What is an atmospheric river?

California is facing more extreme weather this week after days of heavy rain, snow and powerful winds have caused treacherous conditions across the state.

At least 14 people have been killed and a five-year-old boy remains missing as severe storms continue to batter California.

The system swamping California is known as an “atmospheric river” - or a “river in the sky” - a band of water vapor that forms over the ocean and can be hundreds of miles wide.

These weather phenomenon occur globally, but are common on the US West Coast where they drag moisture onshore from the Pacific creating up to half of annual rainfall.

Powerful atmospheric rivers can carry water vapor equivalent to the average flow of water at the mouth of the Mississippi River, according to the National Oceanic and Atmospheric Administration (NOAA).

When they hit the heights of colder mountain ranges, like the Sierra Nevada, the water vapor becomes heavy snowfall. But it’s not necessarily good news for the Sierra snowpack.

While traditional cold winter storms out of the north Pacific build Sierra snowpack vital to the state’s water supply, atmospheric rivers tend to be warm, Bill Patzert, a climatologist at Nasa’s Jet Propulsion Laboratory, told the Associated Press.

That causes the snow at the highest elevations but rain usually falls on snow lower down. That can quickly prompt melting, runoff, and decrease the snowpack needed to supply California with water. This in turn means little impact on the state’s persistent drought.

Along with increasing risks of flash flooding and rock slides, atmospheric rivers have other downsides. A 2016 study found that “rivers in the sky” can virtually wipe out species when they dump vast amounts of rain over a short period.

Researchers at the University of California, Davis, found that rainfall from an atmospheric river wiped out 97 to 100 per cent of wild Olympia oysters in north San Francisco Bay in 2011.

And the problem is worsening due to the climate crisis, scientists warned. As the air heats up, it becomes capable of holding more water and leads to more rainfall.