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

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.

FILE - A man walks by fallen trees after a "bomb cyclone" storm brought heavy winds to Issaquah, Wash., Nov. 20, 2024. (AP Photo/Manuel Valdes, File)

FILE - A tractor-trailer hauling a load of oranges sits on the side of the road after sliding off the Maine Turnpike early on Dec. 11, 2024, in New Gloucester, Maine. (AP Photo/David Sharp, File)

FILE - Mud and debris is strewn on Fryman Road during an atmospheric river Feb. 5, 2024, in Studio City Calif. (AP Photo/Marcio Jose Sanchez, File)

DENVER (AP) — Supporters of a prominent Colorado immigration and labor activist say an immigration judge has ruled that she can post bond and be released after spending nine months in detention.

The judge issued a written ruling Sunday allowing Jeanette Vizguerra to post $5,000 bond, said Jennifer Piper of the American Friends Service Committee, who has been working with Vizguerra's lawyers and her family. Vizguerra's family and a nonprofit group that helps pay bonds for people in immigration detention were working Monday to post the bond, which can take a day or more to process, she said.

Emails seeking comment from U.S. Immigration and Customs Enforcement and the Department of Homeland Security were not immediately returned.

Vizguerra gained prominence after she took refuge in churches in Colorado to avoid deportation during the first Trump administration. She was arrested in the parking lot of the Denver-area Target store where she worked on March 17.

Vizguerra, who came to Colorado in 1997 from Mexico City, has been fighting deportation since 2009 after she was pulled over in suburban Denver and found to have a fraudulent Social Security card with her own name and birth date but someone else’s actual number, according to a 2019 lawsuit she brought against ICE. Vizguerra did not know the number belonged to someone else at the time, it said.

Vizguerra’s lawyers have said ICE was attempting to deport her based on an order that was never valid and challenged her detention in federal court.

A federal judge recently ordered that a bond hearing be held in immigration court to determine whether Vizguerra should be continue to be held in detention facility in suburban Denver as her immigration case plays out.

FILE - Jeanette Vizguerra, center, a Mexican immigrant who has lived in a church to avoid immigration authorities for the past three months, pauses as she speaks after leaving the church early May 12, 2017, in downtown Denver. (AP Photo/David Zalubowski, File)