Jet Stream Essay

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The jet stream is a relatively narrow band of fast, west-to-east flowing air found above the polar front at an altitude of around 6-7 miles (9-12 kilometers) above the surface. The jet stream is produced by the strong pressure gradient that arises from the contrast between the cold air north of the polar front and the warmer air to the south. Winds in the jet stream average about 75 miles per hour (125 kilometers/hour) in winter, when the temperature contrast across the polar front is greatest, and are slower in summer when the temperature contrast is reduced. Wind speeds are variable, however, especially in winter, when segments of the jet stream will often have wind speeds topping 125 miles per hour (200 kilometers/hour). The location of the polar front, and the jet stream above it, also varies seasonally, retreating northwards into Canada in summer and shifting southwards as far as northern Mexico during the winter.

Upper level winds, including the jet stream, tend to flow in sinuous patterns called Rossby Waves, in which the prevailing west-to-east flow meanders north/south in large arcs known as troughs (curves to the south) and ridges (northward bends). These wave patterns gradually change over a period of days, typically with the ridges and troughs slowly migrating to the east and gradually increasing and decreasing in curvature. Jet stream winds flow through the Rossby Waves like water flowing down a curving river, while the shape of the Rossby Waves continuously varies.

The earliest experience of the jet stream came during World War II, as high-altitude U.S. aircraft such as the B-29 bomber encountered extremely fast headwinds as they attempted to fly westward across the Pacific. Today, aviators rely on accurate knowledge of the jet stream position in planning routes in order to avoid the delays, excessive fuel consumption, and turbulence that result from attempting to overcome these strong headwinds. Although the jet stream would assist an eastward bound flight, the associated turbulence can still be hazardous to aircraft and passengers.

The position of the jet stream also has a significant affect on surface weather. Cold air is found north of the polar front, while warmer air is to the south. Troughs in the jet stream guide cold air southward, while ridges bring unusually warm air to the north. More dramatically, the speed and path of the jet stream helps to determine where storm systems such as midlatitude cyclones are able to form. When a deep trough forms in the jet stream, there is a tendency for upper-level air to diverge or spread out on the downstream (east) side of the trough. This upper-level divergence causes air to be drawn up from the surface, which produces a cyclone (a region of low pressure with winds spinning in a counterclockwise direction). As the cyclone spins, cold air is pushed south, producing a cold front, while warmer air is pushed northward, producing a warm front. The entire system is blown generally eastward in the direction of the upper-level flow. Midlatitude cyclones tend to form in the winter when upper-level flow is strongest, and often are associated severe weather, including thunderstorms, heavy precipitation, and tornadoes.


  1. Jay Harman, Synoptic Climatology of the Westerlies: Process and Patterns (Association of American Geographers, 1991);
  2. Frederick K. Lutgens and Edward J. Tarbuck, The Atmosphere, 8th (Prentice-Hall, 2001);
  3. Joseph Moran and Michael D. Morgan, Meteorology: The Atmosphere and the Science of Weather, 5th ed. (Prentice-Hall, 1997).

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