Um, it's dehumidifying the air. Your windows can't fog up if there isn't any moisture in the air. Sure, the wamr or cool air will help, but taking the moisture out of the air (via the A/C) will work more efficiently at keeping your windows from fogging up.
So why not just run the A/C? I mean, why would you have to run it in defrost mode so it blows on the windshield? It's a combo of humidity and the difference in temps that cause the windows to fog. Ever notice when you open a freezer door at the grocery store, how it quickly fogs over? You've just introduced warm air and humidity to the door. Close the door and let it get cold again and it will clear up momentarily.
Either method works (heat or cold) and sometimes one works better than the other. But, whatever - it's really not worth debating.
Well you do need the temp difference as well. This is why the heater system was designed so that all air passes through the evaporator in the A/C system so that you could run the heat in the winter and still get the A/C to dry the air for you.
So why not just run the A/C? I mean, why would you have to run it in defrost mode so it blows on the windshield? It's a combo of humidity and the difference in temps that cause the windows to fog. Ever notice when you open a freezer door at the grocery store, how it quickly fogs over? You've just introduced warm air and humidity to the door. Close the door and let it get cold again and it will clear up momentarily.
Either method works (heat or cold) and sometimes one works better than the other. But, whatever - it's really not worth debating.
It's about surfaces being warmer, colder, or the same temp as the moist ambient air and how that effects the processes of condensation and it's opposite counterpart, evaporation. These processes are all about matter (h20 in this case) changing state from gas to liquid.
Here's how it all works. If the surface in question is the same temp as the moist air then niether process occurs to a noticable extent. For example, the Freezer door that has remained closed.
If the surface in question is the colder than the moist air then the moisture form the air slaps itself on to the cold surface in the form of liguid water, that is the process of condensation. For example, the freezer door that is held open.
Finally, if the surface in question is warmer than the moist ambient air then evaporation will occur from that surface. (Assuming there is moisture on that surface and that the ambient air is not already saturated with moisture.) For example, a hot roadway dries quickly after a brief rain shower. (Sorry, I couldn't make the freezer door analogy on this one. But think of the opposite roadway scenario when dew forms in the morning on a cold roadway as the air becomes warmer than the road.)
Why the AC alone won't do a good job to clear the winshield? Well, while the dry air from AC will encurage evaporation, if it is also making the windshield cooler than the ambient temp will it will encurage condensation to form. AC alone may actually defog the windshield given the right conditions but it is by no means the best way to encurage evaporation. There's a reason the AC is designed such that it's air does not come out of the vents directed toward the windshield.
Think of how the rear window defoggerdefroster works. All it does is heat the glass with a warm embedded wire. The heat alone encurages evaporation and at the same time discurages condensation.
So to put this dead horse to rest. If you wanna clear your windshield of fog or frost, you should blow warm, dry air at it. I think someone said this on the first page of this thread but I've had too much coffee and went on this rant anyway lol. I hope my participation clears up this interesting but nearly meaningless debate.
Later
A thank you goes out to my Chem 101 teacher, Berny Brennan, at SBCC. I couldnt' have explained that without your teachings lmao!
One last note and I'll let 'er die as well. Here in FL, often the inside of the car is warmer than the outside when a quick rain shower or T-Storm blows thru, and I've found the A/C works better in this senario.
But thanks to all for the great info and a friendly debate (quite rare out here).