I have noticed that at the outside bottom of the panes on my double glazing and struggle to explain it.
I guessed it is that the glass (like railway rails) radiates heat to clear skies faster than the conduction from the air round it replaces the heat and then downward convection (replacing the air cooled by contact) brings more slightly warmer and hence more moisture-laden air into contact with the surface again.
It's a mystery!
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