An atmospheric river is a region of very moist air that moves from tropical latitudes and into the middle latitudes. Additional terms that refer to this phenomenon include tropical plume, tropical connection, moisture plume, and water vapor surge. The importance of atmospheric rivers is that they can transport abundant amounts of moisture that can lead to significant precipitation events. These precipitation events can be heavy due to the large amount of moisture that is transported. Once the moisture is lifted by mountains or a storm system then very heavy precipitation can occur.

Atmospheric rivers are seen on water vapor imagery as a plume of moisture in the upper levels that extends from the tropics and extends into the middle latitudes. They have a white appearance since high moisture levels produce white on water vapor imagery. Atmospheric rivers can have regional names. Perhaps the most famous one is called the “Pineapple Express”. This is an atmospheric river that extends from the region around the Hawaiian islands and toward the North American continent. It can transport large amounts of moisture toward North American from the tropical Pacific Ocean that when lifted will generate significant precipitation.