aerostat vs weather balloon

aerostat

noun
  • A moored balloon flown in a semi-permanent manner, such as a border patrol monitoring balloon affixed at 18,000 feet (~6 km). 

  • An aircraft, such as a dirigible or balloon, that derives its lift from buoyancy rather than from wings or rotors. 

weather balloon

noun
  • a balloon that carries meteorological instruments aloft in order to measure, record and return data concerning atmospheric pressure, temperature and humidity 

How often have the words aerostat and weather balloon occurred in a corpus of books? (source: Google Ngram Viewer )