Turbulence in an air jet
Turbulence: spiller of coffee, jostler of luggage, filler of barf bags, rattler of nerves. But is it a crasher of planes? Judging by the reactions of many airline passengers, one would assume so; turbulence is far and away the number one concern of anxious flyers. Intuitively, this makes sense. Everybody who steps on a plane is uneasy on some level, and there’s no more poignant reminder of flying’s innate precariousness than a good walloping at 37,000 feet. It’s easy to picture the airplane as a helpless dinghy in a stormy sea. Boats are occasionally swamped, capsized, or dashed into reefs by swells, so the same must hold true for airplanes. So much about it seems dangerous.
At times like this, pilots will slow to a designated “turbulence penetration speed” to ensure high-speed buffet protection (don’t ask) and prevent damage to the airframe. We can also request higher or lower altitudes, or ask for a revised routing. If you feel the plane climbing or descending midflight, good chance it’s because of a report from fellow pilots up ahead. Air traffic control can’t always honor such requests, though, in which case you have little choice but to endure it.
In the worst of it, you’re liable to imagine the pilots in a sweaty lather: the captain barking orders, hands tight on the wheel as the ship lists from one side to another. Nothing could be further from the truth. The crew is not wrestling with the beast so much as merely riding things out; it’s surprisingly hands-off. Indeed, one of the worst things a pilot could do during strong turbulence is try to fight it. Some autopilots have a special mode for these situations. Rather than increasing the number of corrective inputs, it does the opposite, desensitizing the system.
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Clear Air Turbulence is defined as turbulence which is not associated with cloud and therefore cannot be detected visually or by conventional weather radar.
An aircraft descending for an approach into Milan encounters moderate turbulence associated with a southerly airflow over the Alps. A member of the cabin crew checking the security of the cabin falls breaking an arm.
Turbulence or turbulent flow is a flow regime in fluid dynamics characterized by chaotic changes in pressure and flow velocity . It is in contrast to a laminar flow regime, which occurs when a fluid flows in parallel layers, with no disruption between those layers. 
The onset of turbulence can be predicted by a dimensionless constant called the Reynolds number , which calculates the balance between kinetic energy and viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, and the interactions within turbulence creates a very complex situation. Richard Feynman has described turbulence as the most important unsolved problem of classical physics. 
Via this energy cascade , turbulent flow can be realized as a superposition of a spectrum of flow velocity fluctuations and eddies upon a mean flow . The eddies are loosely defined as coherent patterns of flow velocity, vorticity and pressure. Turbulent flows may be viewed as made of an entire hierarchy of eddies over a wide range of length scales and the hierarchy can be described by the energy spectrum that measures the energy in flow velocity fluctuations for each length scale ( wavenumber ). The scales in the energy cascade are generally uncontrollable and highly non-symmetric. Nevertheless, based on these length scales these eddies can be divided into three categories.