What are atmospheric rivers?

Atmospheric rivers may sound like a description in a travel blog, but these weather systems cause serious damage.

They occur when water evaporates into the air and is carried along by the wind, forming long currents that flow in the sky like rivers flow on land.

They can cause severe rains, mountain snow and winds.

Atmospheric rivers are partly to blame for the severe rains in California last week, and another one is expected to hit early this week.

One of the most well-known atmospheric rivers is the Pineapple Express, a weather system that begins in Hawaii, where warm temperatures help evaporate ocean waters into the atmosphere.

Once in the air, winds carry the vapour swiftly along - as the vapour passes over cooler land, it condenses and falls as rain or snow.

It regularly brings rains to California and other parts of the western United States, especially during winter's cooler months.

Atmospheric rivers can stretch 1,000 miles (1,610km) long and 350 miles wide. The phenomena are actually quite beneficial - the federal government estimates that 50% of California's annual rain and snowfall comes from these rivers.

But the meteorological plumes of moisture affecting California recently have hit especially hard, in part because they have overlapped with other weather systems, such as bomb cyclones, which are storms where a rapid drop in pressure creates a cyclone effect.

The two events have led to severe storms that have killed 12 people in 10 days and left tens of thousands of people without power as of Monday.

More rain, and snow in some regions, is expected to fall over California and western Nevada on Monday and Tuesday, according to the National Weather Service.

Severe droughts in the West have also affected the region's ability to absorb the water, which makes flooding much more likely.