Clear-air turbulence

CAT can be hazardous to the comfort, and occasionally the safety, of air travelers,[1] as the aircraft pilots often cannot see and anticipate such turbulences, and a sudden encounter can impart significant stress to the airframe.

[4] In aviation, CAT is defined as "the detection by aircraft of high-altitude inflight bumps in patchy regions devoid of significant cloudiness or nearby thunderstorm activity".

However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers, Doppler LIDARs, or N-slit interferometers.

However, because this turbulence affects long range aircraft that fly near the tropopause, CAT has been intensely studied.

As of 1965 it had been noted that 64% of the non-light turbulences (not only CAT) were observed less than 150 nautical miles (280 km) away from the core of a jet stream.

[clarification needed] Although the altitudes near the tropopause are usually cloudless, thin cirrus cloud can form where there are abrupt changes of air velocity, for example associated with jet streams.

As is explained elsewhere in this article, temperature decreases and wind velocity increase with height in the troposphere, and the reverse is true within the stratosphere.

When these factors coincide with jet streams, CAT can occur: The tropopause is a layer which separates two very different types of air.

When a pilot experiences CAT, a number of rules should be applied:[12] Because aircraft move so quickly, they can experience sudden unexpected accelerations or 'bumps' from turbulence, including CAT – as the aircraft rapidly cross invisible bodies of air which are moving vertically at many different speeds.