Chapter 19, Turbulence 19-6
Figure 19-6. Wind Shear Turbulence Associated with a Temperature Inversion
19.2.3.2 Clear-Air Turbulence (CAT)
CAT is defined as sudden severe turbulence occurring in cloudless regions that causes violent buffeting of
aircraft. CAT is a higher altitude turbulence (normally above 15,000 ft) particularly between the core of a
jet stream and the surrounding air. This includes turbulence in cirrus clouds, within and in the vicinity of
standing lenticular clouds and, in some cases, in clear air in the vicinity of thunderstorms. Generally,
though, CAT definitions exclude turbulence caused by thunderstorms, low-altitude temperature inversions,
thermals, strong surface winds, or local terrain features.
CAT is a recognized problem that affects all aircraft operations. CAT is especially troublesome because it
is often encountered unexpectedly and frequently without visual clues to warn pilots of the hazard.
19.2.3.2.1 CAT Discussion
One of the principal areas where CAT is found is in the vicinity of the jet streams. There are three jet
streams: the polar front jet stream, the subtropical jet stream, and the polar night jet stream. This handbook
does not address the polar night jet stream, as it is a phenomenon in the stratosphere. (See Chapter 9, Global
Circulations and Jet Streams, and Figure 9-4 and Figure 9-5 for more information and the polar front jet
stream and the subtropical jet stream locations.)
CAT associated with a jet stream is most commonly found in the vicinity of the tropopause. CAT is most
frequently found on the poleward side of the jet stream (over the United States, this is to the left side when
facing downwind). CAT is also common in the vicinity of a jet stream maxima, a region of stronger winds
within the jet stream that translates along the jet stream core.
There are several patterns of upper-level winds that are associated with CAT. One of these is a deep, upper
trough. CAT is found most frequently at, and just upwind of, the base of the trough, especially just
downwind of an area of strong temperature advection. Another area of the trough in which to suspect CAT
Chapter 19, Turbulence 19-7
is along the centerline of a trough area, where there is a strong horizontal wind shear between the jet core
and winds to the poleward side of the jet core. CAT is also found in the west side of a trough in the vicinity
of a wind maxima as the maxima passes along the trough.
One noteworthy generator of CAT is the confluence of two jet streams. On occasion, the polar front jet
stream will dip south and pass under the subtropical jet stream. The wind shear effect between the two jet
streams in the region of confluence and immediately downstream is often highly turbulent.
CAT intensity can vary significantly along any flightpath. Common dimensions of a turbulent area
associated with a jet stream are on the order of 100 –300 mi long, elongated in the direction of the wind,
50–100 mi wide, and 5,000 ft deep. These areas may persist from 30 minutes to one day.
The threshold wind speed in the jet stream for CAT is generally considered to be 110 kt. The probability of
encountering CAT increases proportionally with the rapidity of the decrease in wind speed away from the
jet core. This is known as wind shear. It is not the wind speed itself that ca uses CAT; it is the wind shear
that is turbulent to an aircraft, as the atmosphere bounces in waves or actually overturns. Moderate CAT is
considered likely when the vertical wind shear is 5 kt per 1,000 ft or greater, and /or the horizontal wind
shear is 40 kt per 150 mi or greater.
Jet streams stronger than 110 kt (at the core) have potential for generating significant turbulence near the
sloping tropopause above the core, in the jet stream front below the core, and on the low -pressure side of
the core.
Wind shear and its accompanying CAT in jet streams are more intense above, and to the lee of, mountain
wave ranges. CAT should be anticipated whenever the flightpath traverses a strong jet stream in the vicinity
of mountainous terrain.
Both vertical and horizontal wind shear are, of course, greatly intensified in mountain wave conditions.
Therefore, when the flightpath traverses a mountain -wave-type of flow, it is desirable to fly at turbulence
penetration speed and avoid flight over areas where the terrain drops abruptly, even though there may be
no lenticular clouds to identify the condition.
CAT is also related to vertical shear. If vertical shear is greater than 5 kt per 1,000 ft, turbulence is likely.
Curving jet streams are more apt to have turbulent edges than straight ones, especially jet streams that curve
around a deep pressure trough.
Wind shift areas associated with pressure troughs and ridges are frequently turbulent. The magnitude of the
wind shear is the important factor.
19.2.4 Wind Shear
Wind shear is the sudden, drastic change in wind speed and/or direction over a small area, from one level
or point to another, usually in the vertical (see Figure 19-7). Wind shear occurs in all directions, but for
convenience, it is measured along vertical and horizontal axes, thus becoming horizontal and vertical wind
shear.
