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The Jet Stream by Captain Abraham Ackerman

I find that it is a weather phenomenon that is of interest to all pilots. The pattern of the jet stream is not only beneficial to high altitude flying but is also helpful in predicting several weather events. The fact about westerly wind at a speed that approaches 200 knots, known today as the jet stream, was discovered during World War II by crews of bombers on missions over Europe and Japan

The Earth's atmosphere is classified into various layers. The lowest layer, the troposphere, extends from the surface to an average altitude of 7 miles. The second layer is the stratosphere which is 26 to 29 miles thick. The boundary between the troposphere and stratosphere is called the tropopause. The height of the tropopause varies with latitude, seasonal and diurnal changes. The height of the tropopause varies from about 20,000 feet over the poles to about 65,000 feet over the equator. During the summer and during the daytime the tropopause is higher than during the winter and at night respectively. The tropopause is not continuous; it is often ragged as it slopes from the Equator towards the poles. Abrupt changes in temperature, strong wind, turbulence and wind shear characterize the tropopause. The height of the tropopause and certain weather phenomena are related.

Among these are the height of thunderstorms (thunderstorms reach higher altitudes when the tropopause is higher) and the formation of the jet stream. The temperature lapse in the troposphere is at an average of 2° C per 1000 feet until reaching the tropopause. At this altitude the temperature reach -56.5° C and remains constant for about 10 to 20 miles before it begins to rise. The height of the tropopause has also an effect on radio waves propagation.

The difference in the height of the tropical and polar tropopause causes a "break" in the tropopause. The jet stream forms around this break with maximum velocity at the break area. The general direction of the jet stream is from west to east between latitude 30° N to 60° N. The jet maximum is not constant; it is rather broken into segments in a "bull's horn" shape. The length of the jet stream segment may extend as much
as 1,000 to 3,000 miles with maximum velocity prevailing along 100-400 miles. The thickness of the jet segment at its maximum is between 3,000 to 7,000 feet. A second or even third jet stream at the same time is common.

The adjacent chart shows a cross section of the upper troposphere and the lower stratosphere. 
The core of the jet streams is situated at the break between the high tropical and lower 
polar trpopause. Note the pattern of the isotherms (lines of equal temperatures) and the 
isotachs (lines of equal velocities) around the core of the jet.

The velocity of the jet stream is greater in the winter compared to its velocity in the summer. A mid latitudes jet stream is normally stronger than one near the tropics. To be classified as a jet stream, the wind velocity must exceed 50 knots. The jet maximum velocity is about 200 knots. Jet stream segments move with pressure ridges and troughs in the upper atmosphere. In general, they travel faster than the weather pressure systems, and maximum wind speed varies as the segments progress through the systems.

Information regarding the location of the jet stream is reported in the SIGN WX charts. The location is detected by Radio Sonda signals. PIREPs are another helpful source of information regarding the location of the jet stream. During a flight, entering and exiting the jet can be detected by rapid change in the ambient temperature. An increase in temperature is detected when crossing the jet from south to north while a decrease in temperature is indicated when crossing from north to south, providing the altitude is maintained. Another indication of entering the jet can be obtained from a rapid increase of ground speed as it is indicated in the navigational equipment. Entering the jet stream is accompanied by Clear Air Turbulence. The turbulence is greater at the northern part of the jet compared to southern part. Inside the core itself the air is calm.

To maximize the benefit of the jet stream, flight from west to east should be planned within the jet stream while avoiding it when flying from east to west.

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Last update May 17, 2005
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