There is a gap between the two blocks opposite, about forty centimetres wide, and it lines up with his north window.
There is always wind in it.
Dust settles on the windowsill, and there is one band across the middle of it that stays clean. Two fingers wide, running from the inside edge of the sill straight out to the outside edge.
He sealed the window. Two runs of silicone, foam strip in the gaps, expanding filler injected and left to swell.
The dust still settled. The band was still clean.
He wiped the sill down and dusted a thin layer of flour across it.
In the morning the band was bare. There were fine striations along both of its edges where something had pushed the flour aside.
He tore off a strip of paper, held it by one end, and lowered it slowly onto the clean band.
The paper lifted.
Outward. The wind was going from the room into the gap.