Q What causes the insides of my windshield to frost up overnight?
A Frost accumulates on the inside of the windows when water vapor inside the car condenses and freezes. Most of the water vapor, or humidity, comes from snow or water carried into the vehicle on boots and clothes.
As the air in the car warms, the snow melts and the water evaporates into the air. When a car is parked overnight or for an extended time when it's cold, the water vapor condenses on the glass, which cools very quickly. Then, as the car cools further, the condensation freezes.
To help prevent frost and ice from forming, leave a window cracked open to allow the water vapor to escape.
To remove frost in the morning, direct dry, heated air across the windows by positioning the heating controls.
Most cars today will have the air conditioning kick on when the heater is set to defrost. That will help remove humidity. If your car doesn't do that, turn on the air conditioning or bilevel position, with the temperature level at full hot to get the dry air to the side windows.
When temperatures are higher (30 to 50 degrees), and fogging is the problem, the air conditioner will dry the air and the heater will heat it, resulting in the desired effect of hot, dry air being blown across the windows.
Be sure the heating ventilation system's recirculating button is off. Recirculating air will exacerbate the fogging, frosting problem.
