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Today while driving in the rain, I was not able to get rid of the fog. Outside temperature was 21 degrees and I had set my AC to 23 degrees with front mode on. This tric works sometimes (Outside temperature + 2 degrees) but not all the times.

I then randomly kept on changing the AC temperature from very high to very low. And suddenly about after 30 minutes 24.5 degrees worked for me. Got rid of the fog for a while as fog built up slowly on my windshield again. Next, 23 degrees worked (outside temperature was 20).

My question is, how should I calculate the AC temperature instead of trying random values!?

  • The dryer the air the faster it will work, that is why the AC compressor comes on during defrost, to remove moisture from the air more efficiently. – Moab Jun 6 at 15:50
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Evaporation isn't directly correlated with temperature it's a combination of air temperature and air moisture content.

In general, warmer air can hold more moisture, so assuming that air of a given temperature becomes warmer, it will allow condensation from your windshield to evaporate. The warmer the air is, the more moisture it will be able to hold, and the faster the condensation will evaporate.

However, air also has an upper limit to how much water vapor it can hold, and as temperatures climb the relative temperature will need to increase to provide the same rate of evaporation.

Air Water Content At 100% Humidity Across a Range of Temperatures

Was your car on recirculate?

  • If you were recirculating air from the car's interior, it was probably at a higher moisture content than outside air, due to water vapor in your breath.
  • If you were driving and pulling external air, the evaporation was probably caused by you going into a different microclimate which had dryer air.

While I can't/don't know enough to give you an exact number, the general rule of thumb is that relative humidity matters more than temperature, and a larger temperature difference will cause evaporation faster than a small difference. So for best results, turn on A/C (to demoisturize), turn off recirculate, and turn up the heat.

  • How does this apply when running the A/C or when the defroster is running (and the A/C is working)? – Pᴀᴜʟsᴛᴇʀ2 Jun 5 at 23:12
  • "So for best results, turn off recirculate and turn up the heat" - exactly! But it works only 50% of the times. Any suggestions to try when it doesn't work? – PC. Jun 6 at 16:06
  • Can you tell if your compressor is running when the defrosting is not working? – mike65535 Jun 6 at 16:22
  • How do I make sure of that? AC led is green, that probably means that the compressor is working... – PC. Jun 7 at 18:12
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Turn on your A/C, and turn up the heat.

Dry air will absorb the moisture from your windshield faster than moist air (obviously), and hot air can hold more water, so heating up the air will allow it to absorb some of the extra water on your windshield. Hotter is better, no matter what temperature you're already at.

However, that doesn't mean you should turn off your A/C. A byproduct of air conditioning is drying out the air, and since dry air can absorb more moisture from your windshield, you want the air blowing on your windshield to be as dry as possible.

You'll also (generally) want to turn off recirculate. In some cars, this is equated to the Max A/C setting (as opposed to the Normal A/C setting). The reason to turn off your recirculate is that you're assuming that the air outside your car is going to be able to absorb more water than the air inside your car (either because it's dryer or hotter than what's inside). Generally this is true, but if you're driving through fog and sometimes through rain, the air outside your car might be wetter and colder than what's inside, which will actually hurt your cause. In this case, it would be better to leave your car on recirculate. Often times though, especially in modern cars, the manufacturer does not allow this while on the Defrost airflow setting (the orange one that looks like a windshield with air blowing over it). Manufacturers will turn off the recirculate setting automatically if you shift to the Defrost setting. In this case, you'll simply have to leave your recirculate off, regardless of the conditions outside.

There's not a temperature that performs better than others; hotter is better. So turn up the heat, turn on A/C, and (generally) turn off recirculate.

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