Chapter 19, Turbulence 19-1
19 Turbulence
19.1 Introduction
Aircraft turbulence is irregular motion of an aircraft in flight, especially when characterized by rapid
up-and-down motion caused by a rapid variation of atmospheric wind velocities. Turbulence varies from
annoying bumpiness to severe jolts that may cause structural damage to an aircraft and/or injury to its
passengers. It is important to note that the effect of turbulence varies based on the size of the aircraft.
Turbulence intensities and their associated aircraft reactions are described below:
• Light − Causes slight, erratic changes in altitude and /or attitude (pitch, roll, or yaw). Report as
Light Turbulence. Or it causes slight, rapid, and somewhat rhythmic bumpiness without appreciable
changes in altitude or attitude. Report as Light Chop.
• Moderate − Similar to Light but of greater intensity. Changes in altitude and /or attitude occur, but
the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.
Report as Moderate Turbulence. Or turbulence that is similar to Light Chop but of greater intensity.
It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as
Moderate Chop.
• Severe − Causes large, abrupt changes in altitude and/or attitude. It usually causes large variations
in indicated airspeed. Aircraft may be momentarily out of control.
• Extreme − The aircraft is violently tossed about and is practically impossible to control. It may
cause structural damage.
Chapter 19, Turbulence 19-2
19.2 Causes of Turbulence
Turbulence is caused by convective currents (called convective turbulence), obstructions in the wind flow
(called mechanical turbulence), and wind shear.
19.2.1 Convective Turbulence
Convective turbulence is turbulent vertical motions that result from convective currents and the subsequent
rising and sinking of air. For every rising current, there is a compensating downward current. The downward
currents frequently occur over broader a reas than do the upward currents; therefore, they have a slower
vertical speed than do the rising currents.
Convective currents are most active on warm summer afternoons when winds are light. Heated air at the
surface creates a shallow, absolutely unstable layer within which bubbles of warm air rise upward.
Convection increases in strength and to greater heights as surface heating increases. Barren surfaces such
as sandy or rocky wastelands and plowed fields become hotter than open water or ground covered by
vegetation. Thus, air at and near the surface heats unevenly. Because of uneven heating, the strength of
convective currents can vary considerably within short distances.
As air moves upward, it cools by expansion. A convective current continues upward until it reaches a level
where its temperature cools to the same as that of the surrounding air. If it cools to saturation, a cumuliform
cloud forms.
Billowy cumuliform clouds, usually seen over land during sunny afternoons, are signposts in the sky
indicating convective turbulence. The cloud top usually marks the approximate upper limit of the
convective current. A pilot can expect to encounter turbule nce beneath or in the clouds, while above the
clouds, air generally is smooth (see Figure 19-1). When convection extends to great heights, it develops
larger towering cumulus clouds and cumulonimbus with anvil -like tops. The cumulonimbus gives visual
warning of violent convective turbulence.
Figure 19-1. Convective Turbulence
When the air is too dry for cumuliform clouds to form, convective currents can still be active. This is called
dry convection, or thermals (see Figure 19-2). A pilot has little or no indication of their presence until
encountering the turbulence.
Chapter 19, Turbulence 19-3
Figure 19-2. Thermals
19.2.1.1 Thunderstorms
Turbulence is present in all thunderstorms , and s evere or extreme turbulence is common. A severe
thunderstorm can destroy an aircraft. Gust loads can be severe enough to stall an aircraft at maneuvering
speed or to cause structural damage at cruising speed. The strongest turbulence within the cloud occurs
between updrafts and downdrafts.
Outside the cloud, shear turbulence has been encountered several thousand feet above and up to 20 mi
laterally from a severe storm. Additionally, clear-air turbulence ( CAT) may be encountered 20 or more
miles from the anvil cloud edge. These kinds of turbulence are sometimes referred to as Convectively
Induced Turbulence (CIT).
It is almost impossible to hold a constant altitude in a thunderstorm and maneuvering to do so greatly
increases stress on the aircraft. Stresses are least if the aircraft is held in a constant attitude.
The low-level wind-shear zone between the gust front and surrounding air is very turbulent airspace.
Oftentimes, the surface position of the gust front is denoted by a line of dust or debris along the ground or
a line of spray along bodies of water. Gust fronts often move far ahead (up to 15 mi) of associated
precipitation. The gust front causes a rapid and sometimes drastic change in surface wind ahead of an
approaching storm. Often, a “roll cloud” or “shelf cloud” on the leading edge of the storm (see Figure 19-3)
marks the top of the extreme turbulence zone , which forms as warm, moist air is lifted by the gust front.
Shelf clouds are most common with multicell line thunderstorms.
Chapter 19, Turbulence 19-4
Figure 19-3. Thunderstorm with Shelf Cloud
19.2.2 Mechanical Turbulence
Mechanical turbulence is turbulence caused by obstructions to the wind flow, such as trees, buildings,
mountains, and so on. Obstructions to the wind flow disrupt smooth wind flow into a complex snarl of
eddies (see Figure 19-4). An aircraft flying through these eddies experiences mechanical turbulence.
Figure 19-4. Mechanical Turbulence
The intensity of mechanical turbulence depends on wind speed, surface roughness, size of the obstructions,
and stability of the air. The higher the speed and/or the rougher the surface, the greater the turbulence.
Chapter 19, Turbulence 19-5
The wind carries the turbulent eddies downstream, with the distance dependent on wind speed and stability
of the air. Unstable air allows larger eddies to form rather than those that form in stable air; but the instability
breaks up the eddies quickly, while in stable air they dissipate slowly.
19.2.2.1 Mountain Waves
Mountain waves are a form of mechanical turbulence that develop above and downwind of mountains. See
Chapter 16, Mountain Weather, for information on mountain waves.
19.2.3 Wind Shear Turbulence
Wind shear is defined in Section 19.2.4. Wind shear generates turbulen ce between two wind currents of
different directions and/or speeds (see Figure 19-5). Wind shear may be associated with either a wind shift
or a wind speed gradient at any level in the atmosphere.
Figure 19-5. Wind Shear Turbulence
19.2.3.1 Temperature Inversion
A temperature inversion is a layer of the atmosphere in which temperature increases with altitude.
Inversions commonly occur within the lowest few thousand feet above ground due to nighttime radiational
cooling, along frontal zones, and when cold air is trapped in a valley. Strong wind shears often occur across
temperature inversion layers, which can generate turbulence (see Figure 19-6).