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Thread: Water film on my windshield

  1. #1 Water film on my windshield 
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    Not sure if this should go here or under "physics" but oh well. This totally random question has bugged me a little every winter for the last decade or so Maybe you'll find it interesting, and possibly solve the riddle for me!

    During the winter, when it's cold outside, around here in the San Francisco bay area, CA, say 40-50 degrees or so (and possibly humid) and I get to my car at night after work, I sometimes find my windshield completely covered with water condensation that makes it almost impossible to see through. My question is - what makes this particular water behave the way it does, which is as follows:

    1) I can wipe it with my finger and it will leave a smeary "sheet" of water before it reverts back to bigger droplets + super fine condensation. And by that I mean if you have water droplets in a non-stick/teflon pan and smear a finger across it, the water will temporarily leave a trail behind your finger in a thin sheet before it quickly beads up again and separates to form little droplets again. This is the same, except that in addition to the water beading up again into big droplets, this separates into big droplets AND fine condensation that is blurry to look through*.

    2) Windshield wipers don't work on it - wipers, just like my finger, will make the water smear around, and then immediately bead back up into drops and fine condensation. I'd like to note that my wipers work just fine when it's raining - they don't leave drops or smears anywhere.

    3) Even stranger, windshield wipers + squirting the wiper fluid all over the windshield doesn't even work. Same result.

    4) The fine condensation makes my windshield wipers work worse for the LARGE droplets. My windshield wipers would normally take off all large drops of water. When there is this fancy cold weather condensation it makes my windshield wipers utterly ineffectual - they wipe and after the sheet of water beads up back into large drops, my windshield is still just covered in blobs of water. The wipers do NOT cleanly remove all the large drops, leaving only the blurry condensation.

    5) It's definitely not on the inside - there's no breath moisture or anything on the inside surface.

    6) After driving around with my head out the window, I can get on the freeway, and then 30 seconds later the condensation is completely gone.

    7) It doesn't happen with the morning dew that sometimes shows up at home; it only happens at night. At least I believe this to be the case. I may be forgetting cases in the morning, or perhaps driving around in morning light with condensation is easier than when it is pitch black out with super bright headlights scattering everywhere.


    I have a theory about what it is, but I'd like to get other people's ideas before I throw mine out there and contaminate everybody's thinking


    Enjoy,

    --
    Ken



    * disclaimer: when I say the sheet of water beads back up into droplets and fine condensation, I don't actually know that the condensation part ever got smeared around and turned into a thin sheet by my finger or windshield wiper at all. For all I know, my finger/wiper smeared out the big droplets into a thin sheet, which actually sat on top of the condensation, and then the sheet beaded back up into large droplets, and the fine droplet condensation stayed unaffected by the whole process. Frankly, that seems more likely than a sheet of water beading up into fine condensation much smaller than should be possible when taking into account water tension.


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  3. #2  
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    My guess is that your "clean" windshield is not really clean. There is a fine, invisible layer of oily condensation on it before the moisture condenses.

    You have done a fine job of reporting your observations. Now do an experiment. Do a thorough job of cleaning 1/3 of your windshield. Use a variety of cleaning products on this one section, including commercial degreasing agents. When you are satisfied that the surface is scrupulously clean, see if there is any change in the results.

    Let us know and then we might suggest another treatment for the next 1/3 section. One section remains untreated as a control.
    *


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  4. #3  
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    You might want to put new wiper blades on as well if they are impregnated with oil they will behave ineficiently. does it happen to all the other cars on the lot?
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  5. #4  
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    Well, this morning I applied the only degreaser I owned, which was Windex, to half the windshield. In a moment of staggering stupidity, I did it to the passenger side of the car, so that if it does in fact work, I will still be driving blind on my way to the freeway

    I am skeptical that it is oil on either the windshield or the wipers because the wipers work just fine in the rain. It's just this weird cold night condensation.

    I am not sure if other cars get it, but I imagine so - I will ask some of my coworkers.
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  6. #5  
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    Is there anything in the area that could be mixing with the rain ?
    Like, are you in a lot on three mile island?
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  7. #6  
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    When you park your car outside on a clear night, your windshield will cool off to below the ambient temperature by radiative cooling. Then if the air is saturated with moisture, it wants to condense on your windshield. After you drive for a while with the defroster on, your windshield warms up to ambient or above, and the problem goes away. In the rain, your windshield is the same temp as the surrounding air, so that's why the problem doesn't show up then.

