Jet Streams

Have you ever been white-water rafting? How would you like to shoot the rapids at 300 mph? Jet streams are rivers of wind in the upper atmosphere that can reach these speeds. Because jet streams are found at jet-airline cruising altitudes, flight routes must take account of their location. This article will explain how jet streams form and how they affect our daily weather.

The jet streams were discovered during World War II, when high-altitude bombers and fighters reached the tropopause (about seven miles above the planet) for the first time. During the war, the Japanese put high-altitude balloons rigged with bombs into the jet streams, set to drop automatically on the West Coast of the United States after several days. The accuracy of the bombs was extremely low, but they were quite effective as a psychological weapon.

To understand jet streams, we must first examine the structure of the atmosphere. The lowest level of the atmosphere is the troposphere, where the temperature decreases as altitude increases to around 36,000 feet. At this height, called the tropopause, the temperature remains relatively constant with altitude. The height of the tropopause varies with the seasons and is found at higher levels in the summer. It is also higher at the equator than at the poles.

The change in the height of the tropopause from the equator to the poles is not continuous. At some latitudes, breaks in the tropopause create a large change in air temperature within a small altitude range. This creates what meteorologists call a temperature-induced pressure gradient. The pressure gradient pushes on the atmosphere, creating streams of wind. As an analogy, think of how a watermelon seed squeezed between your fingers shoots out at a high rate of speed. The same thing happens to the airstream when it is squeezed between the broken layers of the tropopause.

Jet streams form in two areas, at 30 degrees latitude, called the tropical jet, and at 60 degrees, called the polar jet. When these two jets remain in their normal latitudes, flowing from west to east, the pattern is called zonal flow, which is most common in the summer and has very little weather associated with it. In winter and spring, when the tropopause is found at lower altitudes, the jet streams may make large bends as they are influenced by low-altitude pressure systems. These bends create significant weather changes.

Areas of high and low pressure are like hills and valleys bending a river. When a river encounters a valley, it is bent downward and its speed increases. If the valley slope is steep, the water accelerates to a white-water turbulent flow. The jet stream is affected similarly. When the jet stream bends, areas of turbulence can develop and must be avoided by airliners. This type of turbulence, called clear air turbulence (CAT), can be dangerous because it occurs with little warning.

When the polar jet steam takes a southerly turn, the "Siberian Express" brings cold arctic air to the southern part of the United States. The "Pineapple Express," named for the proximity of the flow to Hawaii, occurs when the jet stream bends to the south over the Pacific Ocean, picking up warm moisture and then deflecting back to the colder north. When this happens the warm moist air condenses and brings waves of rain to the Pacific Northwest.

From bombs in World War II to clear air turbulence to arctic blasts, jet streams continue to be one of the many weather phenomena that can dramatically affect human lives.

Reference: Ahrens, D.C. (1994). Meteorology today. New York: West Publishing Co.

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