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Turbulence is caused by the relative movement of disturbed air through which an aircraft 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 or the direction of airflow (or both) is changing, although perception of the severity of turbulence which has been 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 occasioned by surface heating.
Turbulence may also arise 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, like turbulence caused by Mountain Waves, which can form downwind of ridges, occurs clear of cloud and in the form of Clear Air Turbulence (CAT) (CAT).
Very localised, but sometimes severe, Wake Vortex 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.
Air moving over or around high ground may create turbulence in the lee of the terrain feature. This may produce violent and, for smaller aircraft, potentially uncontrollable effects resulting in pitch and / or roll to extreme positions.
Relative air movements which involve rapid rates of change in wind velocity are described as wind shear and, when severe, they may be sufficient to displace an aircraft abruptly from its intended flight path such that substantial control input is required to compensate. The consequences of such encounters can be particularly dangerous at low altitude where any loss of control may occur sufficiently close to terrain to make recovery difficult. The extreme down-bursts which occur below the base of cumulonimbus clouds called Microbursts are a classic example of circumstances conducive to Low Level Wind Shear
For the purpose of reporting and forecasting of air turbulence, it is graded on a relative scale, according to its perceived or potential effect on a 'typical' aircraft, as Light, Moderate, Severe and Extreme.
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.
The perception of turbulence severity experienced by an aircraft depends not only on the strength of the air disturbance but also on the size of the aircraft - moderate turbulence in a large aircraft may appear severe in a small aircraft. Therefore pilot reports of turbulence should mention the aircraft type to aid assessment of the relevance to other pilots in, or approaching, the same area.
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