The Frontal Cortex

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Generally, the defroster setting has both the heat and the ac on. The heat warms the window to reduce condensation on the inside, or melt ice on the outside (takes a while for the heat to diffuse through the glass). The ac removes water from the interior air, so there is less to condense on the inside of the window.
Sometimes you have to explicitly turn on the ac and heat at the same time. Some cars, like my Volvo, automatically turn on the ac when defrost is chosen.
Run them both at full blast until the window is clear.

Posted by: DaleP | December 4, 2006 10:08 AM

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These are both very helpful. My problem is "defogging", not defrosting. So now I know: turn the AC on full blast to dehumidify the interior of the car. Thanks a lot.

PS. But wouldn't cooling off the interior of the car cause more water to condense? (You know, like dew.) Is there some de-humidifying element built into the AC? And if the point of the defogger is really just dehumidifying, then why does it blow air onto the windshield? Wouldn't just turning the AC on have a similar effect?

Posted by: Jonah | December 4, 2006 10:59 AM

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Is there some de-humidifying element built into the AC?

Yes, it's called a "refrigeration coil." Moisture will condense out on a cold surface. That's why you will notice recently parked cars dripping in the summer time, and why windown air conditioners also drip. Even your refrigerator has a drip tray for condensation that forms on its coils.

And if the point of the defogger is really just dehumidifying, then why does it blow air onto the windshield? Wouldn't just turning the AC on have a similar effect?

Dry air will help to remove any condensation which has already formed. It would be easy to avoid this situation if you would stop breathing. This is also where the A/C + heating comes into play. You first cool the air to remove moisture, then heat the air, which raises its moisture capacity (dew point).
You can buy products for the home which are known as dehumidifiers. They have a refrigeration coil, a fan, and a drip line. Cooling the refrigeration coil causes moisture to condense out of the air, and it goes down a drain. The refrigeration of course takes energy, and there is a warm coil as well, so a dehumidifier actually heats an indoor space. The difference between a dehumidifier and an air conditioner is that in the latter, the warm coil is placed outside.

Posted by: Mustafa Mond, FCD | December 4, 2006 12:09 PM

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PS. But wouldn't cooling off the interior of the car cause more water to condense? (You know, like dew)

Yes, but that should happen in the A/C unit, not in the car interior. (mostly)

Posted by: Mustafa Mond, FCD | December 4, 2006 12:13 PM

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Unless things have changed, all the cars I have had defrost means divert hot air to the window vents, and AC, means run the air through the AC first. Only is vertain difficult situations, heavy snow/rain get sucked in and evaporated raising the dew point of the air coming out of the blower, have I needed both AC and defrost at the same time.
I had one car (80's model) that had reall stupid engineers. Turning on defrost turned of AC, and vice versa. So in those difficult weather conditions you had to keep wiping off the windshield. Hopefully Detroit learned that lesson.
On a tougher note, I was once driving in Wisconsin, it was 14F and there were supercooled fog droplets in the air. I was driving a 79 Suburu, and the defroster was not strong enough to keep the windshield above freezing. I had to pullover and scrape about every mile. People with V8's had no such problem.

Posted by: bigTom | December 4, 2006 2:14 PM

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I've never seen a defogger that does the AC thing. I suppose it's plausible, but only in a limited set of conditions, and only if applied very selectively.

On my planet, defrost is typically limited to the back window, and involves actual heating elements in/on the window surface. (You could do the same thing with a layer of ITO or something in the windshield, but I don't believe anyone does). Defog/defrost on the windshield circulates air (presumed to be above its dew point) against the windshield, with the intention of both absorbing excess moisture and heating the windshield so that it is not locally the coldest surface around. (And yes, I have also had the experience of sleet pelting against the windshield and interior defog/defrost cranked up full blast to prevent the whole thing from icing over.)

Using the AC in a situation like this seems perilous to me -- you're pretty much guaranteed the air coming out of the AC will be at or near its dew point, and you'll be cooling the glass so that it may fog up on the outside as well. If you had power to burn you could first cool the air to dry it and then heat it to lower the relative humidity further, but that would require a very particular arrangement of ducting not useful for most other purposes.

Posted by: paul | December 4, 2006 7:47 PM