Aircraft experience turbulence in varying intensities due to the 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 is caused by convective currents (called convective turbulence), obstructions in the wind flow (called mechanical turbulence), and wind shear
Turbulence varies from annoying bumpiness to severe jolts, which cause structural damage to an aircraft and/or injury to its passengers
The effect of turbulence varies based on the size of the aircraft
Turbulence Intensities:
Turbulence is classified by its intensity:
Light Turbulence:
Causes slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw). Report as Light Turbulence. Or causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as Light Chop
Moderate Turbulence:
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 Turbulence:
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 Turbulence:
The aircraft is violently tossed about and is practically impossible to control. It may cause structural damage
Convective Turbulence:
Convective Turbulence
Thermals
Convective turbulence is turbulent vertical motions that result from convective currents and the subsequent rising and sinking of air
Low altitude, with updrafts 200-2,000 FPM
For every rising current, there is a compensating downward current
The downward currents frequently occur over broader areas 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
Typically short distance, with possibly severe turbulence below the clouds
Varies widely with terrain
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 [Figure 7]
The cloud top usually marks the approximate upper limit of the convective current
A pilot can expect to encounter turbulence beneath or in the clouds, while above the clouds, air generally is smooth
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
When the air is too dry for cumuliform clouds to form, convective currents can still be active
This is called dry convection, or thermals [Figure 8]
A pilot has little or no indication of their presence until encountering the turbulence
Convective Turbulence
Thermals
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
Can form in stable or unstable air
Presents landing hazards
The intensity of mechanical turbulence depends on wind speed and roughness of the obstructions
The wind carries the turbulent eddies downstream. How far depends on wind speed and stability of the air
Unstable air allows larger eddies to form than those that form in stable air; but the instability breaks up the eddies quickly, while in stable air they dissipate slowly
Mountain Wave:
Mountain Wave
Mountain Wave Clouds
A mountain wave is an atmospheric wave disturbance formed when stable air flow passes over a mountain or mountain ridge, producing hazardous wind conditions [Figure 9]
The waves may extend 600 miles (1,000 kilometers) or more downwind from the mountain range
Mountain waves frequently produce severe to extreme turbulence
Location and intensity varies with wave characteristics
Mountain waves often produce violent downdrafts on the immediate leeward side of the mountain barrier, sometimes exceeding maximum aircraft climb rates
Up/downdrafts increase as wind speed increases
A mountain wave cloud is a cloud that forms in the rising branches of mountain waves and occupies the crests of the waves
The most distinctive are the sharp-edged, lens-, or almond-shaped lenticular clouds
When sufficient moisture is present in the upstream flow, mountain waves produce interesting cloud formations including: cap clouds, cirrocumulus standing lenticular (CCSL), Altocumulus Standing Lenticular (ACSL), and rotor clouds [Figure 10]
These clouds provide visual proof that mountain waves exist
However, these clouds may be absent if the air is too dry
Stable air and wind speeds between 25-40 knots foster turbulence
Mountain Wave
Mountain Wave Clouds
Clear Air Turbulence:
Typically above 15,000 and in vicinity to the jet steam
Drastic changes in wind speed and velocity
Frontal Turbulence:
Occurs in a narrow zone, just ahead of a fast-moving cold front
Updrafts of 1,000 FPM, with significant turbulence
Turbulence Reports and Forecasts:
Turbulence reports can be found through pilot reports
Private Pilot - Weather Information Airman Certification Standards:
To determine that the applicant exhibits satisfactory knowledge, risk management, and skills associated with weather information for a flight under VFR
References: 14 CFR part 91; FAA-H-8083-25; AC 00-6, AC 00-45, AC 00-54; AIM
Weather Information Knowledge:
The applicant must demonstrate an understanding of:
PA.I.C.K1:
Sources of weather data (e.g., National Weather Service, Flight Service) for flight planning purposes
Meteorology applicable to the departure, en route, alternate, and destination under VFR in Visual Meteorological Conditions (VMC) to include expected climate and hazardous conditions such as:
Use available aviation weather resources to obtain an adequate weather briefing.
PA.I.C.S2:
Analyze the implications of at least three of the conditions listed in K3a through K3l above, using actual weather or weather conditions in a scenario provided by the evaluator
PA.IV.C.S3:
Correlate weather information to make a competent go/no-go decision