An air conditioning system of a vehicle including a controller and sensors in communication with the controller. The sensors include an interior temperature sensor for determining the temperature of the vehicle interior and an inner surface temperature sensor for determining the temperature of the inner surface of the vehicle's windshield. The controller determines a dew-point for the vehicle interior based on the sensed interior temperature. The controller adjusts the dew-point or the inner surface temperature based on a correction value, and performs a defogging operation when the temperature of the inner surface of the windshield is less than the dew-point, wherein the dew-point or the inner surface temperature is adjusted.