CLEAR-AIR TURBULENCE

erratic air currents that occur in cloudless air between altitudes of 7 and 12 kilometres (4 and 7 miles) and constitute a hazard to aircraft. The turbulence apparently is caused by the wind velocity gradients around the jet stream, where rapidly moving air is close to much slower air; it is most severe over mountainous areas. Detection of clear-air turbulence for warning purposes is difficult, but in March 1964 a radar device, called cat-spy, was introduced; it indicates probable turbulence 16 kilometres (10 miles) ahead of an aircraft.