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Why Windshields and Windows Fog Up

March 7, 2019

Why Windshields and Windows Fog Up

If you have ever walked out to your vehicle in the early morning hours or late at night only to find the windshield and windows fogged up, you are not alone.  Automobile windows and windshields tend to fog up at the worst possible times: those brutally cold and dark winter mornings and during inclement weather that is inherently stressful to begin with.  The last thing you should have to worry about is trying to see out of foggy windshields and windows.  Let's finally remove the mystery of foggy car windshields and windows by revealing the cause of this incredibly frustrating problem.


Why Windshields and Windows Fog Up

The abundance of water in the air is the primary reason for fog on the windshield.  If there is excessive water in the air, and the interior of the glass is warmer than the exterior, the water will condense along the glass.  The cold outdoor air reduces the relative humidity, making it difficult for outside condensation to occur.  

The vehicle's humidity level increases that much more as occupants inside the vehicle breathe and/or when they get in the car with wet clothes from the inclement weather. Even the heat from food or a hot beverage ramps up the moisture level in the air.  As an example, steam coming from a cup of coffee is full of humidity that remains in the cabin air, causing fog to form along the interior glass.


What to do When your Vehicle's Windows and/or Windshield Fog Up 

If your vehicle's interior glass fogs up, do not panic.  Stay calm, turn on the vehicle's defrosters and remain patient.  Be sure to turn on the defroster for the front windshield as well as the rear window.  Opt for fresh air as opposed to recirculation mode when operating the defroster.  The vehicle's glass needs fresh air from outside rather than the already-moist air in the vehicle.  The worst thing you can do is rub your hands, gloves or coat sleeve on the windshield or windows.  Attempting to manually clear fog with your clothing or hands will leave streak marks that reappear the next time you are in the vehicle.  These streaks will prove incredibly annoying so it is important to resist the temptation to wipe the windows and windshield on your own.

If it is warm outside and your vehicle glass is having fog issues, try using the windshield wipers to remove condensation from the exterior of the glass.  It will also help to turn on the heat to warm up the vehicle interior so it approaches the outdoor temperature.  If it is cold outside and your windows or glass fog up, open a window just a bit.  Do not turn on your vehicle's heat as doing so will cause the windows to fog that much more. 


You Need the Right Tools for the Job 

If you find your windshield or windows is excessively foggy, resort to using a squeegee.  Keep a small squeegee or even a large cloth in the vehicle to eliminate fog as quickly as possible.  It will also help to keep the windows clean on the inside and out so be sure to give your vehicle a thorough cleaning with sponges and squeegees at regular intervals.