The evaporative cooling of air by inlet fogging is a well proven technology for enhancing the power of gas turbine engines since it decreases with the increase of ambient temperature. In this work the inlet fogging process is modeled based on the evaporation of droplets. By considering heat and mass transfer and thermodynamic relations, analytic expressions are obtained for transient behaviors of air and droplet temperatures, and droplet diameter. They depend on ambient conditions, water injection ratio and initial droplet diameter. Results are given for the high fogging case as well as low fogging case.