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Defogging Your Car Windshield with Psychrometrics


A lot of people don’t understand the best way to defog their car windshield in wet, cool weather.  About 30 years ago, I was on a group trip on a hired shuttle bus.  The outdoor temperature was probably in the 40s Fahrenheit and it was raining.  Perfect conditions for fogging up the interior glass, right? 

Unfortunately, the shuttle driver didn’t understand psychrometrics, the study of the mixture of dry air and water vapor.  She chose setting for the bus’s HVAC system that not only didn’t work well, but made us all uncomfortable for the whole five-hour trip.  She did have one setting right, though.

In the decades since then, I’ve figured out the optimal way to defog a car windshield on those days and I’ll share it with you here.  I’ve written about all the principles behind the optimal method in this blog before so let’s put them together in a lesson on applied psychrometrics.  And at the end, I’ve got a video that shows you exactly what’s going on and why the settings work.

1.  Cold surfaces condense water vapor

The air in the car is fairly isolated from the outdoors.  If people are inside breathing, the humidity can rise and the dew point can easily go above the temperature of the front and back windshields and the side windows, too.  That’s when those glass surfaces fog up.

As you know if you’ve been reading this blog for a while, that gives you two pathways to fix the condensation problem:

  1. Warm up the windows.
  2. Reduce the humidity, and dew point, inside the car.

2.  Cold air is dry air

The amount of water in the vapor phase depends on temperature and availability of water.  As the temperature goes higher, more water from liquid and adsorbed phases goes into the vapor phase.  Conversely, as the temperature drops, water vapor condenses out of the vapor phase and the dew point goes down.

And that’s why it’s better to talk about the actual quantity of water vapor with dew point (or humidity ratio for the psychrometric geeks out there).  Cold air can have very high relative humidity.  It can be 100%.  But I’ve shown before that when you bring cold, 100% relative humidity air inside and warm it up, the relative humidity of that air drops tremendously.  And that’s why we say cold air is dry air.

The best way to defog your car windshield

Now we know enough to choose the proper settings.  Ready?

  1.  Set the fan speed as high as it goes.
  2.  Set the car’s HVAC system to the highest temperature setting.
  3.  Set the air direction to blow on the windshield.
  4.  Turn the air conditioner on.  (Yes, really!  Remember, you’ve got the temperature set to high.)
  5.  Set the fresh air/recirculate option to fresh air.
  6.  Open the windows a bit if it’s not raining or too cold.

Those settings will optimize your windshield defogging.  You obviously want to move a lot of air and you want high temperature air so you can heat the windshield surface above the dew point and evaporate the condensed moisture that’s already there.  These settings keep the moisture inside the car but put it back into the vapor phase instead of fogging up your windows.

The reason you want the air conditioner on is that it removes moisture from inside the car.  Once you get that water into the vapor phase, the AC can condense it and remove it from the inside of the car.

You want the fresh air setting because cold air is dry air.  That helps you send humid air to the outside and bring in colder air that has a lower relative humidity when you heat it up.  And that’s the same reason you want to open the windows a bit if you can.

Now, here’s that excellent video I promised.  It’s by a guy named Mark Rober, a mechanical engineer who worked at NASA for nine years.  But he’s not your typical engineer using techno-jargon.  He explains all this really well.

So there you have it.  If you’ve got fogged-up windows in your car, now you know what to do…unlike my shuttle drive 30 years ago.  I don’t know all the settings she used and maybe there was a lockout on one part of it, but she made us all freeze on that long drive from Tallahassee to Clearwater that chilly day.  She did turn the AC on but she made the mistake of thinking that a low temperature setting would work better than a high temperature setting.  She may well have used the recirculate mode, too, because I think she still had a window fogging problem most of the way back to Clearwater.


Thanks to Anthony Cox for sending me the link to the Mark Rober video.


Related Articles

Make Dew Point Your Friend for Humidity

Two Rules for Preventing Humidity Damage

Cold Air Is Dry Air


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This Post Has 21 Comments

  1. It seems that most modern
    It seems that most modern cars have a ‘climate control’ system which will only activate the AC compressor if the temperature in the car is above the temperature selected. Consequently, selecting Heat and AC will not activate the compressor.

    All the other suggestions are excellent.

    1. JA, I’m not sure what kinds

      JA, I’m not sure what kinds of cars you’re referring to, but my current car and all the rental cars I’ve had in recent years do allow simultaneous heat and AC.  In fact, most cars now have controls that automatically turn on the AC when you select the defogging option.  Maybe the confusion is about the setting I was talking about.  I wasn’t talking about selecting “heat” vs. “AC.”  I was talking about turning the temperature setting up high while the mode selected is AC.  Are you sure yours doesn’t do that?

