In the photograph: condensation on the outside some "double-glazing" foil on a Velux skylight window. Where "outside" is, in fact, the inside, or at least, the side facing into the house. I like to think that the swirly pattern depicts the turbulent airflow between the cold outter glass layer and the foil, channeled to the sides by the blind. Because otherwise it's just some damp patches. And who cares about them, anyway.

And yes, I thought it was sufficiently interesting – and aesthetically pleasing, that random air currents could create twists and eddies of such exuberance, enough to make Mr Whippy green with envy – to take several photos of. I don't see why you're acting all surprised at this.

The little stripey thing at the bottom, by the way, was my poking finger, when I checked which side the moisture was on, like a clumsy, pokey chimp who can't see anything without poking it in a pokey way. Pokey, pokey, poke, poke, poke. Tchah.