What Is an ‘Atmospheric River?’

  • California is recovering from an atmospheric river with another expected this week.
  • The term was coined in 1994 and increasingly appears in news stories and research.
  • An atmospheric river is a long, narrow region of moisture that transports heavy amounts of water.

An atmospheric river caused flooding and mudslides in California this week, with another expected in the next few days, spiking West Coast interest in a weather term that’s relatively new, but increasingly part of conversations, news stories and academic research.

Early research studies on the weather phenomenon include a 1994 paper by MIT researchers Yong Zhu and Reginald E. Newell about “atmospheric rivers and bombs.”

The National Oceanic and Atmospheric Administration describes atmospheric rivers as “relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.”

Atmospheric rivers can carry the water vapor equivalent of the water flow at the mouth of the Mississippi River, according to the federal agency. The vapor is released as snow or rain when the rivers make landfall.

They start near the equator when the sun heats water, which evaporates and rises into the atmosphere, according to the US Geological Survey, which also notes they are expected to intensify in California because of climate change.

Since the term first appeared in 1994, it has increasingly been used in academic research and news reports.

In 2019, researchers at UC San Diego announced a new scale that measures the strength of atmospheric rivers. Similar systems exist for tornadoes and hurricanes. In 2021, the LA Times published an explainer on atmospheric rivers, a term which now regularly appears in headlines.

Google Trends shows a spike in searches for the term in California since December 18, when heavy storms started to threaten the state.

Those storms arrived over the holidays, causing flooding, power outages, some fatalities, and flight delays. There are still 44,000 customers without power in California.

AccuWeather meteorologists expect another atmospheric river to hit the state in the middle or later this week. But the repeat deluges aren’t expected to solve the state’s ongoing problems with drought.

“The moisture that we’re getting now is a big help, but we need more — a lot more — to really put a major dent in the drought,” Richard Heim, a meteorologist with the National Centers for Environmental Information and one of the authors of the US Drought Monitor, told the LA Times.

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