No, not ice on car windows this time, but ice on aeroplane windows. John Fouhy has sent this question to me, and I don’t know the answer. I have a couple of ideas, but what do you think? Below I reproduce John’s question in full, along with the picture.  Airpoints Gold Card holders, help us out here…





 I recently flew into Auckland from Singapore. It was a night flight, so we only opened the windows as we prepared to descend. When we did, I looked outside and noticed ice on the outside window. Well, fair enough, it’s cold outside. But if you look at the photo I attached, there is only a small patch of ice. It’s in the shape of an annulus, centred on a small metal pin. The pin appears to be attached to neither the inner nor the outer surface of the window (I imagine aeroplane windows have many layers).

It didn’t take long for the ice to melt in the morning sun. Drops of water remained on the window for a while after, in the same place. So it is possible (from the absence of water elsewhere) that ice did not form elsewhere.   [Marcus - I've seen this before too - is it something to do with the manufacture process?]

As far as I could see, by craning my neck, the next window up from me had the same feature.

So my questions, if you have time: Why would ice form only near this metal pin? Why would ice _not_ form even closer to the pin?