01:20 - Source: CNN
What are atmospheric rivers?
CNN  — 

An atmospheric river is a plume of moisture that helps carry saturated air from the tropics to higher latitudes, delivering unrelenting rain or snow.

Think of it as a fire hose that aims at – then drenches – a particular region.

Typically 250 to 375 miles wide, atmospheric rivers can stretch more than a thousand miles long, the National Oceanic and Atmospheric Administration says.

In the western US, they account for 30% to 50% of annual precipitation.

While atmospheric rivers are an incredibly important source of rainfall, they can also bring flash flooding, mudslides and landslides, sometimes killing people and destroying property.

“When atmospheric rivers pass over land they can cause conditions similar to those of hurricanes with intense and rapid rainfall, cyclone force winds, and significantly increased wave heights,” NOAA says.

Atmospheric rivers happen all over the world

Ten or more atmospheric rivers can be happening at once across the globe.

A well-known and strong one is the Pineapple Express, with moisture transported from the tropical Pacific around Hawaii to the US and Canadian West Coasts.

The eastern half of the US also experiences atmospheric rivers, with moisture pulled from the Gulf of Mexico.

“Atmospheric rivers are more frequent on the East Coast than they are on the West Coast,” said Jason Cordeira, associate professor of meteorology at Plymouth State University. “They’re just not as impactful and don’t usually produce as much rainfall.”

Western Europe and North Africa also experience frequent atmospheric rivers, as do New Zealand and Australia.

Fiona Goodall/Getty Images
Flooding blocks roads in 2023 in Auckland, New Zealand from an atmospheric river.

Most notable atmospheric river events

• In the winter of 2023, a series of atmospheric rivers sent an unprecedented amount of moisture to drought-parched California. Over roughly three weeks, parts of the state got between 2 and 3 feet of rain. The coast was battered by 30-foot waves, and the state endured 100-mph winds. At least 20 people died.

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Ryan Orosco carries his 7-year-old son, Johnny, on his back while his wife, Amanda, waits at the front porch to be rescued from their flooded home in Brentwood, California, on Monday, January 16.
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An abandoned car is trapped on a flooded street in San Diego on January 16.
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Joey Klien salvages items from his house on January 16 after part of it was flooded in Carmel Valley.
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Damage caused by recent storms is seen at the Capitola Pier on Sunday, January 15. The pier was built in 1857.
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Melissa Foley clears debris in her neighborhood as the San Lorenzo River rises in Felton on Saturday, January 14.
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A flooded field is seen in Salinas as the Salinas River begins to overflow its banks on Friday, January 13.
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National Guard troops, sheriff's office personnel and firefighters search for missing 5-year-old Kyle Doan near San Miguel on Thursday, January 12. Doan was pulled from his mother's hands by rushing floodwaters on January 9.
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Kelly Slate packs a mirror in the back of a truck after her home was flooded in Planada on Wednesday, January 11.
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A car drives through floodwaters in Planada on January 11.
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Residents in Piru work to push back wet mud that trapped cars and invaded some houses on January 11.
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A person walks near driftwood and storm debris that washed up in front of the Santa Cruz Beach Boardwalk amusement park.
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A San Diego firefighter rescues a dog from a flooded home in Merced on Tuesday, January 10.
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Pauline Torres carries belongings from her flooded home in Merced on January 10.
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A neighborhood is flooded in Merced.
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Matt O'Brien shovels mud from a driveway on January 10 after the San Lorenzo River overflowed in Felton.
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Several people had to be rescued after two vehicles fell into this sinkhole in Chatsworth on January 10.
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Commuters in downtown Los Angeles are shuttled over a flooded section of a pedestrian walkway leading to train platforms on the main level of Union Station.
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Cleanup takes place in Aptos on January 9 after streets and homes were flooded near the Rio Del Mar State Beach.
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People arrive at an evacuation center in Santa Barbara on January 9.
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A crew member is reflected in pools of water while setting up the red carpet for the Golden Globe Awards in Los Angeles.
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A home is flooded in Gilroy on January 9.
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Colleen Kumada-McGowan stands in floodwaters in front of her home in Watsonville on January 9.
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Crews work to clear a mudslide on Highway 17 in Scotts Valley.
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Naia Skogerson leaves her home in Aptos.
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A road is damaged in Scotts Valley on January 9.
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This aerial photo shows a tree that fell in Sacramento on January 8.
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Santa Cruz residents clear storm debris and stack sandbags near their homes on January 7.
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Sandbages are piled in front of a door in Capitola on January 6.
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Boone White leaps from his car after a large tree fell on it while he was driving near Capitola.
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A support piece from the Capitola Wharf is seen inside the storm-damaged restaurant Zelda's on the Beach.
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Evan Sousa, left, gets help from Calvin Drake as they push water out of his flooded apartment in Pacifica on January 5.
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Henry Valletta cuts up a downed tree in Sacramento on January 5.
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Residents walk in a damaged area of Aptos on January 5.
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Dominic King, owner of the restaurant My Thai Beach, surveys storm damage at his business in Capitola.
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A vineyard is flooded in Walnut Grove on January 4.
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Drivers in San Francisco barrel into standing water on Interstate 101 on January 4.
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Khaled Dajani clears water from his flooded living room in San Francisco on December 31.

• In October of 2021, a strong atmospheric river hit the San Francisco Bay area exceptionally hard. Winds exceeded 80 mph, and 60-foot waves were recorded. In just three days, the storm delivered as much as 15% of annual rainfall to the region.

California’s megaflood of 1861 is the state’s most catastrophic atmospheric river. The 43-day pounding by unrelenting rain turned the Central Valley into an inland sea. Thousands of people died, and downtown Sacramento was submerged in 10 feet of water and mudslide debris.

Climate change and its impact on atmospheric rivers

As the world warms, the atmosphere can hold more moisture – which will lead to rainier atmospheric river events.

Neal Waters/Anadolu Agency/Getty Images
A road in in California's Santa Cruz Mountains was partially washed away by torrential rain caused by an atmospheric river.

“It’s expected that as the air temperatures increase, the air can hold more water vapor, and therefore any storms that are comprised of water vapor will have more of it,” Cordeira explained.

“So, an atmospheric river, which is defined as a region of water vapor, will likely become more intense. Their frequency may not be more common, but their intensity could become larger.”

Atmospheric rivers will be “significantly longer and wider than the ones we observe today, leading to more frequent atmospheric river conditions in affected areas,” a NASA-led study found.

The frequency of the most intense atmospheric rivers will likely double, the study found.

CNN meteorologist Monica Garrett contributed to this report.