I believe Geghis is more right than you may believe.
Glass IS a HIGHLY VISCOUS liquid. That was eve taught to me way back when at school. It takes a LONG time for the glass to deform in one way or another due to gravity, but it DOES happen. SLOWLY....
His circulation points and those of others make sense, including Bombers: "The air at the top of the room is the warm air. At a specific point (in height above ground) the air will be cold enough that it will release its moistue onto the cold window. Above that point the air is warm enough to hold all it's moisture thence no condensation"
What people fail to realise, is that the windows, naturally, are attached to walls. Now, the walls are going to retain heat LONGER than any glass, so even if the wall is below the level of dew point, the wall will be hotter than the glass above it, and therefore NOT condense the air around it. The glass, being relatively thin, will be significantly colder, and cause the water vapour to condense on it.
Now, also, having said that, the glass will warm up MUCH quicker than the walls, so that at a point as the day progresses, the glass will be warmer than the walls....
It's all fairly straighforward thermodynmaics.... an IR camera would undoubtedly show a warmer spot on the underlaying walls, and the glass a bluer spot.
Again, take it as you will...