What is ‘atmospheric river’ which has put millions at risk in California?

California is in chaos. Parts of Los Angeles received a record downpour on Monday after a storm hit the state. Over 700,000 were left in the dark on Monday and the city witnessed mud and boulders sliding down hillsides. Virtually all of Southern California was under flash flood advisories and watches, including the Los Angeles area, where between 12.7 and 25.4 centimetres of rain had fallen and more was expected, according to the National Weather Service. At the downtown measuring station, 17 centimetres of rain had fallen by Monday afternoon, nearly half the yearly average of 36 inches. It was already the third-wettest two-day period since 1877, the service said. This is the second storm to hit California in a matter of days –  both powered by an atmospheric river. Both atmospheric rivers were called a “Pineapple Express” because they originated near Hawaii. But what do we know about this weather phenomenon? Let’s take a closer look: What are they? Atmospheric rivers are relatively narrow plumes of moisture that form over an ocean. As per the National Oceanic and Atmospheric Administration, atmospheric rivers can be between 400 and 600 kilometres wide. They can be over 1,600 kilometres long. So, how do they form? “When atmospheric rivers run up against mountains or run into local atmospheric dynamics and are forced to ascend, the moisture they carry cools and condenses, so they can produce intense rainfall or snowfall,” Qian Cao, a hydrologist at the University of California, told PBS.

The vast airborne current of dense moisture can be carried aloft for hundreds of kilometres.

Known as ‘rivers in the sky’, they are funnelled over land and fall as bouts of heavy rain and snow. Appearing as a trail of wispy clouds, they can produce torrential amounts of rain as they move over land. Some atmospheric rivers are weak weather systems that bring beneficial rain or snows that feed water supplies and are a crucial element to the global water cycle. As per CNN, they comprise 30 to 50 per cent of annual rainfall for the western United States. According to the National Oceanic and Atmospheric Administration, atmospheric rivers can carry up to 15 times the volume of the Mississippi River – which is the 15th largest river in the world. [caption id=“attachment_13695492” align=“alignnone” width=“640”] A storm darkens the sky at the mouth of the Russian River, north of Bodega Bay, California. Image courtesy: NOAA[/caption] As per PBS, atmospheric rivers on average have around twice the regular flow of the Amazon River. These “rivers in the sky” are relatively common, with about 11 present on Earth at any time, as per NASA. “Atmospheric rivers are more frequent on the East Coast than they are on the West Coast,”  Jason Cordeira, associate professor of meteorology at Plymouth State University told CNN. “They’re just not as impactful and don’t usually produce as much rainfall.” Not all bad, but… Most atmospheric rivers are weak and do not cause damage. In fact, they can provide much needed rain or snow. “They also contribute to the snowpack, which provides a significant portion of California’s year-round water supply,” Cao told PBS. However, it’s not all good news.

Atmospheric rivers can cause a lot of damage.

“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. As per CNN, nearly two dozen people in California died in 2023 after California was hit by several atmospheric rivers. The coast witnessed 30-foot waves and bore the brunt of winds around 160 kmph. In 2019, an atmospheric river also nicknamed the “Pineapple Express” slammed into the California. The water vapour from near Hawaii brought rain and triggered mudslides that forced motorists to swim for their lives and sent homes sliding downhill. As per CNN, California in 1861 witnessed a mega flood due to an atmospheric river. The state received 43 straight days of relentless rain and thousands were left dead. Worse, scientists say atmospheric rivers that drenched California and flooded British Columbia in recent years will become larger — and possibly more destructive — thanks to climate change. They will also become more frequent and more likely to arrive as part of series, causing up to four times more economic damage than they would have individually, a study in Science Advances said. “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 told CNN. “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.” History shows they are becoming more frequent. Since last winter, 46 atmospheric rivers have made landfall on the US West Coast, pulling the state out of a years-long drought, according to the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes. Nine were categorised as strong, two were extreme and one was exceptional. ‘Cold and exhausted’ So far officials have attributed three deaths to the storm that first hit Northern California. Crews rescued people from swift-moving water in various parts of Southern California on Monday, including 16 people and five cats in Los Angeles County alone, authorities said. Also rescued were two homeless people who spent the night on a small island in the Santa Ana River in San Bernardino, about 88.51 kilometers east of Los Angeles, authorities said. “They were cold and exhausted from a night out stranded on this little patch of dirt that was in the middle of the river,” said Captain Nathan Lopez of the San Bernardino County Fire Department. A dog and two cats were also saved. At a news conference, authorities said rain would taper off in intensity on Tuesday, but the threat of flooding remained high. “The ground is extremely saturated, supersaturated,” said Ariel Cohen, meteorologist in charge of the National Weather Service bureau in Los Angeles. “It’s not able to hold any additional water before sliding. It’s not going to take much rain for additional landslides, mudslides, rockslides and other debris flows to occur. With inputs from agencies