January 18, 2019 - UPDATED November 11, 2019
Foggy windows can be the ultimate frustration - especially on days when you're in a rush. Knowing what causes window condensation will help you know how to defog your windows more quickly.
Fog on car windows and windshields is condensation created when the temperature of the glass is below the dew point of the air next to that surface.
If it’s humid outside and you’re running the air conditioner inside the car, water will condense on the outside of the window glass.
If it’s cold outside and you have the heat on inside (or lots of people breathing out warm air - LOL), water will condense on the inside of the window glass.
To eliminate (or reduce) window condensation in your car you need to match the air temperature inside the car as closely as possible to the temperature outside the car.
How to defog car windows on a cold day
When the condensation is on the inside of the window glass:
Lower the temperature inside your carThe goal is to get the temperature of the air inside your car closer to the temperature of the foggy windows. Turn on the AC, turn down the heat, or crack the windows.
Turn off the air recirculation mode in your carSince you don’t want to increase or perpetuate the warm air inside your car, make sure the function that recirculates the interior air is turned off.
Turn on the defrost vent with cool airThis function will blow air directly on your windshield. If it’s cold outside, you’ll want to blow cool air on the foggy windows to clear them. Also, most cars built in the last 20 years automatically shut off recirculation when the defrost function is selected.
Remove any wet items (umbrellas, snow boots, gym towels, etc) to the trunk of the carMoisture from these items will turn to condensation when it hits the colder air next to the car windows.
How to defog windows on a hot or humid day
When the condensation is on the outside of the window glass:
Use your windshield wipersSince the fog should be on the outside of the windshield, your windshield wipers will clear the condensation from the glass.
Warm up the air temperature inside your car
- Turn down the AC enough that the temperature inside your car better matches the temperature outside the car.
- Open the windows a bit to allow in some outside air.
Turn off the air recirculation mode in your carYou want the temperature of the air inside your car to match as closely as possible that of the air outside your car, so you don’t want to be recirculating interior air.
Please note that, like most tips, the aforementioned actions won’t work for all cars in all circumstances.
And, finally, try to resist the urge to wipe down your car windows with your hand. Consider keeping some microfiber cloths in your glove box if you need to clear the windshield quickly.
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