Here is a striking photo I snapped in Tubingen, in southern Germany, showing a building with a tile roof… bearing a weird pattern:
The cause of the red circle on the roof is unmistakable, the air flows around the chimney… but it is the opposite of what you’d expect, and the details are a bit unclear (considering the darker soot circle around the cleared out area). If you care to speculate, do it in the comments!
Perhaps the building is located somewhere where it snows frequently? Hot air from the chimney would melt the snow, at once cleaning the area closest to the chimney top, and carrying soot outward, where it would then accumulate as seen the photograph.
Nathan
It is the mildew. It cannot survive the highest temperature close to the chimney but thrive on the slightly higher temperature than the atmosphere when the heat is mixed with the outside air.
YH
Interesting ideas! I can see how Yoram’s may work – it explains the light area as well as the dark ring. SFSlim’f is close (the area does have snow) but the melted snow ought to flow down, not outward, under gravity, so the top part of the dark ring is unaccounted for in this model.
Keep the ideas coming!
I would really like to see the other side of the roof.
If the other side is similarly discolored – minus the cleared circle – then a fair guess is would be that the discoloration is NOT from what is coming out the chimney.
The wind direction is also a question. If the picture is of what is usually the downwind-side, then the air tumbling over the ridge might force warmed air to re-circulate against the roof.
Also possible the roof is warmed from beneath. Whatever is beneath the chimney may “leak” a column of warm air against the underside of the roof.
At first I thought about smokestacks I have seen, where the smoke only appears visible some distance from the stack, where the air cools and the particles begin to condense. But on second thought, this line from dark to light is so sharply defined that my best guess is that they cleaned the area around the stack.