A long, narrow corridor of concentrated water vapor moving through the atmosphere is called an atmospheric river.
Atmospheric rivers are often several hundred kilometers wide and can stretch for thousands of kilometers. They transport large amounts of moisture from the tropics or subtropics toward higher latitudes. When such a corridor reaches land and rises over mountains, it can produce intense rain or heavy snow.
The term became widely used in atmospheric science during the 1990s. One well-known example is the Pineapple Express, an atmospheric river carrying moisture from the tropical Pacific toward the western coast of North America. Similar events occur around the world, including near Europe, East Asia, Chile, and New Zealand.
Atmospheric rivers can refill reservoirs and build mountain snowpack, but they can also trigger floods, landslides, and damaging winds. They are not rivers of liquid water in the sky; the name refers to the focused transport of invisible water vapor.