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by I don't know how many times I've had my driving impaired by significant fog on the inside of my windshield. My car has the ability to blow air on the area, of course, but this sometimes actually seems to make the situation worse. Luckily, I recently was informed of the secret: hot, fresh air. In other words, the key to dissipating that annoying fog is to change the air temperature to as hot as it will go, as high as it will go - and to use fresh air by de-selecting the recycle air button.
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My late model S10 has a problem with that too - even on nice days with only slightly cooler air temps. It was particularly annoying because I have a 4 cylinder and every time I turned the defroster on, the engine's power was greatly reduced (it is designed to automatically turn on the A/C to dry the air with the compressor). I found that Rain-X Anti-Fog works well enough to almost completely eliminate fog. It's instructions call for a reapplication every 30 days, but it's so easy that's no problem. It's not so expensive and usually not hard to find in automotive and department stores (in the US anyway). BTW, if you haven't tried the original Rain-X glass treatment (goes on the outside of the windshield) you're missing out. It causes the rain to bead up on your windshield, and as you drive, the wind blows the drops right off. That's a way cool visual effect and works great (at a moderate speed ... >45 Km/h ~ 30 mph). Link for Rain-X Anti-Fog
Awesome - I had no idea they made a version for inside the car. I also agree that it's a cool visual effect to watch the water beads dance around. Especially on the flat window of a Jeep Wrangler. With Rain-X, I don't even need my windshield wipers.
It was particularly annoying because I have a 4 cylinder and every time I turned the defroster on, the engine's power was greatly reduced (it is designed to automatically turn on the A/C to dry the air with the compressor).
Although your vehicle is different, you can get around that if you're comfortable taking your dashboard apart and soldering. One of the wires leads to the A/C relay and is activated by either choosing A/C or defrost. If you splice a switch inline with that wire, you can have A/C-less defrost or A/C defrost just by flicking the switch. It's a handy feature on a Jeep because the only way you can get top/bottom air simultaneously is with the A/C selected which is pointless in winter. Having the A/C override switch lets you use the vents however you want in a Wrangler. But it would address giving you more control of your defrost in your S10.
Whenever this happens it's usually when the relative humidity is high, but the ambient temperature of the outside air has suddenly cooled down. This causes the humid air in your car to condense onto the windshield. Blowing hot air will always help, because when the windshield heats up, the water vapor can no longer condense onto it; but this takes some time and can initially make it worse as you noticed. I always found that turning the air conditioner on for a few minutes (even in up north here in the winter!) will almost immediately remove the moisture. An air conditioner is essentially a de-humidifier as I'm sure you've noticed from the comfort level they provide in hot humid climates. I wonder if recycling the air has any effect, seeing as the moisture is condensed out on the air conditioner's coil anyway? You'll have to experiment.
In the winter, our cars get terribly wet inside from the snow that gets tracked in and melted on a constant basis no matter how careful we are. Eventually it evaporates inside the car often causing the problem you described. (In extreme cases I've even had to scrape the frost off the inside of my windshield with an old credit card.)
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Dry Air by VnutZ :: NR8 :: on 23 January 2008
I don't know so much that the temperature itself has as much to do with it (although it contributes to condensation) ... but I think it's the forceful dry air that defogs a window. The dry air comes from the A/C unit.
I suppose you could always test the theory using a hair dryer on a mirror after a shower.