JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Jet Stream Turbulence
Figure 4-6 — Jet Stream Diagram
As described in Chapter 2, one of the major sources of wind shear turbulence is the jet stream,
which can sometime reach speeds of over 250 knots (Figure 4-6). 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. Also, a
climb or descent to a different flight level should also 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.
Temperature Inversions
Recall from Chapter 1 the lapse rate where temperature increases with altitude, the 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 on clear, cool nights
when the winds are light and the air is stable. If the winds just above the inversion grow
relatively strong, wind shear turbulence can result.
Figure 4-7 shows a wind shear zone and the turbulence that developed between the calm air and
stronger winds above the inversion. When taking off or landing in near-calm surface winds under
clear skies within a few hours of sunrise, watch for a temperature inversion near the surface. If
the wind at 2000 to 4000 feet AGL is 25 knots or more, expect a shear zone at the inversion. To
prepare yourself, allow a margin of airspeed above normal climb or approach speed if turbulence
or a sudden change in wind speed occurs in order to counteract the effects of a diminished
headwind or increased tailwind at and below the inversion.
4-8 Version 3.2/Dec 08