# Strange pattern on car windows

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?

• 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. – Hot Licks Dec 25 '17 at 15:38