The fog is moisture condensing on the cold windshield surface. Use heat directed at the windshield with AC on. The AC dries out the moisture and the heater warms up the surface preventing additional condensation.
this post was submitted on 02 Sep 2026
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If your windows gets foggy you need to turn up the heat as that lowers the relative humidity and direct the air towards the windshield, also check below the floor mats for water. If your car is a swimming pool your windows will be foggy forever.
Counter intuitively, you want to turn the a/c on as that pulls the moisture out of the car.
During winter when a vehicle is defrosting, it runs a combination of ac and heat to defrost windows and condition the air to limit moisture condensing on the interior.
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When I press the defrost button in my car, it turns both the heat and AC on at the same time. I assume this is to remove water from the air with the AC, then warm up the air increasing its capacity to hold water with the heat.
Your assumption is correct Also your car automatically calculates the condensation point and keeps the temperature and air moisture low enough not to cause any fogging on your windshield. Source: I've been programming AC unit controls for cars for the last 10 years
How upset do you get when your buddy disables auto and proceeds to slam back and forth between full heat and full ac while claiming their shitty dial dj set is somehow waaaaay better than the computer?
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in a lot of newer cars they can actually use the A/C to heat and cool the car, rather than using excess engine heat or an electric heater which is especially helpful when it's cold and you've only just started the engine!
Fun fact (well, I suspect you know this already, but some didn't, I had discussion with someone arguing with me that this is not true):
AC and a heat pump are basically the same thing, AC is just pumping the heat outside instead of inside.
Also when you say modern cars I am assuming you might be talking about EVs. Using a heat pump requires less energy than using resistive heating so it could be energy saving measure.
ICE cars on the other hand can get the heat from engine "for free" (not really, but since the heat will be produced anyway whether you use it or not, so why not use it?) so a heat pump might make a bit less sense in them.
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I assume you mean EVs with heat pumps? I don't know of any ICE vehicles that do this, seems inefficient to waste mechanical or electrical energy when you have perfectly good waste heat to use upon reaching operating temperature (though you may need to use an aux heater before reaching that, particularly on diesels)
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It’s not about temperature, it’s about condensation and humidity. Turn on the AC regardless of temp (e.g. even if it’s cooler out and you’re blasting the heat) and the defog too.
Source: I fucking live in Seattle and it fucking rains all the time
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Always use heat. Anytime you have a fogged up window heat will cause that moisture to evaporate. Cold will always make the window cooler which causes more condensation.
Air conditioning will remove moisture from the air while cooling the cab, so the condensation can only form on the outside, where you have wind and windshield wipers to wipe off the condensation.
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Trick question:
Dew point is the exact temperature at which the current level of humidity forms. Heat is good because it makes the air at the surface stop condensing. AC is good because it dries the air out forming the condensate in your air box. Cracking the window is generally enough warmth to interrupt the process. Most new cars have a defrost mode that turns on both heat and ac to dry the air out.
So the answer is do anything, except just turning the ac colder.
People in comments assume the heat or air conditioning even work.
Air conditioning also rapidly reduces humidity, so that tends to be the ideal option.
But if neither heat nor air conditioner work, well tough it out and roll the windows down...
If neither the heat or A/C work, you shouldn’t be driving that POS on the road.
Just because the ac/heat don't work it's automatically a POS that's not road worthy?
Depends on the part of the world and season. It wouldn't be roadworthy to drive in freezing temps without heat on the windshield
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Clean your window.
Yup. The inside of the windshield gradually gets cloudy from all the plastics inside the vehicle slowly off-gassing. Soapy water, a squegee, and a microfiber do wonders.
Huh, that makes sense. I was wondering what caused that buildup slowly over time.
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crack a window
Temp isn't as important as ac
Remember to pull your sun visors down. My AC doesn't stay charged so I have to rely on heat alone to defog windows. The visors trap the warm air.
Open the window.
Every time. And I'm halfway through life. I could do a web search, but who ever thinks of that except when this happens? Will I do a web search now? No. I have other stuff to think about. And so it goes.
Just the other day I was listening to this song by the Grateful Dead:
"Sometimes the light's all shining on me ...
Sometimes I can barely see ...
And lately it appears to me
What a long strange trip it's been. "
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