air raid vs flak

air raid

noun
  • An attack on a surface target by aircraft. 

  • A passing-heavy offensive scheme based on a shotgun formation with four wide receivers and a single running back. 

flak

noun
  • Ground-based anti-aircraft guns firing explosive shells. 

  • A public-relations spokesperson. 

  • Anti-aircraft shell fire. 

  • Adverse criticism. 

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