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enter image description here

A couple of days ago I was in a friend's car, and I noticed this pattern on the windows; I took a picture of the sun through the window to make it clearly visible.

The night before had been quite cold, but I don't think that the temperature went below $0$ °C, even though I am sure that it did some days before.

I can speculate that the phenomenon originated from some condensation/freezing of humidity on the outside of the car window, so I searched the web for pictures of water condensation and frost patterns (and also water staining) on car windows, but couldn't find anything similar.

What could be the origin of this intricate pattern?

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  • $\begingroup$ I would guess it's a dust/dirt pattern that resulted from frost on the window. Note that the overall air temp does not need to be below freezing for frost to form. $\endgroup$ – Hot Licks Dec 25 '17 at 15:38
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From your question, I can guess that the weather is rainy in your region.

When you drive a car in the rain, the water drops pass your windows at an angle. This, plus wind and other winter stuff causes the path of the drops to twist and jiggle like in this photo

enter image description here

I would also guess that the rain stopped while still driving, so the water could've evaporated in this pattern. The sunlight then makes those residues more pronounced when you took the picture.

Take a look at the following picture from a google search of 'water stains on glass'. To me it looks similar to your photo, just without the effect of moving window (keep in mind that the residues in the water may differ from one place to another due to pollution and etc., so the stains don't have to look the same).

enter image description here

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  • $\begingroup$ Very clever, I hadn't thought about moving raindrops at all! This is probably the correct answer, but I think I will wait a bit more just to see if someone comes out with alternative explanations. $\endgroup$ – valerio Dec 25 '17 at 15:12
  • $\begingroup$ I also thought that maybe the window was dirty, and the signs are not residue coming from inside the droplets, but what remains of the dirt after the droplets passed. This would also be more compatible with the size of the signs, I think. $\endgroup$ – valerio Dec 25 '17 at 15:15
  • $\begingroup$ @valerio92 That's plausible. Maybe the dirt tends to accumulate more in the water. $\endgroup$ – eranreches Dec 25 '17 at 15:23

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