      1. All of the cars that I have
        All of the cars that I have owned in the past decade do automatically turn on the AC and direct the air to the windshield and even side front windows when you switch to Defrost/Defog mode. These are thermostatically controlled heating/cooling systems, so you can raise the temperature to help with defrost/defog by reheating the air. So the car manufacturers have figured this out. I think that most drivers do not know that the AC is running in defrost mode, but don’t care either. I have a few engineering friends that know this, and don’t like the car making this decision for them. (By the way, I am sure that the compressor cuts out on low pressure if it is too cold outside since the additional dehumidification is probably not needed.)

        1. Roy, that’s been my

          Roy, that’s been my experience, too.  And I’m one of those people who gets annoyed by the car making that decision for me.  Sometimes I just want to blow heat up high in the car and don’t need the AC running.

          1. Allison, if you want to just
            Allison, if you want to just blow heat up “higher” in the car, just readjust your dash vents! But why do you want to do that anyway, since “heat rises” on its own? I have always hated that expression because it should be “hot air rises”, at least when it is near cold air. “Heat” goes from hot to cold, regardless of gravity. Oops, kind of got off topic here.

          2. Roy, I guess I didn’t really

            Roy, I guess I didn’t really say what I meant.  Sometimes the windshield’s just a little bit fogged and I don’t want to use the AC because I don’t want to use extra gasoline for the AC.

        2. Here in the UK we have cold
          Here in the UK we have cold damp days and rain at 4c. This calls for the air con compressor to be running. Modern cars however disable the compressor below 5c. Ford do this on my car and their response is you don’t need air con when it’s cold! For demisting you do.

  2. NONE of this is obvious! I
    NONE of this is obvious! I hope I can remember it all next time I need defrosting. Thanks for spelling it out.

    1. You’re welcome, Mark.  Glad

      You’re welcome, Mark.  Glad to help.

  3. I grew up in New England. At
    I grew up in New England. At the time AC was a rare luxury item in cars. It wasn’t until much later that I learned the role and value of car AC for defogging windows during chilly weather.

    Merely heating the glass takes a long time, especially since the glass loses heat to the exterior, especially fast when the car is moving.

    Blowing AC-dried air onto the glass is 10+ times faster since the glass itself need not be warmed for the moisture to be removed.

    1. Curt, I thought everyone knew

      Curt, I thought everyone knew about using heat to defog windows but my shuttle driver on that trip in Florida 30 years ago proved me wrong.  She knew (or guessed) that AC would help but thought that if the AC was running, she had to set it to a low temperature.

  4. Sigh…..once again the video
    Sigh…..once again the video repeats the same lame description of the relationship of water vapor and air we’ve been hearing for decades, but of course the end result happens to work the same under the conditions stated. I was just hoping a scientist might go a little different direction in his description of vapor pressure and phase change, but I guess his audience was better served by keeping it simple.

    Air has no more ability to “hold” more or less water than it does to hold more or less nitrogen, oxygen, argon, or krypton……or any other molecule. There’s more water in vapor form in warmer air because there’s greater water activity; net evaporation is greater than condensation (though both are happening even in a warm air mass). OK, OK, it doesn’t really matter in a practical sense, but the wringing towel analogy really throws me in a tizzy.

    It would have been way cooler for the science guy to have explained condensation on the AC coil** and the effect of increasing the discharge air temperature using the car’s heater to shift the ratio of evaporation to condensation of water in warmer air. But no…let’s just keep telling ourselves that somehow the nitrogen/oxygen/other gas mixture that forms our atmosphere somehow “holds” water. Yes, more water in vapor form might be coincident in warmer air, but not because it’s being “held.”

    **AC Coil – The AC industry calls the indoor or cooling coil the evaporator coil because the system uses a metering device to portion higher pressure liquid refrigerant into the lower pressure coil where it evaporates. This phase change requires a lot of energy, which the refrigerant absorbs from the air passing over the coil. The coil is typically held below the dew point of the air flowing over the coil, which allows net condensation of water vapor int he air.

    So, the evaporator coil condenses water vapor….Maybe a little tough for the little video and probably not nearly as entertaining. Of course the outdoor condenser coil is also an evaporator, and in a heat pump it switches back and forth….evaporator becomes condenser, which evaporates and condenser becomes evaporator, which condenses until it goes into defrost mode which again reverses the process. ACKKKK!

