Hyperborean vs tropical

Hyperborean

tropical

noun
  • A tropical plant. 

adj
  • Pertaining to tropical geometry. 

  • From or similar to a hot humid climate. 

  • Of or pertaining to the tropics, the equatorial region between 23 degrees north and 23 degrees south. 

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