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 the National Oceanic and Atmospheric Administration.
Atmospheric rivers are long and relatively narrow bands of water vapor that form over an ocean and flow through the sky, transporting much of the moisture from the tropics to northern latitudes.
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.
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.
While traditional cold winter storms out of the north Pacific build the Sierra snowpack, atmospheric rivers tend to be warm.
