Genghis, I can't go along with the glass thickening at the bottom theory. It does so at such a slow rate that I can't imagine it being even 1% thicker after a century, which will make negligible difference to thermal resistance. How about air in contact with glass rapidly cools, flows down window, starts depositing condensation?
An even bigger mystery in life to me is why car airconditioning cold air is better at demisting than warm air from the heater. Now that one doesn't make sense to me!