Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Water and cars

  1. Oct 29, 2003 #1
    Sometimes when I am driving I make myself the following questions.

    Why do the front windscreen of cars steam up? What is the physical reason? and what is the best way to eliminate the steam?

    I have heard many theories about the cause and how to act. who knows the right theory?
  2. jcsd
  3. Oct 29, 2003 #2
    It's just condensation. If the inside of the car has moisture added to it (ie breathing), and the windshield cools as you drive, then in the end the moisture condenses on your cooler windshield. The best way to prevent this is to turn on your front defroster vents and turn it to warm to heat up the moisture so that it evaporates off. If you turn off the vents, the windshield cools again and the now warmer water vapor just condenses again.
  4. Oct 30, 2003 #3


    User Avatar

    Staff: Mentor

    Re: Re: Water and cars

    There are two ways of eliminating the condensation. This is one but under certain conditions it requires you to heat the car to an uncomfortable level. When it is around 60F outside and humidity is high, you get better results by turning on the air conditioning which removes moisture from the air.
  5. Oct 30, 2003 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Re: Water and cars

    What's really funny is the large proportion of people who do not know that the best way to deal with windshield fog is to turn on the air conditioning. I've seen people go through very elaborate routines, involving cracking windows, turning on the fan full blast on the hottest setting, then on coldest, then closing the windows, then turning it back hot, etc. etc.

    Turning on the A/C deals with it quite effectively in two seconds.

    Newer cars actually automatically turn the A/C compressor on when the knob is in the defrost position.

    - Warren
  6. Oct 30, 2003 #5
    I have heard the two theories, the Air conditioning and the heat. The best results I had were with A/C, however the drivers book of my car sugest to use the front heat at maximum power.
  7. Oct 30, 2003 #6
    Also, if your car has a setting to allow you to control whether the air is brought in fresh or recirculated, this can be combined to maximize the effects of the other two techniques.

    On cooler days (below 50 deg F ), open the outside air setting to allow as much fresh air as possible and turn the heat on full. This evaporates the humidity into the air and then exchanges the humidity laden air with air of lower humidity. Even if the 50 deg F entering air has nearly 100% relative humidity, when it is heated to just 70 deg F the relative humidity drops to just 40% in the car.

    On warmer days change the setting to recirculate as much of the air as you can and turn on the A/C (there is a default minimum amount of outside air in newer cars, but you can usually drop it to this amount). This will dehumidify the air that's in the car more quickly.
  8. Oct 30, 2003 #7
    Re: Re: Re: Water and cars

    Yeah, I should have explained that it depends on the weather outside too; whether to use a/c or the heater.
  9. Oct 30, 2003 #8


    User Avatar

    Staff: Mentor

    Re: Re: Re: Re: Water and cars

    Even better would be a/c with reheat. You'd get the same reduction in humidity while increasing the surface temperature of the glass (not to mention making yourself warmer when it is cool out).
    Important point - air generally comes off the coils at 55F and 100% relative humidity. So below 55F (50F is close enough) air outside is ALWAYS less humid (absolute humidity) than air inside the car.

    A/C isn't necessarily as intuitive as people would expect. Thats why I get paid the big bucks for it.
  10. Oct 30, 2003 #9
    Re: Re: Re: Re: Re: Water and cars

    I agree, especially in a car because heat is a byproduct of the car's operation and would not have a negative impact on efficiency (the usual reason to avoid reheat).
  11. Oct 30, 2003 #10


    User Avatar
    Science Advisor

    I just keep 200 pounds of drierite scattered on the floor of my car.

  12. Oct 30, 2003 #11
    The best way to deal with it is by applying FOGX. I also use rainx and above 25mph have no need for wipers.

  13. Nov 1, 2003 #12
    In most cases the winshield is clear of dew when you get in your car. On really crisp days (really cold) the last thing you wanna do is breath. Doing so will put you at a standstill from lack of visibility. The first thing I do is open the window and send my cloud to the outdoors rather than on my windshield. I then start my car and go. Getting the car moving allows you to close the window to a crack where you can breath easy and expect that breath to be sucked out the cracked window. In most every case ... your heating system will in short order allow you to close the window.

