knit cap vs woolly hat

knit cap

noun
  • A soft, head-hugging cap usually without a specified front or back, barring those with designs or logos 

woolly hat

noun
  • an item of knitted headwear, without a brim or peak, made from wool or other fabrics. 

How often have the words knit cap and woolly hat occurred in a corpus of books? (source: Google Ngram Viewer )