Chapter 19, Turbulence 19-8
Figure 19-7. Wind Shear Example
It is important to remember that wind shear can affect any flight at any altitude (e.g., at upper levels near
jet steams or near the ground due to convection). Wind shear can subject an aircraft to violent updrafts and
downdrafts, as well as abrupt changes to the horizontal movement of the aircraft. While wind shear may be
reported, it often remains undetected and is a silent aviation weather hazard. Always be alert to the
possibility of wind shear, especially when flying in and around thunderstorms and frontal systems.
Some references or publications may use the term “severe wind shear.” They may define the term as a wind
shear that exceeds the performance capability of the aircraft or a wind shear producing airspeed changes
greater than 15 kt or vertical speed changes greater than 500 feet per minute (fpm).
19.2.4.1 Non-Convective Low-Level Wind Shear (LLWS)
Wind variations at low altitude have long been recognized as a serious hazard to airplanes during takeoff
and approach. These wind variations can result from a large variety of meteorological conditions such as
topographical conditions, temperature inversions, sea breezes, frontal systems, and strong surface winds.
While wind shear can occur at any altitude; non-convective LLWS is especially hazardous due to the
proximity of an aircraft to the ground. Non -convective LLWS is defined as a wind shear of 10 kt or more
per 100 ft in a layer more than 200 ft thick that occurs within 2,000 ft of the surface. So, what does this
mean? It means that within the lowest 2 ,000 ft, the wind speed and/or direction is changing rapidly in a
200-ft layer (see Figure 19-8). Non-convective LLWS is commonly associated with passing frontal systems,
temperature inversions, and strong upper-level winds (greater than 25 kt).
Chapter 19, Turbulence 19-9
Figure 19-8. LLWS Example
19.2.4.2 Convective Wind Shear
See Section 22.7.3 for information on convective wind shear.
19.2.5 The Effects of Orographic Winds and Turbulence on Aviation Operations
19.2.5.1 High-Altitude Operations
Turbine-powered aircraft operating at cruise altitudes above FL180 in the vicinity of mountainous terrain
may encounter moderate or greater turbulence associated with orographic winds. This type of turbulence
may be characterized by relatively rapid onset and can lead to structural damage or airframe failure.
Structural damage is not the only danger associated with high-altitude turbulence encounters. It is possible
to operate some turbine-powered aircraft at such weights and altitudes so that their cruise airspeed is only
a few knots below the onset of Mach buffet and a like speed above stall buffet. In this situation (the so-called
“coffin corner”), turbulent airspeed excursions of moderate or greater intensity (15 kt or more) can quickly
lead to high-speed upset, Mach tuck, and loss of control.
19.2.5.2 Takeoff and Landing
Takeoff and landing concerns include experiencing turbulent air with inadequate stall margins, loss of
directional control on or near the runway, rolling moments that surpass aircraft roll authority, and downdraft
velocities that exceed the climb capabilit y of the aircraft, particularly for airplanes with high wing and
power-loading. Localized gusts in excess of 50 kt, with downdrafts greater than 1 ,500 fpm, are also
possible.
Vortices spawned by the interaction of strong winds and high terrain can lead to severe turbulence and
aircraft rolling moments that may exceed the pilot’s ability to maintain aircraft control.
Chapter 19, Turbulence 19-10
19.2.5.3 Low-Level Mountain Flying
Aircraft that engage in low-level flight operations over mountainous terrain in the presence of strong winds
(20 kt or greater at ridge level) can expect to encounter moderate or greater turbulence, strong updrafts and
downdrafts, and very strong rotor and shear zones. This is particularly true for General Aviation (GA)
aircraft.
Mountain flying literature often cite 20 kt as the criterion for classifying a wind as “strong.” This criterion
refers to the large-scale (or prevailing wind in the area as opposed to a local wind gust) wind speed at the
crest of the ridge or level of the mountain peaks, upwind of the aircraft ’s position. Such an ambient wind
flow perpendicular to a ridge will lead to substantially stronger surface winds, with the likelihood of
turbulence. Similar wind enhancements can be anticipated near the slopes of an i solated peak. In contrast,
downdrafts over forested areas may be strong enough to force aircraft down into the trees, even when the
aircraft is flown at the best rate -of-climb speed. This effect on the aircraft can be exacerbated by loss of
aircraft performance because of the high-density altitude.