Atmospheric rivers are rivers of water vapor low in the Earth’s atmosphere.
They can bring heavy rain or snow to parts of the US, which could 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 floods and mudslides and killing at least nine people, the Los Angeles Times reported.
“An atmospheric river is 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 intensity, according to the National Oceanic and Atmospheric Administration. They can stretch more than 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?
Atmospheric rivers form when moisture moves out of the tropics in narrow channels in the atmosphere.
When they make landfall and hit large mountains like the Sierra Nevadas in California, they can bring heavy rain, snow and flooding, Cordeira said.
“Atmospheric rivers do not form on their own,” Cordeira said. They usually 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.”
This includes California, Oregon, and Washington in the US, as well as parts of Europe, Chile, New Zealand, and Australia. But they can also affect the northeastern US, he said.
Atmospheric rivers are so common that there are typically eight to 10 of them over the northern and southern hemisphere oceans at a time, Cordeira said.
2. Do El Niño and La Niña 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 are less likely to exist. But a relatively high number, 46, landed on the US 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 lifespan of an atmospheric river is between three and seven days, Cordeira said. What usually stops it is rain.
“Things that remove water vapor from the sky, the river of the atmosphere will die,” Cordeira said.
When the rivers run into land masses with large mountains, the mountains cool the air, “and as it cools, it condenses and produces precipitation and sends a lot of that rain out,” Cordeira said.
When the river hits land, it rains out within a day, he said.
4. Is a hurricane like an atmospheric river?
Atmospheric rivers and hurricanes have some things in common, Cordeira said, but “the processes in the atmosphere that lead to hurricanes are different than the processes that lead to atmospheric rivers.”
Both draw tropical moisture from low latitudes and bring them into higher latitudes, Cordeira said. They may experience heavy rain and high winds, but they are not directly related, he said. “Although they share some of the same ingredients, they are distant cousins.”
5. Are atmospheric rivers new?
People who grew up in the 90s probably didn’t hear much about atmospheric rivers when they were young.
It was a phenomenon, but often the media reference to them as a “express pineapple” – a strong atmospheric river that carried moisture from the tropics near Hawaii to the US West Coast.
In 1994, MIT researchers Yong Zhu and Reginald E. Newell published a paper about “rivers and atmospheric bombs.”
The term was widely used in academic research From the time that.
Then, in 2019, the University of California, San Diego researchers went one step further and announced a new scale for measuring the strength of atmospheric rivers, similar to the categories used for hurricanes.
An atmospheric river receives an AR-1 to AR-5 rating based on the amount of moisture and duration of the event with AR-1 being the weakest and least threatening.
6. Are there any advantages to atmospheric rivers?
Some states rely on atmospheric rivers to prevent droughts.
“Most droughts in California and the West Coast of the US occur because of the lack of atmospheric rivers that lead to land,” Cordeira said. Rain or snow can fill reservoirs or add snowpack to mountains.
7. Is there an atmospheric river due to climate change?
It is difficult to attribute the severity of a single weather event to the climate crisis. But Cordeira said as the atmosphere and oceans warm, it is likely that there will be more moisture in the rivers of the atmosphere.
“It’s probably going to be more intense, and it’s going to be a wider storm” because of the increased moisture, he said. The moisture could fall as rain instead of snow, which means that there won’t be as much water coming down from the mountains in the summer.
Warmer temperatures may also mean frequent “families” of atmospheric rivers, where two or three events occur one after the other, Cordeira said. This happened in early 2024 when two heavy rains fell in Southern California.
When that happens, it increases the risk of flooding, Cordeira said. Lands already saturated with water cannot absorb more moisture, and swollen rivers can overflow.
“You can imagine how serious the impacts would be if there were two or three of them back to back,” he said.
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