Weather Words: Atmospheric River

While rivers on the ground carry large amounts of water downstream, rivers in the atmosphere can do the same thing, only high above Earth’s surface.

An atmospheric river (AR) is a long plume of moisture in the atmosphere that acts like a conveyor belt, transporting huge amounts of water vapor from the tropics toward higher latitudes. It stretches from the tropics or subtropics into higher latitudes, often thousands of miles long. This ribbon of moisture can unleash heavy rain or snow, especially along mountains where the air is forced upward and cools, wringing out even more moisture.

While not all atmospheric rivers are extreme, the most intense ones can deliver as much water as the Mississippi River — in the sky. These events can bring both benefits and hazards: They help refill reservoirs and ease drought, but they can also trigger flooding, landslides and damaging winds when they linger or strike the same area repeatedly.

ARs are most common along the West Coast of the U.S., especially in California during the cool season, but they occur all around the world. Thanks to satellite observations, meteorologists can now track and forecast them with greater accuracy, giving communities more time to prepare for both their soaking rains and their much-needed water supply.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.