Ch 11 – Wind Shear
Ch 11 – Wind Shear
• Section A – Wind Shear Defined
• Section B – Causes of Wind Shear
–
–
–
–
–
Microbursts
Fronts and Shallow Lows
Airmass Wind Shear
Elevated Stable Layers
Jet Streams
Ch 11 – Wind Shear
• Strong wind shear is a hazard to aviation because it can
•
•
cause turbulence and large airspeed fluctuations and,
therefore, serious control problems.
It is a threat especially to aircraft operations near the
ground because of the limited altitude for maneuvering,
particularly during the takeoff and landing phases of
flight.
In this chapter, we examine wind shear and its causes
Ch 11 – Wind Shear
• When you complete the chapter, you will know what
•
wind shear is and what its critical values are.
You will also know how, why, and where it develops in
the vicinity of thunderstorms, inversions, developing
extratropical cyclones, fronts, and jet streams.
Ch 11 – Wind Shear
• Section A: Wind Shear Defined
– Wind shear – a gradient in wind velocity.
• It is interpreted in the same sense as a pressure
gradient or temperature gradient
• that is, it is a change of wind velocity over a given
distance.
Ch 11 – Wind Shear
– Horizontal wind shear – it is convenient to
visualize wind shear as being composed of two parts:
a horizontal wind shear (a change in wind over a
horizontal distance) being one part.
– Vertical wind shear - a change in wind over a
vertical distance
Ch 11 – Wind Shear
• ***Wind shear is best described as a change in
•
wind direction and / or speed within a very short
distance
***During departure under conditions of
suspected low-level wind shear, a sudden
decrease in headwind will cause a loss in airspeed
equal to the decrease in wind velocity
Ch 11 – Wind Shear
• Section B: Causes of Wind Shear
– ***An important characteristic of wind shear is
that it may be associated with a thunderstorm,
a low-level temperature inversion, a jet
stream, or a frontal zone
Ch 11 – Wind Shear
• Downburst – Professor T. Fujita, an atmospheric
scientist from the University of Chicago, coined the term
downburst for a concentrated, severe downdraft that
induces an outward burst of damaging winds at the
ground
Ch 11 – Wind Shear
• Micro bursts
– Microburst - Professor T. Fujita, an atmospheric
scientist from the University of Chicago, introduced
the term microburst for a downburst with horizontal
dimensions of 2.2 n.m. (4km) or less.
Ch 11 – Wind Shear
– Vortex ring – The microburst is characterized by a
strong core of cool, dense air descending from the
base of a convective cloud.
• As it reaches the ground, it spreads out laterally as
a vortex ring which rolls upward as a vortex ring
which rolls upward along its outer boundary.
Ch 11 – Wind Shear
• ***An aircraft that encounters a headwind of 45
knots with a microburst may expect a total shear
across the microburst of 90 knots
• ***The duration of an individual microburst is
seldom longer than 15 minutes from the time the
burst strikes the ground until dissipation
Ch 11 – Wind Shear
• ***When a shear from a headwind to a tailwind is
encountered while making an approach on a
prescribed glide slope, the pilot should expect
airspeed and pitch attitude decrease with a
tendency to go below glide slope
• ***If there is thunderstorm activity in the vicinity
of an airport at which you plan to land, you
should expect wind shear and turbulence on
approach
Ch 11 – Wind Shear
• Low-level wind shear systems (LLWAS) – These
•
alert systems have been installed at many large airports
around the U.S. where thunderstorms are frequent.
Terminal Doppler Weather Radar (TDWR) – These
systems are being installed across the U.S. at many
vulnerable airports to provide more comprehensive wind
shear monitoring.
Ch 11 – Wind Shear
• Fronts and Shallow Lows
– Frontal wind shear – a front is a zone between two
different air masses and frontal wind shear is
concentrated in that zone
Ch 11 – Wind Shear
• ***With a warm front, the most critical period for
LLWS is before the front passes
Ch 11 – Wind Shear
• Air mass Wind Shear
– Air mass wind shear – occurs at night under fair
weather conditions in the absence of strong fronts
and/or strong surface pressure gradients.
– It develops when the ground becomes cooler than the
overlying air mass as a result of radiational cooling.
If the cooling is strong enough, a ground-based or
surface inversion will result.
– In this case, the temperature increases with altitude
from the surface to an altitude of a few hundred feet.
Ch 11 – Wind Shear
– Nocturnal inversion – low-level soundings taken
throughout the day and night during fair weather
conditions have revealed stable layers developing at
night due to radiational cooling of the ground.
• By sunrise the stability has increased to a
maximum as indicated by the nocturnal inversion.
Ch 11 – Wind Shear
• ***A pilot can expect a wind shear zone in a
surface-based temperature inversion whenever
the wind speed at 2,000 to 4,000 feet above the
surface is at least 25 knots.
Ch 11 – Wind Shear
• Elevated Stable Layers
– Elevated stable layers – In addition to fronts and
surface-based nocturnal inversions, wind shears may
be found in the free atmosphere, in elevated stable
layers.
– These layers are frequently found over shallow,
relatively cool air masses. Convection from the
ground concentrates wind shear at the base of the
stable layer.
Ch 11 – Wind Shear
• ***When a climb or descent through a stable
layer is being performed, the pilot should be alert
for a sudden change in airspeed
Ch 11 – Wind Shear
• Jet Streams – Certain patterns of upper level, short
wave troughs and ridges produce significant wind shear.
– The strongest shears are usually associated with
sharply curved contours on constant pressure
surfaces and / or strong winds.
– Stable layers near jet streams and within a few
thousand feet of the tropopause have the highest
probabilities of strong shears.
– Occasionally, the shear is strong enough to cause
large airspeed fluctuations, especially during climb or
descent.
Summary
• Wind shear is one of the most serious low-level flight
•
•
hazards in the atmosphere.
Significant wind shear not only occurs with microbursts,
but also with fronts and nocturnal inversions.
Wind shear is also found in elevated stable layers in the
free atmosphere, especially capping cold air masses and
in the vicinity of jet streams and the tropopause.
Summary
• Failure to be aware of all causes and weather conditions
•
•
that produce wind shear can lead to catastrophic results.
An encounter with LLWS, in particular, is unforgiving
because of the proximity of your aircraft to the ground.
You now have some useful conceptual models and basic
rules of thumb to help you recognize and, where
possible, avoid potential wind shear conditions.
Summary
• In the next chapter, you will become aware of a number
of situations where wind shear and turbulence are
present at the same time in the same location (Lester,
2006).