    If you could manage to park in the shade, or maybe put a piece of cardboard on your windshield under the wiper, that could help by reducing the radiative cooling.
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  8. #7  
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    What is radiative cooling, and how does it cool the windshield to below the temperature of the air? I was actually trying to figure out why I had condensation at night at all. I had assumed that the car would be warmer than the air (since the temperature is dropping at night), but then it didn't make sense that I had condensation at all.

    It had occurred to me that the condensation went away after 30 seconds of the freeway because of the defroster instead of just being evaporated away faster by the 80 mpg (since gunning the car up to 80 mph is what finally gets the engine hot enough to warm the air coming out of the vent), but I did not think so because the entire window clears at the same time (instead of from the bottom up, as it usually does when clearing interior condensation with the air conditioner). But I don't know anything about the insulation properties of glass; perhaps the whole thing warms up more evenly on the other side of the glass, or perhaps it is a warmth issue, but it is the air moving by at 80 mph on the outside that more quickly warms the glass back up to ~air temperature.

    In any case, the next time this happens (hopefully tonight!) I will test this in two ways: 1) I'll warm up two hand-print shaped sections of the windshield by pressing my hand (hopefully not too oily) to a spot for a good minute or so (one section by pressing from the inside and one section by pressing on the outside) and see if that clears it up and 2) I'll leave the heat/defroster off completely.

    BTW, your explanation makes more sense than my theory! Excellent!
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  9. #8  
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    Quote Originally Posted by ribs
    What is radiative cooling, and how does it cool the windshield to below the temperature of the air?
    http://en.wikipedia.org/wiki/Radiation_cooling
    Radiation cooling is the opposite of what happens when the sun heats things up in the daytime. Things on earth emit infrared radiation into outer space, and that tends to cool them off. A cloud layer will reflect some of it back. That is why the temperature drops more sharply on a clear night.
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  10. #9  
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    I agree with Harold with one minor difference - the air doesn't have to be saturated. The windshield has to cool below the dewpoint, which might be quite a few degrees lower than the dry bulb temperature.

    I'm just jealous that you can say it's cold when it's 40 to 50 degrees. Here it's been 10F all day and going to drop to zero tonight.
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  11. #10  
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    update:

    No condensation last night. Supposedly it was the same temperature out as two nights ago, but, even though I could see stars, it was definitely not as clear out as the night before. Maybe this crazy radiative cooling isn't so crazy after all!


    So this was my completely non-scientific theory:

    I thought that water was condensing on my windshield (now I know the window gets cold enough through radiative cooling), and in particular condensing in the micropits in the windshield caused from driving too fast all the time. The pits are so small that water that is already in drop form (e.g. rain) cannot enter the the pits (no way for the water in a big blobby drop to displace the air inside). However, once water fully fills the micropits (from being deposited molecules at a time during condensation), it similarly resists being pulled out (suction, water tension).

    My belief was that the water collected in these micropits all over the windshield is what was making my wipers work so poorly even on the large drops - water tension is encouraging the surface water being pushed around by the wipers to adhere to bazillions of little pinprick spots all over the windshield, instead of letting the water bead up and be cleared off the windshield.

    After driving 30 seconds on the freeway, the windshield heats up enough or outside air is blowing by fast enough that it evaporates (or simply blows out) the miniscule amount of water in all the micropits while the wipers clear everything else away.

    What do people think of that idea? Totally stupid? I suppose I can test it by just looking with a strong magnifying lens. If there are no little micropits I should be able to heat the glass with my hand for a minute or two, and then turn the windshield wipers on and see if they leave a clear handprint shaped region on the glass

    The only thing that my silly theory doesn't mesh with is my 7th observation, that it doesn't happen with morning dew, although, again, I'm not actually sure about that one. I'll have to pay more attention on the cold, wet mornings...
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  12. #11  
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    I think your theory holds water.
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  13. #12  
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    Does the wind direction have any bearing (no pun intended) ie maybe a dry desert/land air from one direction, moist sea air from another, could account for any odd 'mysterious' days.
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  14. #13  
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    Quote Originally Posted by Bunbury
    I think your theory holds water.
    Har

    Quote Originally Posted by Megabrain
    Does the wind direction have any bearing...
    I don't think it's a weird wind - it's just a cold weather thing. I'm in the San Francisco Bay Area. It's not very moist, and it's not very cold, so it just doesn't happen that often. Since I posted this last Tuesday it still hasn't been cold or clear enough for any condensation to show up at all. Which is kind of nice, because I am cold-weather challenged. But on the other hand, I really want to see if 1) the half of the windshield I cleaned with behaves differently than the other and 2) if I can get the film to go away by heating the window with my hand. Oh well!
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