What causes a windshield to fog up on the inside? I understand it’s related to the differences in temperature, but that’s as far as I understand. What can one do to resolve it?
Sorry if the flair is wrong 🙂
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if the temperature outside the car is colder than inside the car, the glass itself will be cold and any warm moist air inside the car will condense on that cold surface. Just like if you take a ice cold drink outside on a warm humid day, the cup will get all wet on the outside of the glass, that’s just the warm moist air condensing on the surface of the cold glass.
Most cars have a “defogging” function for the front windscreen (part of the temperature controls). The heater vents will be directed towards the windscreen and this air movement will evaporate the water from the inside surface of the windscreen and remove the condensation.
u/mr-photo gave a good answer as to why it happens. I’ll add that while the car is not fully sealed to the exterior, it can still take some time for the conditions inside the car and outside to equalize. Add humans in the car adding moisture from breathing, etc. and you will have slightly more humid air in a car quite often, even after the humans are gone from the car. If it gets cold enough outside, you’ll get the condensation on the inside of the windshield.
>What can one do to resolve it?
That’s what the defrost setting on your car’s climate control is for. Then you wait for it to defog before you are on your way to drive.
You could also leave the car windows cracked open, but if there’s snow rain, wildlife, etc. around, it isn’t necessarily a great idea.
When it’s cold outside the car and warm inside the car, the windscreen itself gets cold.
This means that moisture in the air inside the car – usually from the breath of people sat inside – condenses into liquid water when it hits the cold windscreen. Which then sticks to the windscreen, creating that fog. It’s the same principle as a cold drink getting wet condensation on the outside.
You can fix it two ways – either by equalising the temperature between inside and out, or by removing the moisture.
Most of the time, you don’t want the inside of the car to be as cold as the outside. 🙂
Older cars have vents which blow air at the windscreen, which both warms up the glass a bit and helps dry the area out. Other cars have things like heated windscreens, which keeps the windscreen too warm for water to condense onto it.
Here’s an article I found by Google searching “Why do windshields fog up on the inside?” https://www.allstate.com/resources/car-insurance/foggy-windows
Air usually has some amount of water vapor in it. When this air + water is exposed to something cold and it cooled below the [dew point](https://en.wikipedia.org/wiki/Dew_point) water condenses. This is seen also with a cold object like a drink in a humid summer day.
Here’s something I found by Google searching for “how to use defroster” https://magazine.northeast.aaa.com/daily/life/cars-trucks/ultimate-guide-defog-windows-car/
Blowing warm and dry air past the windshield causes water condensed to evaporate again. On some cars it’s turning knobs. On some it’s pressing a defroster button. Either way, reading your car manual gives you the manufacturer’s recommended method. If your car does not have a printed manual, Google searching your car name and manual, like “ford focus manual” will bring up a link.
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