What Are Atmospheric Rivers?
Atmospheric rivers represent extensive corridors of moisture that traverse the atmosphere, capable of delivering substantial rainfall or snowfall when they encounter landmasses.
Origins and Formation
These phenomena typically originate in tropical zones, where elevated temperatures facilitate the evaporation of water into the air. According to the National Oceanic and Atmospheric Administration, upper-level winds then transport this moisture toward higher latitudes.
While atmospheric rivers occur around the globe, they hold particular importance along the U.S. West Coast. NOAA notes that they contribute 30% to 50% of the region's yearly precipitation, playing a crucial role in water resources while also posing risks through potential flooding and landslides.
Characteristics and Scale
Generated by cyclone-associated winds, atmospheric rivers usually span widths between 250 and 375 miles. They travel influenced by broader weather systems.
Although many atmospheric river occurrences are mild, the more potent ones can convey immense volumes of moisture. Research from the U.S. Geological Survey indicates they may transport seven to fifteen times the daily average water flow of the Mississippi River.
A 2025 study suggests these systems are growing larger, wetter, and more frequent as global atmospheric temperatures rise.
Landfall Effects
When moisture-rich air approaches mountain ranges like the Sierra Nevada, the vapor ascends and cools, converting into heavy precipitation—either rain or snow, as explained by NOAA.
Unlike colder winter storms from the north Pacific that accumulate Sierra snowpack, atmospheric rivers tend to be warmer. Snow might still form at higher altitudes, but rain often falls on lower-elevation snow, potentially triggering rapid melting, runoff, flooding, and reducing the snowpack essential for California's water supply.
Special Types and Terminology
The term "pineapple express" refers to a robust atmospheric river that begins in the tropical Pacific near Hawaii.
The phrase "atmospheric river" emerged from research conducted in the 1990s by MIT scientists Yong Zhu and Reginald E. Newell. These systems are commonly abbreviated as ARs.
