The Collaborative International Dictionary of English v.0.48:

humidify \humidify\ v. t.
   to render (gases, especially air) humid or to increase the
   moisture content of; as, We have a machine that humidifies
   the air in the house.

   Syn: moisturize, make humid, make more humid.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

humidify
    v 1: make (more) humid; "We have a machine that humidifies the
         air in the house" [syn: humidify, moisturize,
         moisturise] [ant: dehumidify]