I found this online:
"The defroster automatically and forcefully turns on the A/C and blows in outside air. Unfortunately, it knows better than you and you have no choice.
The designers figured 99% of the time blowing fresh dried air is the optimum way to defrost the windshield and getting complaints like yours is better than getting complaints from lots more people who leave the recirc button on and can't get their windshields to defrost quick enough."
"Outside air, especially on a cold day, is always lower in humidity than what is inside the cabin, because you're inside, presumably exhaling. That's why the windows fog up in the first place."
"...the defrost is also a dehumidify. As we get in and out of our [cars] in the winter we drag snow and ice into the cabin with us, adding to the humidity factor. As we where taught in school the recipe for steam (mist) is water (ice) and heat."
"I was under the impression that if the outside temperature is cold enough, or the temperature of the evaporator approaches freezing (32F), the ECU will disengage the A/C compressor clutch. This is to prevent ice from developing on the evaporator, and both blocking the airflow and stressing the A/C system.
If this is the case, then when the outside air temperature is below freezing, the compressor might not be running as much as we might be assuming here.
But when the outside temperature is above freezing, and especially when the outside air is damp, (e.g. rain, foggy conditions etc), the added dehumidifying effect of the A/C will have a significant benefit."
