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Atmospheric rivers are rivers of water vapor low in the Earth's atmosphere.
They can cause heavy rain or snow in parts of the US, leading to flooding.
Milder versions can relieve droughts, but stronger versions can be dangerous.
In late January and early February, two atmospheric rivers struck 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 air," Jason Cordeira, an atmospheric scientist at 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, the paper said National Oceanic and Atmospheric Administration. They can stretch more than 1,200 miles long and 600 miles wide and carry more water than a dozen Mississippi rivers.
Below we answer some frequently asked questions about atmospheric rivers.
1. What causes an atmospheric river?
Atmospheric rivers form when moisture in narrow channels in the atmosphere leaves the tropics.
When they make landfall and encounter large mountain ranges such as the Sierra Nevadas in California, they can cause heavy rain, snow and flooding, Cordeira said.
"Atmospheric rivers don't form on their own," says Cordeira. They typically develop alongside extratropical storms.
The cold fronts of these cyclones help accumulate water vapor, which the atmospheric rivers can then transport thousands of miles, Cordeira said, adding that "the eastern side of ocean basins or the western 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 could also affect the northeastern U.S., he added.
Atmospheric rivers are so common that between eight and 10 of them typically flow at a time across the oceans in the Northern and Southern Hemispheres, Cordeira said.
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2. Do El Niño and La Niña have an influence?
Different weather patterns like El Niño and La Niña could 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 would be less likely. Still, a relatively large number, 46, have made landfall on the US west coast, according to the Scripps Institution of Oceanography.
"But there are also relatively dry years when very few atmospheric rivers make landfall," Cordeira said.
3. How long does an atmospheric river last?
The average life cycle of an atmospheric river is three to seven days, Cordeira said. What usually stops it is rain.
"Things that remove water vapor from the air will cause an atmospheric river to die," Cordeira said.
When the rivers encounter landmasses with large mountains, the mountains cool the air, "and as it cools, it condenses and produces precipitation and it reverberates much of that rainfall," Cordeira said.
Once the river makes landfall, it usually rains out within 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 take tropical moisture from low latitudes and bring it to higher latitudes, Cordeira said. They can both have heavy rains and strong winds, but they are not directly related, he said. "Even though they share some of the same ingredients, they are distant cousins."
5. Are atmospheric rivers something new?
People who grew up in the 1990s probably didn't hear much about atmospheric rivers when they were young.
The phenomenon existed, but the media often did referred to them like a 'pineapple express' - a strong atmospheric river that transported moisture from the tropics near Hawaii to the west coast of the US.
In 1994, MIT researchers Yong Zhu and Reginald E. Newell published one paper about 'atmospheric rivers and bombs'.
The term has come into widespread use in academic research since.
Then, in 2019, University of California, San Diego researchers went one step further and announced this a new scale for measuring the strength of atmospheric rivers, similar to categories used for hurricanes.
An atmospheric river is rated from AR-1 to AR-5 based on the amount of moisture and duration of the event, with AR-1 being the weakest and least threatening.
6. Are there benefits to atmospheric rivers?
Some states rely 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 snow to the mountains.
7. Is an atmospheric river the result of climate change?
It is difficult to attribute the severity of a single weather event to the climate crisis. But Cordeira said that as the atmosphere and oceans warm, atmospheric rivers will likely hold more moisture.
"It will most likely be more intense and it will be a broader storm" because of the increased moisture, he said. The moisture may fall in the form of rain instead of snow, meaning not as much water trickles down from the mountains in the summer.
Warmer temperatures could also indicate frequent "families" of atmospheric rivers, where two or three events occur one after the other, Cordeira said. This happened in early 2024, when Southern California was inundated by two heavy rain events.
When that happens, it increases the risk of flooding, Cordeira said. Soils that are already saturated with water cannot absorb more moisture and swollen rivers can overflow.
"You can imagine how serious the consequences would be if we had two or three in a row," he said.
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