cold spell vs cold wave

cold spell

noun
  • A short period during which temperatures are much lower than usual. 

  • Coordinate term: dry spell 

  • A period of poor performance. 

cold wave

noun
  • A short period of very cold weather. 

  • A permanent wave in the hair set by special solutions without the aid of any heating machine. 

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