Clear-air turbulence

In meteorology, 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 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 occasionally the safety, of air travelers. In meteorology, 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. 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". It was first noted in the 1940s. Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with a 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. At typical heights where it occurs, the 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. As of 1965 it had been noted that 64% of the non-light turbulences (not only CAT) were observed less than away from the core of a jet stream. Jet stream produces horizontal wind shear at its edges, caused by the different relative air speeds of the stream and the surrounding air.