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Building Science

Defogging the Windshield with Psychrometrics

Understanding the properties of water vapor is useful in cars, too

A fogged-up car windshield is easier to clear if you apply psychrometrics. (Image credit: Energy Vanguard)

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.

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:

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…

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  1. User avater GBA Editor
    Martin Holladay | | #1

    You have accurately described the best way to remove moisture from a windshield when the water droplets are on the interior side of the glass. But in many climates, it's far more common for the water droplets or frost to be on the exterior side of the glass -- in which case the moisture content of the interior air is irrelevant, and the use of air conditioning or the tactic of opening the windows doesn't help.

    If the moisture or frost is on the exterior, the only thing that helps is heat (or manual removal by scraping or wiping).

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #3

      Yes, indeed that's true, Martin. This article addresses only the interior defogging of car windows, not the defrosting that's sometimes needed, so thanks for pointing that out here in the comments.

  2. Trevor Lambert | | #2

    This is great advice for people driving old, or entry level cars . For everyone else, step number 4 (turn on air conditioner) is going to be a problem. Most modern cars have automatic climate control and have no method of turning on the a/c independent of the temperature setting. You can turn it off (usually called econ mode), but the only other option is auto, or just nothing (i.e. turn econ mode off). In such a car, if you turn the temperature to the highest setting, is the a/c going to be on? Probably not; only in an incredibly sophisticated system in which it's monitoring interior humidity and glass surface temperature. In those cases, the system is probably smart enough to just pick the required settings to defog the windows. I have found in most cases, simply pushing the defog option manages to get the job done. I suspect it is doing exactly what is listed here, but without any operator oversight required.

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #4

      Trevor, most cars I've driven in the past twenty years automatically turn on the AC when you select the front windshield defogging mode. Whether the compressor runs or not will depend on the outdoor temperature because they're locked out below a certain temperature to prevent damage. (I don't know what that temperature is and it probably varies a bit from car to car.)

      As Martin said above, though, if it's the outside of the windshield you're trying to defog or defrost, you don't need the AC and can push the AC button to turn it off.

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