I'd say Genghis is spot on with his suggestion of the cooling air flowing downwards and cooling further until dew point is reached, it was the exact same one which popped into my head before I scrolled down and read his reply.
The glass thickening adding to this effect I admit did not occur to me. As a point of interest though one or two panes are likely to have been replaced and be identifiable by their superior optical correctness, are these less prone to this pattern of condensation build up?
Have to say although the reasoning is spot on I would imagine that the glass would react to changing external air temperature through its thickness so quickly as to rule out a significant pre condensation temperature difference, the thickness difference being miniscule in relation to the surface area of the pane.
However once the condensation has formed due to the downward air flow on hitting the (relative to room air temperature) cold glass it would have the effect of cooling that area of the glass still further by the fact that some of that moisture would be evaporating which as we all know has a cooling effect.
Oh dear! Is that possible evaporation AND condensation happening in the same place at the same time? Sounds like a contradiction now I've written it!
I think it can, even if only locally, say at the top of the condensation band where the glass is in its trasition between wet and dry. This in turn could provide an additional reason why the condensation 'grows' up the glass from the bottom as the top of the band is actually more vigorously 'self cooling' than the rest, couldnt it?
Ouch, brain fade! Its always the simple thing that do it!
Is any of that plausible, or complete nonsense?
Any thermodynamics experts care to quell my self inflicted confusion?