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Condensation On Car Windows

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Condensation On Car Windows

Postby Tyronne » Fri Dec 02, 2016 10:25 am

Not sure if this is a physics question but I have observed 2 contrasting things happening recently & wondered about the cause. On some days, as soon as I pull into the parking lot of a building, the windows inside the car all "fog up". On other days, I'll be sitting in the car in a parking lot for a few minutes before going anywhere, with clear windows, but as soon as I start driving, then they fog up almost immediately. In neither case was the heat on until I had to put on the defroster. Temperatures probably in the 30-40 degree range, generally high pressure and no wind. In the one case, the condensation appears when I start the forward motion of the car, in the other, when the forward motion stops. Any explanation?

ANSWER: I can only pass on some simple facts about what causes condensation on windows(or on any other solid object, like dust particles or soda cans). The amount of water that can remain in the gaseous form in air depends on the air temperature. If the amount of water vapor in the air is above what air, at that temperature, can sustain, then the vapor condenses into liquid water. The presence of solids makes this condensing go more quickly, it gives the vapor something to "latch onto" as it condenses.

If a solid is colder than the air with the vapor, then condensation becomes much more likely on that solid. This is why cold soda cans will get wet on a hot summer day. The hot air is easily able to keep a certain fraction of water in vapor form, but the air right at the surface of the can is not able to do so -- thus you get a lot of condensation on the can.

Also note that, if a window is a little colder than the indoors, but not so cold as to cause condensation by the normal level of vapor for the indoor, one can still cause parts of the the window to fog up by blowing on it. Why? Because our exhaled air has a lot of water vapor in it -- thus, exhaled air directly on a cool window can often cause condensation simply by a rapid increase in the amount of vapor in the air near to that window.

So, even though I can't tell you EXACTLY why you would get condensation in the way and timing that you observe, I can tell you that any condensation you see is the result of a level of water vapor on the inside of the car being higher than the air near the windows can "handle." And by the word "handle," I mean the ability to prevent water vapor from condensing. If you see fogging on the inside, then the vapor level has increased(mostly from your exhaled air) or the temperature of the glass has decreased(which happens from driving the car, which creates a wind on the glass), or some combination of the two. Bottom line, the air near the window must have too much vapor in it for the temperature of the air near the glass.

What causes this? I can't say. I regret I can offer no further help on this subject.

---------- FOLLOW-UP ----------

From what you say about water condensing out more easily on solids, I'm thinking there might be a greater build up of both moisture & pollutants in some parking lots(& along roads with lots of businesses & traffic)  particularly as the air is almost always dead calm lately. Last year even though it was WAY colder this time of year it was almost always very breezy at ground level & I almost never had to put the defroster on.
Tyronne
 
Posts: 46
Joined: Wed Apr 02, 2014 7:52 am

Condensation On Car Windows

Postby rock91 » Sat Dec 03, 2016 2:27 am

Not sure if this is a physics question but I have observed 2 contrasting things happening recently & wondered about the cause. On some days, as soon as I pull into the parking lot of a building, the windows inside the car all "fog up". On other days, I'll be sitting in the car in a parking lot for a few minutes before going anywhere, with clear windows, but as soon as I start driving, then they fog up almost immediately. In neither case was the heat on until I had to put on the defroster. Temperatures probably in the 30-40 degree range, generally high pressure and no wind. In the one case, the condensation appears when I start the forward motion of the car, in the other, when the forward motion stops. Any explanation?

ANSWER: I can only pass on some simple facts about what causes condensation on windows(or on any other solid object, like dust particles or soda cans). The amount of water that can remain in the gaseous form in air depends on the air temperature. If the amount of water vapor in the air is above what air, at that temperature, can sustain, then the vapor condenses into liquid water. The presence of solids makes this condensing go more quickly, it gives the vapor something to "latch onto" as it condenses.

If a solid is colder than the air with the vapor, then condensation becomes much more likely on that solid. This is why cold soda cans will get wet on a hot summer day. The hot air is easily able to keep a certain fraction of water in vapor form, but the air right at the surface of the can is not able to do so -- thus you get a lot of condensation on the can.

Also note that, if a window is a little colder than the indoors, but not so cold as to cause condensation by the normal level of vapor for the indoor, one can still cause parts of the the window to fog up by blowing on it. Why? Because our exhaled air has a lot of water vapor in it -- thus, exhaled air directly on a cool window can often cause condensation simply by a rapid increase in the amount of vapor in the air near to that window.

So, even though I can't tell you EXACTLY why you would get condensation in the way and timing that you observe, I can tell you that any condensation you see is the result of a level of water vapor on the inside of the car being higher than the air near the windows can "handle." And by the word "handle," I mean the ability to prevent water vapor from condensing. If you see fogging on the inside, then the vapor level has increased(mostly from your exhaled air) or the temperature of the glass has decreased(which happens from driving the car, which creates a wind on the glass), or some combination of the two. Bottom line, the air near the window must have too much vapor in it for the temperature of the air near the glass.

What causes this? I can't say. I regret I can offer no further help on this subject.

---------- FOLLOW-UP ----------

From what you say about water condensing out more easily on solids, I'm thinking there might be a greater build up of both moisture & pollutants in some parking lots(& along roads with lots of businesses & traffic)  particularly as the air is almost always dead calm lately. Last year even though it was WAY colder this time of year it was almost always very breezy at ground level & I almost never had to put the defroster on.
rock91
 
Posts: 46
Joined: Thu May 23, 2013 5:55 am

Condensation On Car Windows

Postby Lyza » Sat Dec 10, 2016 11:00 pm

You are correct.

Condensation occurs much more readily on dirt particles.

Indeed, without such dust particles in our atmosphere, clouds(and thus rain) would occur only in freak conditions like a super-fast temperature drops.http://en.wikipedia.org/wiki/Cloud_condensation_nuclei
Lyza
 
Posts: 31
Joined: Sun Feb 16, 2014 5:02 pm


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