more water than a dozen Mississippi Rivers.
Below, we answer some FAQs about atmospheric rivers.
1. What causes an atmospheric river?
Just a few atmospheric rivers are responsible for 30% to 50% of the West Coat’s annual precipitation.
Atmospheric rivers form when moisture moves out of the tropics in narrow channels in the atmosphere.
When they make landfall and meet with large mountain ranges like the Sierra Nevadas in California, they can bring heavy rains, snow, and flooding, Cordeira said.
“Atmospheric rivers don’t form by themselves,” Cordeira said. They typically develop alongside extratropical storms.
These cyclones’ cold fronts help accumulate water vapor, which the atmospheric rivers can then transport thousands of miles, Cordeira said, adding that “the east side of ocean basins or the west side of continents are really good locations for these storms.”
That includes California, Oregon, and Washington in the US, as well as parts of Europe, Chile, New Zealand, and Australia. But they can affect the northeastern US too, he added.
Atmospheric rivers are so common that there are typically between eight and 10 of them over the oceans in the northern and southern hemispheres at a time, Cordeira said.
2. Do El Niño and La Niña have an affect?
Different weather patterns like El Niño and La Niña may make the West Coast more or less likely to see atmospheric rivers, Cordeira said, but it’s not necessarily a guarantee.
For example, 2023 was a La Niña year, when atmospheric rivers are supposed to be less likely. Yet a relatively high number, 46, made landfall on the US’s West Coast, according to the Scripps Institution of Oceanography.
“But then there are relatively dry years when very few atmospheric rivers make landfall at all,” Cordeira said.
3. How long does an atmospheric river last?
The average lifecycle of an atmospheric river is between three to seven days, Cordeira said. What usually stops it is rain.
“Things that remove water vapor from the sky will cause an atmospheric river to die,” Cordeira said.
An atmospheric river brought heavy rain to Bolinas, California, in January 2024.
When the rivers run into land masses with big mountains, the mountains cool the air, “and as it cools, it condenses and produces precipitation and it rings out a lot of that rainfall,” Cordeira said
Once the river hits land, it usually rains itself out in about a day, he added.
4. Is an atmospheric river like a hurricane?
Atmospheric rivers and hurricanes have a few things in common, Cordeira said, but “the processes in the atmosphere that give rise to a hurricane are different than the processes that give rise to atmospheric rivers.”
Both pull tropical moisture from low latitudes and bring them into higher latitudes, Cordeira said. They can both have heavy rains and high winds, but they’re not directly related, he said. “Even though they share some of the same ingredients, they’re distant cousins.”
5. Are atmospheric rivers a new thing?
People who grew up in the ’90s likely didn’t hear much about atmospheric rivers when they were young.
The phenomenon existed, but the media often referred to them as a “pineapple express” — a strong atmospheric river that carried moisture from the tropics near Hawaii to the West Coast of the US.
In 1994, MIT researchers Yong Zhu and Reginald E. Newell published a paper about “atmospheric rivers and bombs.”
The term has gained widespread use in academic research ever since.
Researchers updated the scale in 2023. An atmospheric river’s intensity depends on how long it lasts and the amount of moisture it moves over one meter each second.
AGU, after Ralph et al. (2019)
Then, in 2019, University of California, San Diego researchers went one step further and announced a new scale for measuring the strength of atmospheric rivers, similar to categories used for hurricanes.
An atmospheric river gets a rating of AR-1 to AR-5 based on the amount of moisture and the duration of the event with AR-1 being the weakest and least threatening.
6. Are there any benefits to atmospheric rivers?
Some states depend on atmospheric rivers to prevent droughts.
“Most droughts in California and on the West Coast of the US actually occur because of a lack of atmospheric rivers making landfall,” Cordeira said. The rain or snow can fill reservoirs or add snowpack to mountains.
7. Is an atmospheric river due to climate change?
It’s difficult to attribute the severity of a single weather event to the climate crisis. But Cordeira said as the atmosphere and oceans warm, atmospheric rivers will likely contain more moisture.
“It will most likely be more intense, and it will be a wider storm” because of the increased moisture, he said. The moisture could fall as rain instead of snow, which means there won’t be as much water trickling down from mountains in the summer.
Warmer temperatures could also mean frequent “families” of atmospheric rivers, where two or three events happen one after another, Cordeira said. This happened in early 2024 when two heavy rainfalls drenched Southern California.
When that happens, it increases the risk of flooding, Cordeira said. Grounds already saturated with water can’t absorb more moisture, and swollen rivers can overflow.
“You can imagine how serious the impacts would be if we had two or three of them back to back,” he said.