VERIFY answers five top questions about atmospheric rivers, including how they are defined and rated on a scale of 1 to 5.

Series powerful storms bringing flooding, damaging winds and landslides to areas across California, leaving tens of thousands of people without power.

National Weather Service warned “relentless parade of atmospheric rivers” in the state.

But what is an atmospheric river? VERIFY answers five questions about the weather system, including how it is defined and how atmospheric rivers are rated on a scale of one to five.



What is an atmospheric river?

Atmospheric rivers are long, narrow regions of the atmosphere that carry water vapor across the sky, according to NOAA.

USGS says atmospheric rivers “always flow somewhere on Earth, even if they don’t stay permanently in one place like rivers on Earth.” These rivers in the sky can be hundreds to thousands of miles long and hundreds of miles across.

Atmospheric rivers usually occur from autumn to spring – especially from December to February – and can carry as much water as rivers, FEMA’s National Flood Insurance Program reports.

What kind of weather do atmospheric rivers bring?

When atmospheric rivers reach land, they often release water vapor as rain or snow.

“In fact, only a few atmospheric river events each year cause up to half of the annual rainfall on the West Coast,” says NOAA.

Although atmospheric rivers are often harmless and can provide beneficial precipitation, they can also cause extreme weather conditions that cause floods, landslides, and other damage.

Strong atmospheric rivers cause about $1 billion in flood damage in the western states each year and account for more than 80% of the 40-year flood damage. according to FEMA’s National Flood Insurance Program.

How are atmospheric rivers formed?

Atmospheric rivers usually form in tropical regions near the equator, where high temperatures cause water to evaporate and rise into the atmosphere.

The USGS explains that atmospheric rivers typically flow in the lowest part of the atmosphere about half a mile to one mile above the ground. Atmospheric rivers are driven by strong winds.

As atmospheric rivers move inland and flow over mountains, they rise. This causes water vapor to condense into droplets that fall as precipitation as rain or snow.

More from VERIFY: What is a bomb cyclone? 5 quick facts about a winter storm

Are atmospheric rivers rated according to their severity?

Like hurricanes and other storms, atmospheric rivers are rated according to their strength.

The scale, created by a group of meteorology experts, divides atmospheric rivers into five categories based on water vapor content and duration. This scale has been published American Meteorological Society (AMS) in February 2019

According to an article published by AMS, an atmospheric river that lasts less than 24 hours in an area is downgraded by one category and upgraded by one category if it lasts more than 48 hours.

The scale describes “a range of scenarios that could prove beneficial or dangerous depending on the strength of atmospheric rivers,” experts from the Scripps Institution of Oceanography at the University of San Diego to say. It ranks atmospheric rivers from 1 to 5 and classifies them as “weak”, “moderate”, “strong”, “extreme” and “exceptional”.

Here’s a breakdown of those rankings:

  • AR 1 (Weak): Primarily useful.
  • AR 2 (Moderate): Mostly useful, but also somewhat dangerous.
  • AR 3 (Strong): Balance of good and bad.
  • AR 4 (Extreme): Mostly dangerous, but also useful.
  • AR 5 (Exclusive): Primarily dangerous.

“The scale admits that it is weak [atmospheric rivers] are often mostly useful because they can improve water supply and snowpack, while stronger ARs can become mostly dangerous, for example if they hit an area with conditions that increase vulnerability, such as burn scars or already wet conditions,” experts who created the scale said. “Increased durations may increase the impact.”

What is Pineapple Express?

“Pineapple Express” is the name of a certain type of strong atmospheric river. This atmospheric river got its name because moisture accumulates in the tropical part of the Pacific Ocean around Hawaii, where pineapples are grown.

If the Pineapple Express makes landfall in the western United States and Canada, it could produce heavy rain and snow, according to NOAA. In California, the Pineapple Express could bring up to 5 inches of rain in one day.

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