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CHAPTER FIVE
AVIATION WEATHER
change in windspeed or direction over a short distance in the atmosphere. The greater the
change in windspeed and/or direction in a given area, the more severe the turbulence will be.
These turbulent wind shear flight conditions are frequently encountered in the vicinity of the jet
stream, where large shears in both the horizontal and vertical planes are found, as well as in
association with land and sea breezes, fronts, inversions, and thunderstorms. Strong wind shear
can abruptly distort the smooth flow of wind, creating rapid changes in aircraft performance.
Jet stream turbulence is described in Chapter Two as one of the major sources of wind shear
turbulence, which can sometime reach speeds of over 250 knots (Figure 5-7). The highest wind
speeds and probable associated turbulence is found about 5000 feet below the tropical
tropopause and closer to the tropopause in the polar regions. The rapid change of wind speed
within a short distance of the jet core is particularly significant. The vertical shear is generally
close to the same intensity both above and below the core, and it may be many times stronger
than the horizontal shear. The horizontal shear on the cold air side of the core is stronger than on
the warm air side. Thus, if it is desired to exit jet stream turbulence, a turn to the south should
result in smoother air. Additionally, a climb or descent to a different flight level should help, as
jet stream turbulence often occurs in patches averaging 2000 feet deep, 20 miles wide, and
50 miles long. If changing altitude, watch the outside air temperature for a minute or two to
determine the best way to exit the CAT quickly. If the temperature is rising, climb; if the
temperature is falling, descend. This maneuver will prevent following the sloping tropopause or
frontal surface and thereby staying in the turbulent area. If the temperature remains the same,
either climb or descend.
Figure 5-7 Jet Stream Diagram
Temperature Inversions
Recall from Chapter One the lapse rate where temperature increases with altitude, there is a
temperature inversion. Even though this produces a stable atmosphere, inversions can cause
turbulence at the boundary between the inversion layer and the surrounding atmosphere. The
resulting turbulence can often cause a loss of lift and airspeed near the ground, such as when a
headwind becomes a tailwind, creating a decreasing-performance wind shear. It is important to
know how to recognize and anticipate an inversion in flight so you can prepare and take
precautions to minimize the effects. If you are caught unaware, the loss of lift can be
catastrophic because of your proximity to the ground. Inversions often develop near the ground
5-10
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds


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