algor vs ground frost

algor

noun
  • cold, chilliness 

ground frost

noun
  • The condition of air temperature dropping below the freezing point at or near ground level. 

  • Ice produced by the direct deposition of water vapor on grass, trees and other objects in freezing conditions. 

  • The frozen upper level of soil, caused by the air temperature dropping below the freezing point for a lengthy period of time; in temperate climates the soil thaws completely for the summer and in cold climates it may develop to permafrost. 

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