California weather: What are the atmospheric rivers?

Atmospheric rivers may sound like a description in a travel blog, but these phenomena can 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 and mountain snow.

Atmospheric rivers are also partly to blame for the torrential rains in California this winter.

One of the most well-known atmospheric rivers is the Pineapple Express, a ribbon of water vapour that begins in Hawaii, where warm conditions help evaporate ocean waters into the atmosphere.

Once in the air, winds swiftly carry along the vapour, which, if lifted by a front or passing over mountains, 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 (1,600km) long and 350 miles wide. They are actually quite beneficial - the federal government estimates that 50% of California's annual rain and snowfall comes from atmospheric rivers.

But the meteorological plumes of moisture affecting California recently have also overlapped with other severe weather, including "bomb cyclones", which is a term for a rapidly deepening area of low pressure.

These severe storms have killed at least 35 people this year in California and left tens of thousands of people without power.

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

Forest fires have also destabilised areas, as burn scars make landslides more likely.