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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 (emphasis mine)

I can come up with reasons why this should help and I can come up with reasons why keeping the visors up would be better. So which is it?

Does putting down the visors indeed help in defogging the windshield?

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I think the visor is right…

When you put the visors down (or at the "right" angle) they stop the airflow from the defroster vents from being able to move across the headliner. They may even deflect the heated air down in a way that causes a "rotor" of warm air in the vicinity of the windshield. This both helps to warm the windshield and gives more time for the heated air to pick up moisture.

By heating the air you effectively lower the relative humidity and increase the capacity of the air to hold moisture, so I think this is a win in both ways.

I've never heard of this before, but it makes sense. Seems like it would also be reasonably easy to test – put the visor down on one side and leave it up on the other and see if there is a noticeable difference in the rate at which the two sides of the windshield clear.

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I would assume the visors would help to create / trap a column of slightly warmer air at the top the of screen instead of is dissipating across the headliner and into the cabin of the car.

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    Like I said, I can come up with pros and cons. This the main pro I have. But does it actually work like that? Perhaps letting the air flow freely to carry off the humidity is better.
    – SQB
    Dec 20, 2016 at 14:25

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