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Q & A: Car windows fogging up

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Most recent answer: 07/29/2017
my mom’s car fogs up a lot... it is a "99 VW Golf and i think it is desinged badly because it fogs up very very ferquestly i also live in the north east (CT) so i dont know if the climate has anything to do with it (it happens year round so it may or may not be weather) so what i want to know is what causes this fog and more importantly what is best way to stop it? Thanks
- James
The windows in my car get foggy too, sometimes. It depends a lot on the weather and also the people inside and whether the air conditioning/heater is running.

Car windows fog up when water condenses on them. This can happen both on the inside of the window and on the outside. You can check to see in your mom's car where the condensation is because it will be wet to the touch.

It sounds like outside humidity might be a problem (especially on the East coast). It is here in Illinois, particularly on hot, stormy days in the summer. Water will condense on a surface if the temperature of the surface is below the dew point of the air next to that surface. So you need warm, humid air next to a cooler surface to fog up.

If it is very humid outside and you're running the air conditioner inside the car, water can condense on the outside of the windows. Using the windshield wipers and the rear defroster can get rid of condensation there by wiping it off or making it evaporate.

Often, the air in the car will be warmer than the air outside the car (this happens in winter here). Moisture will condense on the inside of the windows if the outside air is cooler than the dew point inside the car. This happened to me a lot when driving through a thunderstorm caused by an incoming cold front. In this case, running both the air conditioner, the heater, and the vents for defrosting the front window worked very effectively. The air conditioner dehumidifies air because water condenses inside it when the air goes past the cold tubes with the refrigerant inside. Heating it up way past its dew point makes it feel dry, and it is dry -- it'll help evaporate any condensation that's on the windshield -- and even more quickly the hotter it is because heat is needed to cause the liquid water to change phase to a gas.


(published on 10/22/2007)

Follow-Up #1: Window Fog

Please help me out here. The front window fogs on our 97 Ford Taurus. Makes visibilty bad and itís also on the inside of the car. Do we need to have the car looked at? What could possibly be wrong? Please help me out!
- Carole (age 64)
Sun City, Az U.S.
Tomís old answer pretty much covers both the physics and the practical aspects of this. For any outside fog, the wipers and the rear-window heater should work.  Since AZ is usually dry, I guess thatís not your main problem. For inside fog, I guess that breath is the main source of moisture. Maybe, at least on typical dry AZ days, having the air-conditioning on the setting that lets in some outside air might help.
Mike W.

(published on 10/22/2007)

Follow-Up #2: followup on car fogging

sounds like you may have a problem with your heater coil. if you lose coolant from radiator then you have a leak somewhere and it needs to be looked at.
- Ivo (age 33)

  If it's the coolant that's evaporating after leaking and recondensing on the windshield, there should be a pungent odor from the engine coolant, and a detectable drop in the coolant level over time.  Also, it can be checked to see if this happens when the air conditioner and heater are turned off.  If the condensation is on the outside of the window, it sounds like the cause is the weather and that the car is cooler.  If it's on the inside, it could be any source of warm, moist air inside the car (usually it's the people, but it could be something else too, like the air handling system, as you say).

(published on 10/22/2007)

Follow-Up #3: fog inside car window

OK, at this time of year, I get in the car, and it starts to fog up on the inside of the windows. I turn on the defroster ( heat ) and the problem gets worse. I turn on the AC and the problem resolves. What is going on?
- Joseph Boggi (age 53)
One part makes definite sense. When you turn on the a/c, that helps. That's because as the air goes past the cold a/c coils, a lot of the water condenses out. That dries the air before it gets to the windows.

Why does turning on the defrost make things worse? I'm not sure, but it may blow more air past the windshield. If the windshield is still cold enough to make water condense out, you could get more condensation than you would otherwise. After a while the windshield will heat up and the condensation will stop.

Mike W.

(published on 09/09/2011)

Follow-Up #4: foggy cars and air recirculation

Ok here is the deal. It is a fact that running your heater on recirculate blows harder then vent. But. Your body heat and your breath mix in with air that is being recirculated, therefore adding moisture to the inside air of the vehicle hence creating fog on the windows. If you run your heat on vent then the moisturizers air can escape the vehicle whereby letting fresh air in. So don't run yur heat on recirculate in the cold months.
- Kevin Winston (age 35)
Aurora, Illinois
Yes, as we noted above, recirculation can be a problem because of moisture from breath.  Mike W.

(published on 01/31/2012)

Follow-Up #5: condensation outside car

Ok that makes sense , if there is condensation on the inside of the car windows then get rid of the moisture by running the a/c because it dehumidifies the air. I have the opposite situation. When I get up after a cold night the outside of the car windows have condensation. Does this mean the car is sealed properly and there is just more moisture outside than in ? Is this normal ? The vehicle does have automatic climate control and the a/c is always on with that feature when he car is on. Thanks
- Dave (age 32)
Bangor, Maine, USA

Your guess sounds very reaonable. Your climate control may keep the inside drier than the outside. When you think about it, the ground is often covered with dew in the morning. That's because as the water vapor cools at night, it starts to condense on any available surface. Your car is also available.

Mike W.

(published on 09/21/2014)

Follow-Up #6: car fog thoughts

Look people over complicate stuff way to much to sound smart and for self satisfaction. Simple answer. If it's cold outside and your window fogs up, put the cooler on in your car. If it's warm outside and your windows fog up, put the heater on. Try match the outside temp to clear the fog quickly and effectively.
- Colin (age 27)
Singapore, singapore

People don't usually like to turn on the car heater when it's hot and don't like to turn on the cooler when it's cold, for obvious reasons. Also sometimes people are curious about how things work.

Mike W.

(published on 08/28/2015)

Follow-Up #7: fog inside windshield

My 2015 Kia Souls windshield fogs up not on the inside or outside but in between the glass. I can't do anything until it clears up on it's own. Is there any help out there.
- Michael A Petix (age 69)

That sounds terrible. I thought that the inner layer of a windshield was filled with a polymer material to hold the windshield together. That would also keep water from condensing. Something sounds very wrong with your windshield.

Mike W.

(published on 07/29/2017)

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