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THE TROPOPAUSE.— The mid-latitude or polar-front jet streams are associated with breaks in the tropopause. North of a jet the tropopause is low, and south of the jet it’s high. These jets can exist without a break in the tropopause, and when this happens, the tropopause shows a very steep slope in the region of maximum wind. There will also be times when two tropopauses overlap, and at these times, the jet is often found between them. The mid-latitude or polar-front jets are associated with the break between the subarctic and mid-latitude tropopauses (30° to 50° lat.).

CLOUDS.— Cirrus is the cloud most often associated with jet streams. It usually forms on the warm-air side of the axis where lateral shear is relatively weak and extends to within several thousand feet of the tropopause. Occasionally, a sharp discontinuity in the cirrus takes place near the jet core; the skies become cloudless immediately north of the core. If cirrus extends north of the core, there is usually a narrow break in the cirrus at the core itself. Most of the time there are no clouds at or above the level of the jet core. On the average, jet stream cirrus is 500 feet thick, but can range from a hundred feet to several thousand feet.

Four cloud patterns are associated with the mid-latitude and polar-front jets, and at least three of the four must be present for a jet to exist. They often completely cover the sky and have bands at right angles to the air flow. The cloud patterns are as follows:

1. Lines of cirrus in bands (H4, 5, and 6)

2. Patches of cirrocumulus (H9) or alto-cumulus castellanus (M8)

3. Lenticular clouds in waves (M4 or M7)

4. Bands of altocumulus (M3 or M5)

Because bad weather often accompanies polar-front jets, lower cloud shields will often obscure the higher clouds. Your ability to recognize a jet stream’s existence by cloud patterns will be dependent on your seeing the clouds. With pilots it’s a different story. Flying above the bad weather, they’ll get a first hand look at the higher clouds. It’s important that they be able to recognize the clouds associated with jet streams, in order to warn passengers and crew of possible impending turbulence. The turbulence that accompanies jet streams is known as clear-air turbulence. Don’t let the name fool you; this type of turbulence occurs both inside and outside clouds.

CLEAR-AIR TURBULENCE.— Pilots expect turbulence in clouds, but they are not always expecting it in cloud-free skies. Pre-flight brief-ings, which include jet stream locations and altitudes in relation to flight paths and altitudes, give pilots advanced warning as to when and where they may experience clear-air turbulence. The wind shear within the jet stream causes this rough cobblestone-type of bumpiness. Clear-air turbulence buffets the crew, passengers, and equipment, and because of the stresses it places on the airframe, it decreases an aircraft’s useful life.

Horizontal Wind Shear.— Along the axis of a jet stream, wind speeds decrease very rapidly on the cold-air side of the core and more slowly on the warm-air side. Therefore, horizontal wind shear is much greater on the cold-air side of jet streams. For example, the wind speeds may drop off 100 knots within 100 miles on the cold-air side of the core, while only dropping off 25 knots over the same distance on the warm side. Figure 8-3-10 illustrates the packing of isotachs on the cold-air side of the jet axis.

Vertical Wind Shear.— The wind speeds above and below the core can also decrease very rapidly over short distances. A decrease of 30 to 40 knots in 1,000 feet is not uncommon. Normally, the shear above the core is much greater than that below the core. However, when the jet core is above a frontal zone, the shear beneath the core exceeds the shear above the core. Figure 8-3-11

Figure 8-3-10.—Vertical cross section of a model of the jet stream—winds and fronts.

Figure 8-3-11.—Probable areas of clear-air turbulence.

 illustrates the most probable areas for aircraft to encounter clear-air turbulence. The two most probable locations are just below and to the left of the core between 22,000 and 28,000 feet and directly above the core in the vicinity of the tropopause between 35,000 and 50,000 feet. Even though there’s the possibility of en-countering clear air turbulence, many flights take advantage of these jet stream currents. Flying with the jet saves time and money. The savings in fuel dollars can be considerable. On the other hand, going against the jet requires more fuel, possibly more stops enroute, and additional flight planning.

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