    There are many different weather conditions. Some situations will give you a fogged window when you turn on your system. This would be due to a variation in temp. The outside temp can change rapidly while the overall mass of your system is lagging temperature wise. This difference can cause the ripe conditions for that cumbersome fog.
  14. Dec 30, 2003 #13
    Summary of discussion & question


    I'd like to summarize the discussion up to this point (liberally borrowing from previous text without use of quotation marks), and then add a clarification question.


    Two ways to eliminate windshield condensation (fog):

    1) In cold weather (below 50 deg F):

    Turn on your front defroster vents, turn the air temperature to its highest setting, and turn the fan to maximum, to quickly heat up the moisture so that it evaporates off the windshield. Also, set the system to bring in fresh outside air, so that it exchanges the humidity-laden air with air of lower humidity. Even if the 50 deg F entering air has nearly 100% relative humidity, when it is heated to just 70 deg F, the relative humidity drops to just 40% in the car.

    2) In warmer weather (e.g., 60 deg F or above) with high outside humidity:

    Turn on your front defroster vents, turn on the air conditioning to its coldest setting, which removes moisture from the air, and turn the fan to maximum. Also, set the system to recirculate air inside the car so that the AC can dehumidify the air that's in the car more quickly.

    I hope all the above is correct.

    Here is my follow-up question. The more interesting case seems to be in warmer weather with high outside humidity (e.g., a rainy night in summer). In this case, is the use of the AC for the reason that (a) the cold AC air warms up when it hits the warmer windshield, and this newly warmed air is therefore able to hold more humidity, thus evaporating the condensation on the windshield, or (b) the operation of the AC involves a heatpump, which removes moisture from the air, or (c) both of the above?

  15. Aug 27, 2004 #14
    In 1993 I began a lawsuit against Lexus/Toyota because the interior of the windshield in my brand new LS400 would sometimes suddenly and spontaneously (seemingly) fog over so completely that I had virtually no forward vision.

    Over more than 50 years of driving I thought I had learned how to deal with a fogged over windshield.....

    So with the Lexus I followed my instincts and to clear the initial minor fogging occurances I would always turn up the heat and the blower and switch the system to the defrost/defog/demist mode. In virtually every instance that would immediately result in even a greater level of condensation. So the car sat in our garage for the remainder of the winter of 92-93.

    At the trial an expert witness for Lexus testified that my Lexus had spontaneously fogged over the windshield due to an operational mistake on my part, "turning up the blower speed".

    I sat there in stunned silence just barely restraining myself from getting up and shouting....

    "What, You IDIOT!"

    But you know what?

    He was right.

    The automatic climate control systems on Lexus and Toyota vehicles rely EXCLUSIVELY on the ability of the A/C to prevent and/or remove condensation from the interior surface of the windshield. The A/C is used to dehumidify the incoming (fresh) airflow and thereby prevent the cabin atmosphere from reaching the dewpoint of the cold interior surface of the windsheild. If the balance is somehow upset, someone wet and sweaty entering an already warm, dry and setpoint acclimated Lexus, the windshield will likely fog over but Lexus believes the A/C can still produce enough dehumidification capability and vapor pressure to quickly evaporate the condensation.


    They are wrong. TOTALLY.

    But let's go back to the courtroom for just a moment.

    "turning up the blower speed was the mistake that Mr. West made that caused the windshield to fog over more seriously than the initial level of fogging he was reacting too."

    To make a long story short...

    If you put a warm quart bottle of milk in a 40F refrigerator and then remove it 5 minutes later you will in all likelihood still have a warm bottle of milk.

    Therefore if you turn the Lexus blower speed up as high as it will go the incoming fresh airflow will move through the A/C evaporator (assume 33F) so rapidly that it will not be significantly chilled. To dehumidify this airflow its temperature must be reduced to dewpoint, or something very close thereto.
  16. Aug 27, 2004 #15
    i always thought it fogged up because of unequal temperatures between outside and inside the car (like condensation on the outside of a cold cup of water). so i open the windows to equal the temperature. it eventually goes away but takes quite a few minutes with the wipers on.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook