We demonstrated that an increase in the gas humidity in a two-zone drier enhanced the formation of polymer droplets via a secondary phase separation. Optical microscopy observations revealed that, with an increase in humidity, the separating polymers contacted and coalesced with the neighboring droplets to form characteristic double phase structures superposed in the films. We also demonstrated that the morphology transition was induced by the variation in the phase equilibrium thermodynamics rather than the variation in the solvent drying rate, which was independent of the humidity.