airmail vs surface mail

airmail

noun
  • The system of conveying mail using aircraft. 

  • The items of mail so carried. 

verb
  • To send mail by air. 

  • To (unintentionally) throw the ball well over a fielder's head where that fielder is unable to make a play on the ball. 

surface mail

noun
  • Mail which is transported by motor vehicle, by railway, or by seacraft, but not by aircraft. 

How often have the words airmail and surface mail occurred in a corpus of books? (source: Google Ngram Viewer )