Then if the clutch is cycling and it's 2 degrees outside, where does all that moisture go? It would have to stick to the evaporator. After a few minutes the evaporator will ice over and no air will flow across it. And because it's so cold, it won't thaw either. Additionally, for R-134a to reach 2 degrees F, the pressure in the evaporator would have to be under 8 PSI. That's well below the cutoff point for the cycling switch in these cars. The switch cuts off at 22-28 PSI, which is between a 25 and 32 degree refrigerant temperature.
Originally Posted by http://en.wikipedia.org/wiki/Gas_laws
If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.
Which means that as temperature drops, so does pressure. The converse is true with regards to a refrigerant; as pressure goes down (or up) so will the temperature, that's the basic principle behind which A/C operates. The cycling switch opens when the pressure is below 22-28 PSI and closes above 40-47, so that means if the pressure in the system with an even pressure on both high and low sides is under 40 PSI, the cycling switch won't close and the clutch won't engage. Simple as that.
Again, a quote from the FSM:
"Ambient temperature below approximately 7 - 10*C (45-50*F), during cold weather seasons, prevents the A/C cycling switch contacts from closing.
This is due to the pressure / temperature relationship of the refrigerant and the requirement of the system pressure to reach 276-324 kPa (40-47 psi) to close the A/C cycling switch contacts."
The defroster works in winter by heating the air, not by removing moisture. As the temperature of the air increases, its ability to hold more moisture does as well. The warm air carries the moisture away from the surface with a higher moisture content than the air, and the moist air passes out of the car. What good is blowing freezing dry air at a frozen iced over windshield? Evaporation is much faster than sublimation...
And besides, I have verified all this myself. When I had my system charged after I replaced the compressor two years ago, it was mid April and the temps were still around 40 degrees. The A/C clutch wasn't cycling then, but as the temperature rose things started to run. Same goes for now. It's 15 outside and the clutch isn't moving.