hoar vs hoar frost

hoar

noun
  • A white or greyish-white colour. 

  • Hoariness; antiquity. 

adj
  • Hoarily bearded. 

  • Of a white or greyish-white colour. 

hoar frost

noun
  • Originally, any frozen dew forming a white deposit on exposed surfaces. 

  • Water vapour which has undergone deposition or desublimation (“transformation directly into ice crystals without first turning into liquid water”) when the air is cold and moist to form a white deposit on exposed surfaces. 

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