Explainer: Storms in California: What are 'atmospheric rivers?'

The latest in a series of atmospheric river storms soaked California on Tuesday, causing flooding and mandatory evacuations for residents in 10 counties.

HOW COMMON ARE ATMOSPHERIC RIVERS?

These "rivers in the sky" are relatively common, with about 11 present on Earth at any time, according to NASA.

Most atmospheric rivers are weak and do not cause damage. In fact, they can provide much needed rain or snow.

One such storm last year in drought-stricken California triggered mudslides, toppled utility poles and blocked roadways, but also helped replenish depleted reservoirs and reduced the risk of wildfires by saturating the state's parched vegetation.

HOW BIG ARE THEY AND CAN THEY BE DANGEROUS?

Atmospheric rivers can carry up to 15 times the volume of the Mississippi River, according to the National Oceanic and Atmospheric Administration. They appear as a trail of wispy clouds that can stretch for hundreds of miles.

In 2019, an atmospheric river nicknamed the "Pineapple Express" hit California. The water vapor from near Hawaii brought rain and triggered mudslides that forced motorists to swim for their lives and sent homes sliding downhill.

In 2021, an atmospheric river dumped a month's worth of rain on British Columbia in two days, prompting fatal floods and landslides, devastating communities and severing access to Canada's largest port.

WHAT IMPACT WILL CLIMATE CHANGE HAVE ON THEM?

Atmospheric rivers of the kind that drenched California and flooded British Columbia in recent years will become larger — and possibly more destructive — because of climate change, scientists have said.

Columns in the atmosphere hundreds of miles long carry water vapor over oceans from the tropics to more temperate regions in amounts more than double the flow of the Amazon River, according to the American Meteorological Society.

There are projected to be 10% fewer atmospheric rivers in the future, but they are expected to be 25% wider and longer and carry more water, according to a 2018 research paper.