Jet streams are fast-moving rivers of air in the upper levels of the atmosphere. You may have heard about jet streams on television, where weathercasters track their paths on weather maps. Weather forecasters pay lots of attention to the positions of the jet streams because they are the boundary between warm and cold air and have much to do with how weather systems form. Jet streams have a strong effect on weather because they steer movement of high and low pressure systems, and block the movement of upper level moisture and energy. Jet streams do not usually move in straight paths; the path of a jet stream typically has a meandering shape. Each large curve or meander of the jet stream is known as a Rossby wave.

Jet streams follow the boundaries between hot and cold air. Since the temperature changes in the atmosphere are greatest around 30° and around 50°–60° in the Northern and Southern Hemispheres, this is where the jet streams form. The major Northern Hemisphere jet streams are the polar jet, which forms at 50°–60° north latitude, and the subtropical jet, near 30° north latitude. The strength and position of the polar jet is important because most large winter storm systems form and move along the polar jet. In the Northern Hemisphere in the winter, areas may get colder than normal periods as the polar jet stream dips south, bringing cold air in from Arctic regions.

A climate study conducted between 1979 and 2001 found that the positions of the jet streams in both the Northern and Southern Hemispheres have shifted toward the poles, a trend some scientists believe will continue. This change of position is important to projecting future climate conditions because it may affect the formation and severity of storms in mid-latitude regions. Jet streams over sub-tropical regions typically slow the development of hurricanes, therefore the movement of jet streams away from these regions may result in more frequent and severe hurricanes. The tracks that storms follow will also change as a result of shifting positions of the jet streams. Scientists are currently studying why the positions of the jet streams are changing. It may be due to natural patterns of variation, to human impacts on the climate system, or to a combination of the two.

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