Search Result for "clear-air turbulence":
Wordnet 3.0

NOUN (1)

1. strong turbulence in an otherwise cloudless region that subjects aircraft to violent updrafts or downdrafts; 



WordNet (r) 3.0 (2006):

clear-air turbulence
    n 1: strong turbulence in an otherwise cloudless region that
         subjects aircraft to violent updrafts or downdrafts