A higher altitude (6–15 km) turbulence phenomenon occurring in cloud-free regions, associated with wind shear, particularly between the core of a jet stream and the surrounding air. It is most common near upper air fronts and the tropopause, and can often affect an aircraft without warning. Clear-air turbulence also frequently occurs close to towering cumulus clouds (usually within 30 km), and near mountains. Airflow disrupted by mountains and other terrain can undulate in waves of turbulence for 1000 km or more. See also aircraft turbulence.