Its winter here in New England and I park my Miata outside for the duration. Obviously a garage would be better, but that luxury isn't available. Snow is ALWAYS brushed off the car asap even though its parked. The car isn't driven and just hangs out, watching my old corolla catch the winter abuse.
A little background- the top is fairly new, it never leaks regardless of how much/how hard rain falls, and is adjusted pretty well. There is a slight bow in the center where it meets the windshield, but nothing awful that scoops air or anything of the sort. The first time I saw the windows ice from the inside, I thought "shoot, I need some damp rid." I did this, and while it works for most days, when the temp drops below 20, some condensation somewhere still forms on the windshield and ices up. The kind of icing you need an ice scraper for.
The heater core isn't leaking, the inside never fogs up when I'm driving and it rains, I don't drive the car much if at all in the winter so the argument about having ice or melted snow on the floor mats/floor pan isn't valid.
Is this just characteristic of leaving a convertible outside in the winter? The same thing (in the form of fogging) doesn't happen in the summer after rain and warm up/cool down events.
Any ideas? I'm stumped.

