The Collaborative International Dictionary of English v.0.48:

Hygrometry \Hy*grom"e*try\, n. [Cf. F. hygrom['e]trie.]
   (Physics)
   That branch of physics which relates to the determination of
   the humidity of bodies, particularly of the atmosphere, with
   the theory and use of the instruments constructed for this
   purpose.
   [1913 Webster]