why is my windshield fogging up on the outside

A windshield fogs up on the outside because of condensation forming when warm, moist air from outside meets the cooler surface of the windshield. This typically happens in warm, humid weather when the temperature of the glass is cooler than the dew point of the outside air, causing moisture to condense on the outside surface. In other words, the glass acts as a cold surface causing the moisture in the warm air to turn into tiny water droplets, creating fog on the exterior of the windshield. This phenomenon is often observed when air conditioning inside the car cools the windshield glass below the outside dew point temperature, while the outside air remains warm and humid. The external condensation creates the foggy appearance on the windshield. To manage and reduce outside windshield fogging, using windshield wipers to clear the condensation or adjusting the car’s air conditioning to reduce cooling of the glass can help. Setting the temperature inside the car to be closer to the outside temperature and maintaining proper ventilation can also prevent or lessen fogging on the outside of the windshield.