Atmospheric rivers are narrow, high-water-density bands within the atmosphere which transport large amounts of water vapour around the planet. At any given time, there are numerous large atmospheric rivers around the world, stretching thousands of miles in length and as much as several hundred miles wide.
The Earth Systems Research Laboratory at the National Oceanic and Atmospheric Administration (NOAA) states that atmospheric rivers "are responsible for most of the horizontal transport of water vapour outside of the tropics." Somewhere between one-third and one-half of precipitation in the western United States is directly caused by the rivers, and each one can carry several times the volume of an actual large river like the Mississippi River.
Atmospheric rivers are not, of course, actual "rivers" in the sense of bodies of water which run downhill. Instead, the term came about in the 1990s. Two researchers working at MIT, Reginald Newell and Yong Zhu, were poring over data on water vapour in the atmosphere when they realized that a high proportion of the planet's water vapour was actually concentrated in a minority of the atmosphere. The vapour trails took the form of twisting filaments. Because the filaments were made of water, Newell and Zhu started calling them "rivers." In the case of atmospheric rivers, wind rather than gravity propels the water through the air. The atmospheric river is caused when a region begins accumulating an unusually high amount of water vapour, which is then moved over a long distance by the wind.
Since Newell and Zhu published their research, weather forecasters in some regions have started working the idea into how they predict short-term changes in the weather. Knowledge of atmospheric rivers has strengthened forecasters' ability to predict strong rain over one-week periods - although, even so, weather forecasting is still more about probabilities than hard and fast predictions.
They have also confirmed that when atmospheric rivers form, rainfall is likely to be especially sudden and extreme, leading to flash floods and other extreme conditions. NOAA researchers looked at all of the atmospheric rivers that passed over California in a recent 10-year period, for example, and noticed that every flood of the Russian River (near San Francisco) during that decade occurred when an atmospheric river was in the vicinity. The so-called "pineapple express," when heavy precipitation moves from Hawaii to the Pacific coast of the United States and Canada, is a regularly occurring form of atmospheric river.
So far, most of the research into atmospheric rivers has been based on weather satellite data gathered on America's west coast. That area seems to feature particularly common and significant atmospheric rivers. However, the same phenomenon also occurs elsewhere in the world. In 2009, for instance, satellite images revealed a massive atmospheric river forming over the North Atlantic and causing precipitation in the United Kingdom and Northern Ireland. Research into the phenomenon is now going on around the world. One question which scientists are particularly interested in answering is whether climate change will have an effect on how frequently atmospheric rivers form or how large they become. In July 2013, for example, climate researchers predicted that climate change will increase the frequency and size of atmospheric rivers, causing more frequent and more severe floods as a result of increased precipitation.