Probing Question: What is the the jet stream?


If you’ve seen the movie Finding Nemo, you probably recall the depiction of the "EAC," a fast-moving ocean current that the film’s surfer-dude sea turtles ride with flair.

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The jet stream could be viewed as something like that, except in the upper atmosphere instead of the ocean, allows climatologist Paul Knight, senior lecturer in meteorology at Penn State.

“The jet stream is a current or river of wind, usually about 200 miles wide and about two miles deep, that flows about five to seven miles above the earth’s surface,” says Knight, Pennsylvania’s state climatologist and longtime host, writer, and producer of the University’s television show Weather World.

“Like a river, it meanders with bends and curves and sometimes even odd-shaped wiggles. This air current directs the movement of high and low pressure at the surface.”

What most people don’t realize, says Knight, is that the jet stream “is the major player in how our weather changes.” We know the most about two main currents of jet stream winds, he notes: the polar front jet and the subtropical jet, both of which travel around the earth from west to east.

Jet streams form where air masses of different temperatures meet in the upper atmosphere, Knight says. The greater the difference in temperature, the faster the jet stream moves—up to 200 miles per hour, with 110 mph as the average. The faster the winds, the more they’re able to push weather systems from one area to another, bringing storms to some regions and periods of fair weather to others.

The strongest jet streams are found during winter, when the temperature contrast is greatest between the cold air near the poles and warm air masses surrounding the equator. Explains Knight, the flow of air often shifts from the north to the south along these thermal boundaries. In the Northern hemispheric winter, the polar jet dips south across the lower third of the United States, bringing in colder air sometimes preceded by strong storms. The Southern Hemisphere has a matching pair of polar and subtropical jet streams. Whenever a front passes through a region, Knight says, you can be sure that the jet stream is overhead.

The jet stream is a fairly recent discovery, he adds. “It was found during World War II when fighter pilots flying from U.S. bases in the Pacific were attempting to reach Japan, but at times, encountered ferocious head winds. In some cases, despite having air speeds of more than 150 miles an hour—remember these were propeller-driven planes—they were not making any forward progress. The phrase ‘the jet stream’ was coined since overcoming these winds would require jet speed. Prior to that, there was evidence of strong winds aloft from weather balloons, but the exact nature of the jet stream was unknown.”

While there’s still a lot to learn about this superhighway of wind, “today, atmospheric scientists have many tools to detect and predict the jet stream,” Knight notes. “Satellite imagery and the abundance of airplanes which monitor the winds as they travel have contributed enormously to our understanding.”

The day-to-day movements of the jet stream are very well forecast and the airline industry relies heavily on these accurate predictions to keep flights on time, says Knight. “The meanderings of the jet stream are attributable to large-scale temperature contrasts over the earth’s surface, including the oceans. So as we better understand the reasons for changes in ocean temperature, our ability to predict the jet stream farther into the future should improve.”

Paul Knight is senior lecturer in meteorology and host, feature writer and producer of Weather World. He can be reached at

Last Updated May 02, 2012