I have an '01, and I can turn off the A/C. My wife had a Nissan that always ran the A/C in defrost mode (with no override) and it drove me nuts.

In theory, yes it does dehumidify the air - SOMETIMES. It makes sense if the wet bulb temperature is around 40-70F. But, if it is 10F out and you are trying to melt ice from your windshield, I don't want my compressor running. When it is that cold, the dewpoint is below the evaporator temperature, therefor no moisture is being removed from the air by the A/C. An override would be helpful here.

But, some manufacturers like to tie the A/C and the defrost together to keep the compressor cycling throughout the year to prevent seals from drying out. It's a good idea to run the A/C on any car at least a couple of times a month to keep things working properly.

 

I have always solved this problem this way. BTW, I used to install aftermarket a/c systems on cars in the 1980's so i can't claim to know if any modern ecu controlled systems will complain. The compressor usually has a wire that engages the clutch to enable the a/c. Cutting this wire and adding a toggle on/off switch in the cockpit is a good way of manually controlling things. You can usually find this circuit under the dash. I am also unhappy with the 02 arrangement especially the bug that engages the a/c by default if you have used the a/c in a previous setting. I will probably check things out this winter. If anyone traces down this wire physically, chime in!

 

Both my earlier Hondas let me decide when to engage the A/C. I always preferred that to the way other cars did it automagically, whether you want it or not. I was disappointed that Honda reverted to the "you're too dumb to know how to do it, so we'll just take care of it" mindset. Especially in this car. Oh, well.

Some cars don't allow the A/C to actually run below some outside temp - like 40F or so since the compressor doesn't run too well. Does anyone know if the S2000 behaves this way? Can't say running the A/C when it's -20F or -30F makes a lot of sense. (BTW, at low temps, when you add any heat the relative humidity is very low - no dehumidification is needed.)

Tony

 

You are correct - the ability of cold air to hold moisture is very low. So, if you hold the moisture content constant and raise the temperature, you are lowering the humidity. The problem is, the moisture content is rising when you are in the car because of your breathing (it's worse with with a passenger). The relative humidity can actually apporach 100% very quickly in cold weather - the air can't hold much moisture, so if you add a little bit from breathing it will quickly fog the windows.

To get technical, the best way to clear the windows is to raise the temperature in the cabin as quickly as possible to get the dewpoint up higher than 45F or so. Then, turn the A/C on to wring out whatever moisture it can.

Hope this helps.