atmospheric river

A kind of relatively narrow corridor or passage, flowing through the lower part of the atmosphere, that transports a large quantity of water vapor from the tropical and subtropical realm to mid and high latitudes. It is a fairly recent term, as it was not coined until the early 1990s, in the USA. Atmospheric rivers are several hundred kilometers wide and several thousand kilometers long. They play an important role in the hydrological cycle, as they are the main mechanism for the horizontal transport of water vapor in the atmosphere. On average, there are on the order of a dozen of them simultaneously present on Earth, each of which moves a quantity of water (in a gaseous state) greater than the flow of the Amazon River. They are associated with the low-level jet streams that arise ahead of cold fronts and mid-latitude low-pressure systems, being potential precursors and generators of intense, persistent rains in continental areas exposed to them, such as the west coast of the USA, the Atlantic facade of Europe, and North Africa. The rains linked to them and amplified by the orographic forcing of mountains sometimes culminate in catastrophic flooding.