Turbulence is caused by the relative movement of disturbed air through which an aircraf is flying. Its origin may be thermal or mechanical and it may occur either within or clear of cloud.
The absolute severity of turbulence depends directly upon the rate at which the speed orthe direction of airflow is changing, although perception of the severity of turbulence which hasbeen encountered will be affected by the mass of the aircraft involved.
Significant mechanical turbulence will often result from the passage of strong winds over irregular terrain or obstacles. Less severe low level turbulence can also be the result of convection caused by surface heating.
Turbulence may also happen from air movements associated with convective activity, especially in or near a thunderstorm or due to the presence of strong temperature gradients near to a Jet Stream. Jet Stream Turbulence, occurs clear of cloud and in the form of Clear Air Turbulence (CAT).
Very localized, but sometimes severe, Wake Turbulence may be encountered when following or crossing behind another aircraft. This turbulence is due to wing tip trailing vortices generated by the preceding aircraft; however, this phenomena is distinctively transient.
SEVERITY OF TURBULENCE
For the purpose of reporting and forecasting of air turbulence, it is graded according toitsperceived or potential effect on a 'typical' aircraft, as Light, Moderate, Severe and Extreme.
- Light turbulence is the least severe, with slight, erratic changes in attitude and/or altitude.
- Moderate turbulence is similar to light turbulence, but of greater intensity - variations in speed as well as altitude and attitude may occur but the aircraft remains in control all the time.
- Severe turbulence is characterized by large, abrupt changes in attitude and altitude with large variations in airspeed. There may be brief periods where effective control of the aircraft is impossible. Loose objects may move around the cabin and damage to aircraft structures may occur.
- Extreme turbulence is capable of causing structural damage and resulting directly in prolonged, possibly terminal, loss of control of the aircraft.
In-flight turbulence assessment is essentially subjective. Routine encounters involve light or moderate turbulence, although to inexperienced passengers (or pilots), especially in small aircraft,these conditions may seem to be severe.