    By the way, I’m not at all shy about taking control of the climate control system in shuttle busses. I’m more polite about it in Ubers so I don’t lose my 5-star customer rating 😉

    1. Thanks, Mac.  We’re

      Thanks, Mac.  We’re definitely on the same page regarding air’s inability to hold water vapor.  I wrote about this a few years ago in an article titled, Are You Making This Mistake With Humidity?    (That article annoyed a couple of readers, who accused me of obfuscation because I tried to make the case using condensation on grass, but my larger point was still valid.)  In the article here, I did talk about air with relative humidity but was careful not to associate any holding capacity with the air.  And I wrote, “The amount of water in the vapor phase depends on temperature and availability of water.”  As you say, the dry air components have nothing to do with how much water goes into the vapor phase.

      And yeah, Rober didn’t really explain why the AC setting helps.  Still, I thought it was a good video for general audiences.

      1. Yep, I cite your article
        Yep, I cite your article frequently, and agree that the video is generally useful and hopefully sparks curiosity in your readers. Yours is the most valuable blog in cyberspace and deserves to be shared with all who enjoy learning about the way our world around us really works.

  5. Whether or not it indicates
    Whether or not it indicates it, every car I’ve owned built since the late ’90s turns on the AC when on defrost.

    And by the way, if it’s cold enough for condensation, PLEASE let your car warm up for a minute or two before turning the AC on. AC parasitic loads can be as much as 10%, and the last thing your cold engine needs is more load. Besides, you won’t get any heat until your engine comes up to temperature and starts circulating warm water into the heater core.

  6. Most vehicles sold in the USA
    Most vehicles sold in the USA since the late ’80s automatically turn on the AC when you turn on the defrost setting – EXCEPT when the air temps go below a certain pre-set (varies between makes/models but is typically just above freezing). The problem with turning on the AC is that the conditioned air only goes through the passenger face/foot vents – which is fine for dehumidification but can preclude the defrost air blast on the inside of the windshield as these functions are sometimes connected to each other. A stop-gap solution if this is a great issue is to have an auxiliary fan plugged into a 12V outlet and directed to the windshield (maybe by sitting on the dash). Now when you change the passenger settings the cabin air can be directed to the windshield instead of trying to get the OEM controls to cooperate.

  7. I was once intrigued by a
    I was once intrigued by a separate mystery of car windshield physics, whose answer came to me as I starting thinking about building science more regularly. I used to wonder why on some cold winter mornings one of our family cars would have ice on the windshield but the other car windshield would be dry, when both are parked next to each other and conditions seem mostly the same. The way our driveway is set up, the one that does not have ice in these situations is parked in front of our house, the one that does have ice is parked in front of our yard. I’ve now realized that the windshield of the car in front of our house must be partially shielded from the cold night sky, as our driveway is on a hill and the silhouette of the house looms above and roughly normal to the angle of the windshield. Thus, that windshield experiences less radiative heat loss and does not get quite as cold as the windshield of the car with only open sky in front of it, which in some conditions can mean the difference between condensation or not.

  8. Excellent. I can’t tell you
    Excellent. I can’t tell you how many people I see this time of year driving with fogged up windows. It’s amazing that this isn’t common knowledge.

  9. Ahhh Good and Honorable Folks
    Ahhh Good and Honorable Folks,

    As the driver of EVs now for 6 years(done with petroleum fur the sake of Global Warming and National Security…Hmmmm last year we got 11% of our Oil from Saudi)…I really like the crack the two front windows a bit(in Georgia) to allow the outside air and the just inside the windshield air be one the same? Using the HVAC in EVs visually penalizes the range per Charge. Adding the wind deflectors seems like a good idea to get that outside air inside and keep the rain out!

    1. Andrew Lane, where do you get
      Andrew Lane, where do you get your electrons from? PVs or Dinos? There is no escaping it, if we burn energy outside of independent alternative generation, we are using fossil fuels. Possibly more efficiently but fossil fuels nonetheless. And if your utility is hydro power, additional load usually consumes electrons that would be exported on the grid to displace the most carbon rich generation elsewhere.

      1. Indeed Chris…currently I
        Indeed Chris…currently I get my electrons from the Grid which is infinitely more efficient than burning anything under the hood! The sheer size of the Grid Generation clearly makes anything one does under the hood quite small and horribly inefficient in comparison! Indeed with EVs that have HVACed Batteries there is also no silly sad messy maintenance with which to contend. Ahhh and with EVs many Car Dealerships give away free fuel some even 24/7 as electricity is so cheap some folks just give it away. Ohhh and when was the last time someone spilled electricity in the Gulf of Mexico, Alaska, or the Kalamazoo River in Michigan? Of course EVs are the only cars that get more efficient as they get cleaner as the Grid gets cleaner! Hmmmm oil burning cars seem to get worse with age!

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