Few things are more annoying or dangerous than driving a car with foggy windows. According to the University of Illinois Department of Physics, car windows fog over when water condenses on them or when warm, humid air meets a cooler surface. Fog can occur on the inside and outside of a window. Consider combining or practicing a variety of different methods to prevent your car windows from fogging over.
Make sure your air conditioner and heater unit is in good working condition and is in the correct "fog-fighting" position. DeAnza College recommends keeping your air conditioner or heater on the fresh air position rather than the recirculation setting, as the recirculation setting reuses the humid or moist air already prevalent in the car and doesn't help rid the fog from the car windows.
Wipe your shoes clean of any snow or wet dew with a dry rag before entering your car. The less moisture you have in your car, the less fog you'll have on your windows.
Crack one of your windows open as soon as you enter your car. According to the Hamilton County Environmental Services, opening a window as soon as you enter your vehicle will prevent your car windows from fogging up. When the air outside is cooler than the air inside your car, moisture will condense on the inside of the windows.
Clean your car windows with an ammonia-based spray. The ammonia-based spray will remove any build-up or film present on the windows which in turn helps prevent the windows from becoming foggy. You will need to spray the windows on a weekly or bi-weekly basis for lasting success.
You can use a liquid dish washing soap in place of an ammonia-based spray. You can also put a light coat of shaving cream on the windows instead of using an ammonia-based spray or liquid dish washing soap. Place a small amount of shaving cream on a dry rag and rub your windows in a vertical or horizontal pattern. Let the shaving cream sit on the windows for a few minutes. Take another dry rag and rub the windshield in the same direction as before. You will need to reapply the shaving cream to your car windows on a weekly or as-needed basis for lasting results.
Don't put too much shaving cream on your windows as it could affect or block your view.