Method of preventing windshield fogging by determining windshield fog boundary humidity

A method is provided of determining the windshield fog boundary humidity for automatically controlling a heating, ventilation and air conditioning (HVAC) system of a vehicle. The method includes the steps of determining a windshield parameter for the vehicle and determining a temperature difference based on temperature within a cabin of the vehicle and ambient temperature. The method also includes the steps of determining a fog boundary relative humidity based on the windshield parameter and the temperature difference and determining a fog probability based on the difference between the fog boundary relative humidity and the measured cabin relative humidity. The method further includes the steps of determining anti-fog actions to be taken based on the fog probability and taking the anti-fog actions determined to control the HVAC system.