[Clear Air Turbulence] Bumpy conditions in the upper atmosphere when no clouds are present to betray the possibility of such. Caused by sharp vertical and horizontal shear of wind, often (but not exclusively) in association with upper-level jet streams .Can occur in, or be enhanced by mountain wave activity.
CAT
Meaning of CAT in English
Weather and meteorology English vocabulary. Английский словарь погоды и метеорологии . 2012