The Science of Defrosting

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.

Defogging and defrosting are two different things.

Defogging is relevant when your windshield is cold. In this case, moisture from the vent will condense on the windshield. When this happens, you will notice that (at least some of) the fog is on the inside. The solution is to run the A/C to remove moisture from the vented air. You may run the heater at the same time to heat the air. this is not contradictory; remember that the A/C is for moisture removal, not for cooling.

Defrosting is a different thing. Frost is generally on the outside of the windows, and if the temps are that low the air doesn't have much moisture in it anyway. You need heat, and A/C won't help any.

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?

One problem with A/C for defogging: If it's really humid outside, cooling off the window can cause water outside to condense on your cool windshield. So sometimes heat is necessary, too. Or just use your windshield wipers for that.

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.

Jonah: "My problem is "defogging", not defrosting. So now I know: turn the AC on full blast to dehumidify the interior of the car."
Turn on the AC _and_ the heat, as adjusted for comfort.
"But wouldn't cooling off the interior of the car cause more water to condense? (You know, like dew.)"
The condensation on the window is determined by the temperature of the window and the amount of water in the air. If you have water in the air, and cool it, when the dew pooint is reached, the water condenses. If all the air is cooled, it condenses everywhere. If the window is cold, it condenses on the window, as the air next to the window cools. In the summer, if the air is very humid, you can see the air next to the ac exhaust vents fog, as the cold ac air condenses the water in the humid air (I was very impressed when I first saw a jet of fog in my van.)
"Is there some de-humidifying element built into the AC?"
Yes, exactly. The cooling coil for the AC is usually about 30 degrees cooler than the general temperature. So, as the air goes over it, it is cooled and the water in the air condenses out. (A drain tube carries the water out of the passenger compartment. In summer, you can see the puddle of water on the ground underneath the car.)
"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?"
When you select Defog/defrost, the air is sent out vents right onto the window, rather than into the rest of the passenger compartmet. So, with the ac running on defrost, you have cool dry air going over the window, which will dry it. If you add heat, it will also heat the window, which will speed the evaporation of the fog. Running ac in the general setting will eventually remove the water from the air, but it will take a lot longer than directly blowing dry air onto the windshield.

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)

You guys are amazing. Thanks so much.

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.

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

If you live some place with extreme heat and humidity, say Florida, you might see some cloud formation when you first start the car and turn the A/C on max. It should dissipate in a minute or two as the cabin air gets replaced.

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.

Four controls on my car: blue - red lever for temperature; selective vent lever (windshield, floor, face, etc); fan speed; and inside-outside recycling air.

Fogging happens when I add the colder ouside air to the inside of the car when I open the door.

When I want to defog, I use red, windshield, and rapid and close the outside recycling vent.