Clear-air turbulence

Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues such as clouds, and is caused when bodies of air moving at widely different speeds meet.

The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT.

CAT can be hazardous to the comfort, and rarely the safety, of air travelers.

Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with conventional radar, with the result that it is difficult for aircraft pilots to detect and avoid it. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers, Doppler LIDARs, or N-slit interferometers.

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. Lines of cirrus perpendicular to the jet stream indicate possible CAT, especially if the ends of the cirrus are dispersed, in which case the direction of dispersal can indicate if the CAT is stronger at the left or at the right of the jet stream.

Detecting and predicting CAT is hard for meteorologists because it is at such heights that even when caused by factors that can be measured, intensity and location cannot be determined precisely. However, because this turbulence affects long range aircraft that fly near the tropopause, CAT has been intensely studied. Several factors affect the likelihood of CAT. Often more than one factor is present. 64% of the non-light turbulences (not only CAT) are observed less than 150 nautical miles (280 km) away from the core of a jet stream.

A jet stream alone will rarely be the cause of CAT, although there is horizontal wind shear at its edges and within it, caused by the different relative air speeds of the stream and the surrounding air.

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Wikipedia