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erratic air currents that occur in cloudless air between altitudes of 7,000 and 12,000 metres (23,000 and 39,000 feet) and constitute a hazard to aircraft. This turbulence can be caused by small-scale (i.e., hundreds of metres and less) wind velocity gradients around the jet stream, where rapidly moving air is close to much slower air. It is most severe over mountainous areas and also occurs in the vicinity of thunderstorms.