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What Produces a Jet Stream?
The jet stream is a product of forces. It is formed and transformed by the coming together of air masses with large differences in temperature and pressure. One such gradient occurs between arctic and more southerly air masses (latitudinal or horizontal differences). Another source of temperature gradient (vertical differences) is the interface between Earth's troposphere and its stratosphere called the tropopause. These factors mold and manipulate the shape and cross-section—the structure—of the jet stream. The Coriolis effect, on the other hand—caused by Earth's rotation—actuates and accelerates it.
Credit: National Weather Service
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A Writhing Snake
Imagining some huge snake with the tip of its tail in its mouth, writhing about, gives a fair visualization of the jet stream. The artic and more southerly air masses increase and decrease in size and shape and so does the jet stream. Part of its coil can dip further south and another part or parts can veer north. The speed at which its winds move can vary as well. This is because air with differences in temperature and pressure is inherently unstable air. The U.S. Centennial of Flight Commission suggests the north to south movement is not fully understood at this time. Is the jet stream controlled by the movement of the air masses or does the jet stream drive that movement? Perhaps both.
Credit: U.S. Centennial of Flight Commission
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Inside the "Snake"
In addition to its overall snakelike shape, a cross-section of the jet stream reveals its internal structure. The flow of air within the jet stream can be likened to the flow of fluid through a pipe. The fluid just inside the wall of a pipe moves slower than the fluid found more toward its center. The air constituting the outer edge of the jet stream moves more slowly than the air found deeper within. Friction and other factors are doubtless contributors. Fluid velocity is greatest in the centermost portion of the stream.
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But Wait - There's More
Although even most well-educated individuals think of the jet stream as being unique, in reality such is not the case. Not only do other planets reveal evidence of possessing a jet stream1—the earth possesses more than one. In fact, there are two major polar jet streams (one north and one south of the equator) as well as other, subtropical jet streams (see the NWS diagram).
Where is the jet stream? In the Northern Hemisphere, the polar jet stream is located generally about 60 degrees North Latitude. In the South, it is at 60 degrees South Latitude. Naturally, the subtropical jets in both instances are located closer to Earth's equator. In addition to the ordinarily east-traveling jet streams themselves, between these defining lines contained air circulates in north-to-south and south-to-north fashion, defining alternating cells.
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Defining Weather and Influencing Flight
Weather broadcasts often refer to the jet stream, polar air and other factors when predicting cooler temperatures, drier conditions and so forth. On rare occasion, the polar jet stream has been known to reach south into Florida. When it does so, it has been blamed for extreme weather conditions, including severe thunderstorms and even tornadoes, such as those seen in Kissimmee, Florida in 2003. Even if not directly the cause of such storms, the polar jet stream may haved influence the subtropical jet stream to intensify otherwise milder bad weather. While violence such as this may not be the norm, the jet stream is a force to be reckoned with.
Fortunately, the force of the polar jet stream can be beneficial in some instances. Pilots sometimes deliberately hitch a ride with the jet stream. In so doing, they save both time and fuel. When they are able to do so, the speed of the airplane in the stream may not be much different from what it may ordinarily be, but the speed of the plane in comparison to the ground will be much greater. The two speeds are approximately additive. Visualize it this way: an airport may be quite large inside. The traveler needs to get from one endto the other. To quicken the process, flat escalators are provided. The traveler walks along the escalator. The traveler's walking speed is added to that of the escalator. The time taken to get from point A to point B is thus significantly reduced.
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References and Resources
1 See, for instance, "Jet Streams on Giant Planets"
U.S. Centennial of Flight Commission: Jet Stream
National Weather Service: JetStream – Online School for Weather
Aviator College of Aeronautical Science and Technology: Principles of Flying – Understanding Jet Streams
Also from Bright Hub: Understanding the Structure of the Atmosphere