azure vs upper atmosphere

azure

adj
  • sky blue; resembling the clear blue colour of the unclouded sky 

  • cloudless 

  • In blazon, of the colour blue. 

noun
  • The clear blue colour of the sky; also, a pigment or dye of this colour. 

  • The unclouded sky; the blue vault above. 

  • azure 

  • azure (heraldry) 

  • Any of various widely distributed lycaenid butterflies of the genus Celastrina. 

  • Lapis lazuli. 

  • Any of various Australasian lycaenid butterflies of the genus Ogyris. 

  • A blue colour on a coat of arms, represented in engraving by horizontal parallel lines. 

verb
  • To colour blue. 

upper atmosphere

How often have the words azure and upper atmosphere occurred in a corpus of books? (source: Google Ngram Viewer )