stratosphere vs welkin

stratosphere

noun
  • The region of the uppermost atmosphere where temperature increases along with the altitude due to the absorption of solar ultraviolet radiation by ozone. The stratosphere extends from the tropopause (10–15 kilometers) to approximately 50 kilometers, where it is succeeded by the mesosphere. 

welkin

noun
  • The upper atmosphere occupied by clouds, flying birds, etc. 

  • The place above the Earth where God or other deities live; heaven. 

  • The sky which appears to an observer on the Earth as a dome in which celestial bodies are visible; the firmament. 

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