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So i had this experience when driving in a cold night, where my windshield just got all blurry. I knew that on the drivers manual they say you should turn the hot air and soon it will go off, but my friend that was on the passenger seat said the cold air will work too. We turned the AC off and re-experienced the fact. I got the chronometer and and got the time of both situations, defog with hot and cold air and there were really small diference btween them. How can i explain this in terms of thermodynamics?

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  • $\begingroup$ You want a thermodynamic answer, but are you sure that is the correct reason? Most modern cars turn on the air conditioner when the defrost is selected. This is done to dry out the air. $\endgroup$ – hazzey Jun 13 '15 at 23:30
  • $\begingroup$ Too many variables in provided data. Does not compute. ... :-). (1) Hotter air has a greater moisture capacity and (2) will increase temperature of water layer and increase its vapour pressure so both enhance evaporation rate. (3) Reducing the air water content with the AC before heating it further increases it's water handling ability. | My observations over many years are consistent with the above.All these Your experimental results seem to differ but more data points may be required. $\endgroup$ – Russell McMahon Jun 14 '15 at 1:28
  • $\begingroup$ To test this, you have to make sure that all parameters are the same (temperatures, humidity, ...). $\endgroup$ – Karlo Jan 11 '16 at 8:28
  • $\begingroup$ The auto makers have automatically turned on the AC to aid defogging for decades because the general public can't figure it out. ( It lowers the humidity of the air). $\endgroup$ – blacksmith37 Jun 28 '17 at 0:30
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The reason the cooled air appears to work about as well as the heated air is that the cooled air is also de-humidified.

For the fastest de-misting, you want warm and dry air. Car owner manuals often tell you to turn on the air conditioner and the defroster at the same time in these conditions. The air conditioner cools the air, which forces it to dump much of whatever moisture it contained. The heater then warms the air again, but without adding any moisture back. The result has high capacity for obsorbing more moisture, so quickly removes the condensation from the inside of the windshield.

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  • $\begingroup$ Whoever downvoted this, please explain what you think is incorrect. I'm still not seeing it. $\endgroup$ – Olin Lathrop Jan 6 '17 at 15:01
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My experience over many years has been similar, except I find, the cold air from the air conditioner directed to the windscreen demisted the windscreen faster than hot air similarly directed.

Currently it is winter were I am. On cold humid nights when the car heater is turned on, the inside of the windows mist/fog up. By turning on the air conditioner while the heater is on allows for the car interior to be warm and for the windows to be mist/fog free, particularly if air directed to the windscreen.

While living in a tropical climate I had a similar experience, but in reverse; warm humid air, turn on the air conditioner and the outside of the windscreen misted/fogged up. That was easily fixed by clearing it with the wiper blades.

This is all due to the dew point of the air and water vapour mixture in contact with the car windows. “The dew point is the temperature at which water vapour condenses into liquid at the same rate at which it evaporates”. Sufficiently change the temperature of the air in contact with the windows by either heating it or cooling it and the dew point changes. Water vapour, from the air no long condenses on the windows and the windows do not mist/fog up and what mist/fog condensed out onto the windows evaporates back into the air.

This happens on car windows because they are the only part of the car which are in contact with the interior air of the car and the exterior atmospheric air. They are the barriers between the two. Car roofs have lining which acts as insulation and another barrier. Car doors and pillars have air gaps in them which acts as another form of weak insulation.

As to which is the better way to demist car windows,it depends on your circumstances at the time and whether you want to be cold or warm at the same time.

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I just came from a heavy rain in a cold day, and wanted to know if any body had posted about this. My facts are that using only the heater, is more delayed and besides that all other car windows get fogged, There after I left the heater on, and turned on the AC, and the result is amazing, not only the windshield cleared, but all the windows around, to the point that I didn´t need to use the rear defogger.

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For anyone who doesn't want to over-think every variable, the long story short is to use HOT air, regardless of any other circumstance. You want to get above the dew point, and dry it out. Warm/Hot air will do this the best.

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    $\begingroup$ They aren't "over-thinking" it; this answer is just "under-thinking" it. Hot air is usually the good choice; but it is not the best in any circumstance. In a scenario where your hot air is fully saturated with water vapour and your cold air is completely dry, there's a good chance the cold air is going to be more effective. A good answer shouldn't ignore important factors on purpose. $\endgroup$ – JMac Jun 27 '17 at 16:46
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    $\begingroup$ Also its based on experience, go back 20 or so years and all cars had heaters but not all had a/c, so people learnt to demist the screen using heat. Now most cars have a/c and there is a choice - in fact some cars suggest leaving the a/c on all year and just controlling the temperature as desired.@JMac $\endgroup$ – Solar Mike Jun 27 '17 at 18:36
  • $\begingroup$ @SolarMike I just realized how abused the term "air conditioning" is. Generally you hear AC and just think "cooling"; but there's more to conditioning than just cooling the air down, and when people simplify things people don't really consider that factor enough. $\endgroup$ – JMac Jun 27 '17 at 20:48

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