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]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "humidify":
   asperge, bedew, bespatter, besprinkle, dabble, damp, dampen, dash,
   dew, douche, hose, hose down, humect, humectate, irrigate, moisten,
   paddle, slobber, slop, slosh, sparge, spatter, splash, splatter,
   sponge, spray, sprinkle, swash, syringe, water, wet, wet down