A twitter acquaintance asks:
...why does the ice form in feathery streaks like this?
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The common term for those is 'dendrites'. It's caused by imperfections and deposits on the glass, which alter the process of condensation.
Here's one way in which that happens: if a bit of the glass is sticking out beyond the surface, the water vapor which is moving towards the glass through diffusion will deposit there and condense. That'll increase the size of the protrusion, which makes it grow even faster. And if there's an imperfection or unevenness in this protrusion, it can cause a similar structure to grow out from that area of the deposition, and you get a fractal-like 'feathery' pattern with branches.
It's similar enough to the mechanism in which snowflakes are formed; you see this kind of pattern where a small imperfection magnifies and then the pattern repeats in a lot of different systems.