Method of preventing windshield fogging in presence of rain

A method is provided of preventing windshield fogging for automatically controlling a heating, ventilation and air conditioning (HVAC) system of a vehicle. The method includes the steps of determining whether rain is occurring on a windshield of a vehicle, determining a windshield parameter as a function of the rain occurring 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 fog boundary relative humidity and a nominal base relative humidity for rain conditions. 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.