certain vs up in the air

certain

adj
  • Fixed or stated; regular; determinate. 

  • Unfailing; infallible. 

  • Known but not specifically named; indeterminate; indefinite; one or some; sometimes used independently as a noun, and meaning certain persons; see also "one". 

  • Not to be doubted or denied; established as a fact. 

  • Sure, positive, not doubting. 

  • Actually existing; sure to happen; inevitable. 

det
  • Having been determined but not specified. 

pron
  • Unnamed or undescribed members (of). 

up in the air

adj
  • Not yet resolved, finished, answered, decided or certain. 

adv
  • Used other than figuratively or idiomatically: see up, in, the, air. 

How often have the words certain and up in the air occurred in a corpus of books? (source: Google Ngram Viewer )