The Collaborative International Dictionary of English v.0.48:

Hygrometer \Hy*grom"e*ter\, n. [Gr. ? wet, moist + -meter: cf.
   F. hygrom[`e]tre.] (Physics)
   An instrument for measuring the degree of moisture of the
   atmosphere.
   [1913 Webster]

   Daniell's hygrometer, a form of hygrometer consisting of a
      bent glass tube terminating in two bulbs, the one covered
      with muslin, the other of black glass, and containing
      ether and a thermometer. Ether being poured on the muslin,
      the black ball, cooled by the evaporation of the ether
      within, is soon covered with dew; at this moment, the
      inclosed thermometer gives the dew-point, and this,
      compared with the reading of one in the air, determines
      the humidity. Hygrometric

WordNet (r) 3.0 (2006):

hygrometer
    n 1: measuring instrument for measuring the relative humidity of
         the atmosphere

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "hygrometer":
   aerological instrument, aneroid barometer, aneroidograph,
   barograph, barometer, barometrograph, glass, hair hygrometer,
   humidor, hurricane-hunter aircraft, hygrodeik, hygrograph,
   hygroscope, hygrostat, hygrothermograph, psychrometer, radiosonde,
   recording barometer, sling psychrometer, vacuometer,
   weather balloon, weather instrument, weather satellite,
   weather vane, weatherglass