Q: What causes fog to form on the inside of my car’s windshield during cold weather?
A: Fog forms from humidity in the air. In the winter the air outside the vehicle is generally very dry. Moisture is added to air inside the car from passengers breathing. Exhaled air is very moist. To reduce the moisture in the air inside the car you need to keep air flowing in from the outside. Many car heating and cooling systems have two settings: (1) to recirculate the air or (2) to bring air in from the outside. Since the cold air is less humid, turn the setting so the air is brought in from the outside. You can also use the air conditioning (AC) with the temperature control set to hot to heat the inside passenger compartment. The AC will pull excess moisture from the air.