Atmospheric river is a name for a long, narrow channel of moist air that occurs over all ocean basins. These plumes of water vapor (water in the form of a gas) are typically several thousands of miles or kilometers long and only a few hundred miles or kilometers wide. They are generally found in the lower troposphere, the layer of the atmosphere closest to Earth’s surface. Atmospheric rivers transport moisture mostly from south to north in the Northern Hemisphere and from north to south in the Southern Hemisphere.
Atmospheric rivers have a central role in the global water cycle. They account for more than 90 percent of the movement of moisture from the subtropics, near the equator, to the midlatitudes, roughly halfway between the equator and the poles. Atmospheric rivers are also a major cause of extremely heavy precipitation events. They often produce severe flooding in midlatitude, westerly coastal regions of the world, including Western Europe and the West Coast of North America.
A single atmospheric river can carry as much water as the world’s largest river, the Amazon. The moisture is drawn from the evaporation of ocean water. There are typically 3 to 5 atmospheric rivers present within each hemisphere at one time.
