For a properly functioning auto, what's the quickest way to defog the windshield? There are lots of air control options - hot vs. cold air, exterior vs. recirculated air - but which are the most effective (assuming the auto is functioning correctly)?

  • You've been asking quite a few foggy window questions. Does your car have air conditioning? – Scott Hillson Nov 6 '12 at 2:04
  • Yup.(I figured that both of these were different questions.) – KatieK Nov 6 '12 at 3:34
  • Yes they are certainly different questions, I'm just wondering what the root cause is of all your fog. Let us know when it started, if it happens during a certain time of day, are there any strange engine noises? – Scott Hillson Nov 6 '12 at 18:37
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    @hillsons - The root cause of my fog is a heater core leak (details in other question) as diagnosed by the shop. – KatieK Nov 6 '12 at 18:39

It will depend on your car, the outdoors humidity, the air temperature and the engine temperature. As an example, for me on a cold icy morning starting with a cold engine:

  • aircon on
  • fan on low
  • use external air, not recirculator!
  • front windscreen demister (covers wiper rest area)
  • rev engine until at working temp
  • then put fan on high

But in high humidity areas in the summer I would use:

  • aircon on
  • fan on high
  • use recirculator
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  • Just to be clear, you use air conditioning on cold icy mornings? – Tim Nov 6 '12 at 14:59
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    Definitely - it dries the air very quickly. Without it I have to wait until the turbo heats up to properly dry incoming air. The air still won't be warm for the first few minutes, but at least it is dehumidifying. – Rory Alsop Nov 6 '12 at 15:06
  • Interesting, where I live the humidity in the air is so low during the winter that I typically don't worry about that. I just worry about frost and ice on the windshield. :) – Tim Nov 6 '12 at 15:49
  • Heh - this is Scotland :-) – Rory Alsop Nov 6 '12 at 16:19

Hot air from the exterior with your a/c on is the quickest. Though in general having the a/c on is the most important piece, as it removes moisture from the air.

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My old Fiat Marea Weekend had instructions for defogging the windshield on the inside of the driver's visor:

  • Turn the blower on max
  • Turn the heat on max
  • Turn the recirculation off
  • Put down the visors

I was wondering about that last bit, so I've asked a follow-up:

Does putting down the visors help defogging the windshield?

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