Atmosphere is a very unstable mass of air. Think of turbulence as of disturbed or rough air. Sometimes it provokes irregular motion of an aircraft in flight.
There are different reasons for causing turbulence and also different parts of atmosphere where it occurs.
Types of turbulence
Turbulence is caused by convective currents (convective turbulence), obstructions in the wind flow (mechanical turbulence), jet streams and wind shear (wind shear is covered by a separate article).
Imagine today is a warm summer afternoon with a very light wind. Air mass heats from the surface and absolutely becomes an unstable layer rising upward. Because of uneven heating, the strength of heated air mass can vary within short distances. For every rising mass, there is a compensating downward mass of air. Therefore, vertical motions appear as a result of rising and sinking of air. It is called convective turbulence.
So, this is a good sign for pilots: expect to encounter turbulence beneath or in the clouds, while above the clouds air generally is smooth.
Mechanical turbulence is caused by physical obstructions to the normal flow of air, such as trees, coasts, buildings, mountains and so on. The intensity of mechanical turbulence depends on wind speed and roughness of the obstructions. The higher the speed and/or the rougher the surface, the greater the turbulence.
Clean air turbulence
Clear Air Turbulence (CAT) is a higher altitude (20,000 to 50,000 feet) turbulence mostly associated with jet streams. The turbulence will be more severe with curved jets and in mountainous areas, particularly when mountain waves are present. CAT frequency and intensity are maximized during winter when jet streams are strongest.
Turbulence remains an important factor, especially above FL150. Pilots are usually expected to report turbulence using standard criteria and reporting form.
|Light||Slight changes in altitude and/or attitude.
Slight, rapid and rhythmic bumpiness.
IAS changes by 5-10 kt.
|Moderate||Like light but of greater intensity.
Changes of altitude and/or attitude.
IAS fluctuates 15-25 kt.
|Severe||Large, abrupt changes in altitude and/or attitude.
Aircraft may be momentarily out of
A standard turbulence report should state:
- Aircraft location
- Time in UTC
- Turbulence intensity
- Whether the turbulence occurred in or near clouds
- Aircraft altitude or flight level
- Type of aircraft
- Duration of turbulence if applicable.
Example: Five zero miles south of Ostrava, 1025Z, moderate chop at Flight Level three three zero, Cessna Citation.
Weather briefing is an important part of every flight preparation. Pilots should become aware of any significant weather phenomena regarding the flight. Two most useful resources for turbulence zones are prognostic chart and SIGMETs.
Here is an example of a prognostic chart issued by Washington WAFC (World Area Forecast Center).
Turbulence is marked using the following symbols:
|Moderate to severe turbulence in Clean Air above 10000 ft|
The vertical extent of turbulence layers is specified by top and base heights in hundreds of
feet and those values are pressure altitude.
On the chart above two different turbulence zones are depicted with yellow dashed lines:
- Moderate turbulence above Newfoundland island from FL250 or below to FL390 (because "XXX" means extending the lowest layer of the chart)
- Moderate turbulence East of Newfoundland island from FL250 or below to FL360.
SIGMET (Significant Meteorological Information) is a weather advisory that contains different meteorological information concerning the safety of all aircraft.
Here is a common map view for SIGMET:
Let's pick up one of the zones and check its information:
Impact on aircraft
Turbulence affects an aircraft in different ways.
- During the cruse phase of flight an aircraft tends to operate towards the top end of permitted speeds range. In turbulence conditions speed should be reduced to provide a safety margin from the dangers of overstressing the airframe and to maintain aircraft controllability and stability.
- Holding procedure regulations define speed limits at or below certain numbers. Those speeds are often close to the bottom end of aircraft permitted speeds range. However, regulations define holding speed restrictions under turbulence conditions which are higher, to prevent speed drop below the permitted range and stall occurance.
- VID 531824 - Creation
DATE OF SUBMISSION
- 00:37, 14 May 2021
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