What's an atmospheric river? AP explains the weather phenomenon

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Atmospheric rivers are massive plumes of moisture carried across the sky that can dump heavy rains or snow over land.

Here’s a look at the phenomenon:

Atmospheric rivers generally form in tropical regions, where warm temperatures can cause water vapor to rise into the atmosphere, according to the National Oceanic and Atmospheric Administration. The winds aloft then carry that moisture to northern and southern latitudes.

They occur globally but are especially significant on the West Coast of the United States, where they create 30% to 50% of annual precipitation and are vital to water supplies but also can cause storms that produce flooding and mudslides, according to NOAA.

Formed by winds associated with cyclones, atmospheric rivers typically range from 250 miles to 375 miles (400 to 600 kilometers) in width and move under the influence of other weather.

Many atmospheric river events are weak. But the powerful ones can transport extraordinary amounts of moisture. Studies have shown they can carry seven to 15 times the average amount of water discharged daily by the Mississippi River, according to the U.S. Geological Survey.

They're also getting bigger, wetter and more frequent as Earth's atmosphere warms, according to a 2025 study.

When the moisture-laden air moves over mountain ranges such as the Sierra Nevada along the California-Nevada line, the water vapor rises and cools, becoming heavy precipitation that falls as rain or snow, according to NOAA.

While traditional cold winter storms out of the north Pacific build the Sierra snowpack, atmospheric rivers tend to be warm. Snow may still fall at the highest elevations but rain usually falls on the snowpack at lower elevations. That can quickly prompt melting, runoff and flooding and decrease the snowpack needed for California’s water supply.

It is a nickname for a strong atmospheric river that originates in the tropical Pacific near Hawaii.

The name came from research published in the 1990s by scientists Yong Zhu and Reginald E. Newell of the Massachusetts Institute of Technology. Atmospheric rivers are often referred to as ARs.