An atmospheric river is a term used to describe a relatively narrow region in the atmosphere that transports water vapor outside of the tropics northward. They are typically a few thousand miles long and 100 miles wide. There can be three to five of these “rivers” at any time covering the hemisphere.
Atmospheric rivers are apparent in satellite images and are often in the vicinity of fronts over the oceans. They transport large amounts of water. Atmospheric rivers that transport water vapor from Hawaii to North America are sometimes nicknamed the “pineapple express.”
While these atmospheric rivers are made of water in the gas form, or water vapor, they can carry as much water as a dozen Mississippi Rivers. The features can transport water into storms and result in extreme precipitation events that cause severe flooding in western coastal regions of the world, including the West Coast of North America.
In early December 2012, San Francisco received about 10 inches of rain from water vapor transported across the Pacific Ocean by one of these rivers of vapor. While some of these events lead to flooding and mud slides, most precipitation events simply provide beneficial precipitation important to a region’s water supply.
Because of these features’ importance to the water supply of a region, their movements are important to follow and forecast. They are monitored using satellite measurements, aircraft reconnaissance and a new set of instruments along the coast of California. These observations, combined with better numerical modeling, will likely improve forecasts that eventually will help improve water management.