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Q & A: Car windows fogging up

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Q:
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
U.S
A:
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.

Tom

(republished on 07/24/06)

Follow-Up #1: Window Fog

Q:
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.
A:
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 08/03/06)

Follow-Up #2: followup on car fogging

Q:
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)
Australia
A:

  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 08/29/06)

Follow-Up #3: fog inside car window

Q:
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)
Maryland
A:
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 09/10/11)

Follow-Up #4: foggy cars and air recirculation

Q:
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
A:
Yes, as we noted above, recirculation can be a problem because of moisture from breath.  Mike W.

(published on 02/01/12)

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