The Collaborative International Dictionary of English v.0.48:

Atmometer \At*mom"e*ter\, n. [Gr. ? smoke, vapor + -meter: cf.
   F. atmom[`e]tre.]
   An instrument for measuring the rate of evaporation from a
   moist surface; an evaporometer. --Huxley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

atmometer
    n 1: an instrument that measures rate of evaporation of water
         [syn: atmometer, evaporometer]