Atmospheric rivers are long, moisture-rich air currents that can produce heavy rain or snow when they reach land. They form mostly in tropical regions and are steered by upper-level winds, supplying 30%–50% of annual precipitation on the U.S. West Coast. Strong events can carry many times the daily flow of the Mississippi River and, because they tend to be warm, can cause rapid snowmelt, runoff and floods. Studies show ARs are growing larger, wetter and more frequent as the climate warms.
What Is an Atmospheric River? How Giant Moisture 'Rivers' Shape Weather, Water and Flood Risk
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)
Atmospheric rivers are long, concentrated bands of moisture in the atmosphere that can deliver intense rain or heavy snow when they move ashore. These "rivers" of water vapor play an outsized role in regional water supplies and flood risk, especially on the U.S. West Coast.
Where Do Atmospheric Rivers Come From?
Atmospheric rivers typically form in tropical or subtropical regions, where warm temperatures help lift large volumes of water vapor into the atmosphere. Upper-level winds then steer that moisture toward higher latitudes. While they occur worldwide, they are particularly important to the U.S. West Coast, where they supply roughly 30%–50% of annual precipitation and sustain reservoirs and snowpack.
Size, Strength and Trends
Driven by winds associated with cyclones, atmospheric rivers commonly span about 250 to 375 miles (400 to 600 kilometers) across. Many events are relatively weak, but the strongest can transport extraordinary volumes of moisture — research shows some carry seven to 15 times the average daily discharge of the Mississippi River. A 2025 study indicates atmospheric rivers are becoming larger, wetter and more frequent as the planet warms.
What Happens When an Atmospheric River Hits Land?
When moisture-laden air encounters mountain ranges — for example, the Sierra Nevada on the California–Nevada border — it is forced upward, cools, and releases heavy precipitation as rain or snow. Unlike colder Pacific storms that build snowpack gradually, atmospheric rivers tend to be relatively warm. That means rain often falls on existing snowpack at lower elevations, which can trigger rapid melting, runoff and flooding and reduce the snow reserves that supply water later in the year.
Special Names and Origins
A particularly powerful atmospheric river that originates in the tropical Pacific near Hawaii is nicknamed the "Pineapple Express." The term "atmospheric river" came into common scientific use following research published in the 1990s by Yong Zhu and Reginald E. Newell of the Massachusetts Institute of Technology. Scientists often abbreviate the phenomenon as "AR."
Why it matters: Atmospheric rivers are vital to replenishing water supplies but can also produce destructive flooding, mudslides and rapid snowmelt. Understanding their behavior is critical for water management and disaster planning as the climate changes.
