Source: USA TODAY research by Chad Palmer, Graphic by John Herne

Jet Stream Roars Along High Aabove Earth

By Jack Williams, USATODAY.com

The term "jet stream" is often used to refer to the rivers of wind high in the atmosphere - above about 20,000 feet - that steer storms.

These winds not only steer storms, but also help determine the locations of areas of high and low air pressure at the Earth's surface. (Related graphic: What's happening in highs and lows)

Strictly speaking, as noted in the American Meteorological Society's Glossary of Weather and Climate, a jet stream is "a relatively narrow river of very strong horizontal winds (usually 50 knots or greater) embedded in the planetary winds aloft."

The "planetary winds aloft" are the part of the Earth's general circulation of air that's not at the surface.

Jet streams form along the upper air boundaries of large masses of warm and cold air. (Related graphic: Air masses)

During major cold outbreaks over the USA, the jet stream often dives south - staying above the warm-cold boundary - sometimes moving well over the Gulf of Mexico. During unusually mild winter weather and during the summer, the jet stream retreats northward into Canada.

One of the things that meteorologists look for are the formation of "jet streaks," upper air winds that are faster than the surrounding winds. These are important weather makers. (Related graphic: jet streaks)

While the term "jet stream" refers to high-altitude winds, meteorologists also talk about "low-level jet," which are high-speed winds that might be only a few hundred feet above the surface. (Related: Low-level jets.)