@ericphelps Yeah, I was thinking along those lines as well, but that inner frost-free circle... The window is double-glazed, so it can't be thermally related to the inside. I was speculating about some kind of weird airflow down the roof, or even infrared reflection off the inner pane of glass. Another window like that, 1 m to the left and next to the wall, does not get that frost pattern. Strange, but nice.