cool down vs hot up

cool down

verb
  • To become cooler; to be reduced in temperature. 

  • To become less agitated or excited. 

  • To cause (something) to become cooler; to reduce the temperature of. 

  • To decrease in degree or intensity. 

  • To cause (someone) to become less agitated or excited. 

hot up

verb
  • To become more heated. 

  • To make or become more sexually appealing, enticing or exciting. 

  • To increase in temperature. 

How often have the words cool down and hot up occurred in a corpus of books? (source: Google Ngram Viewer )