Atmospheric rivers are massive plumes of moisture carried across the sky that can dump heavy rain or snow on the ground.
Here's a look at this phenomenon:
Where do atmospheric rivers come from?
According to the National Oceanic and Atmospheric Administration, atmospheric rivers typically form in tropical regions where high temperatures can cause water vapor to rise into the atmosphere. Winds then transport this moisture to northern and southern latitudes.
They occur worldwide but are especially important on the West Coast of the United States, where they account for 30% to 50% of annual precipitation and are vital to water supplies but can also cause storms that cause flooding and landslides, according to NOAA.
Atmospheric rivers formed by winds associated with cyclones are typically 250 to 375 miles (400 to 600 kilometers) wide and move under the influence of other weather conditions.
Many atmospheric river events are weak. But the most powerful ones are able to tolerate extraordinary amounts of moisture. Studies have shown that they can carry 7 to 15 times the average amount of water released daily by the Mississippi River. US Geological Survey.
They are also becoming larger, wetter and more frequent as the Earth's atmosphere warms. according to 2025 study.
What happens when an atmospheric river reaches land?
When moisture-laden air moves over mountain ranges such as the Sierra Nevada along the California-Nevada line, water vapor rises and cools, turning into heavy precipitation that falls as rain or snow, according to NOAA.
While traditional cold winter storms in the North Pacific create the Sierra's snowpack, atmospheric rivers tend to be warm. Snow can still fall at the highest elevations, but rain usually falls on the snowpack at lower elevations. This can quickly cause melting, runoff and flooding, and reduce the amount of snow cover needed for Californiawater supply.
What is Pineapple Express?
This is the nickname for a strong atmospheric river originating in the tropical Pacific Ocean near Hawaii.
Where does the term “atmospheric river” come from?
The name comes from a study published in the 1990s by scientists Yun Zhu and Reginald E. Newell of Massachusetts Institute of Technology. Atmospheric rivers are often called AO.
