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I went to my car a couple days ago in the morning, and I was amazed at the patterns I found on all the side windows, shown in the attached picture. No amount of googling revealed anything similar, so I am stumped as to a possible cause. To me, these frost crystals remind me of arrows on a vector field that (just guessing here) may align with aerodynamic flow over the vehicle. Knowing a bit about crystal nucleation, I can imagine some dust particles may have been deposited in these patterns while driving around, and encouraged the crystal growth seen here... Any other ideas? Frost pattern on car windows resembling a vector field

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It seems to be window frost.
Quoted from Wikipedia - Frost - Window frost:

Window frost (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is a bad insulator (for example, if it is a single-pane window), water vapour condenses on the glass, forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double-pane energy-efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles, the frost usually forms on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates. The patterns in window frost form a fractal with a fractal dimension greater than one, but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake, which is shaped by a similar process, but forms in three dimensions and has a fractal dimension greater than two.

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I don't really have answers. Just thoughts and more questions.

If you zoom in on the rear door, you can see different patterns on the window, the panel, and the plastic around the window. All are dendritic as you might expect. But the substrate seems to play a role.

The panel is thin and metallic. Perhaps it cooled faster? It is more covered.

On the window, the individual dendritic patterns seem to be aligned into a branching tree like structure. Perhaps this is a larger dentrite that was scoured into pieces by the wind? Perhaps oil on the window prevented ice from nucleating? Or sticking? Or perhaps the window is smoother than the paint/wax?

The depression under the door handle doesn't have much ice on it, especially on the upper half. What it sheltered from the wind? Snowfall?


I noticed similar patterns a few years back on the inside of the glass on my hearth heater. The glass probably has scratches because I cleaned it a few times with a razor blade. Most recently it was cleaned with newspaper dipped in a slurry of ashes and water. This didn't do a perfect job.

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  • $\begingroup$ I'm noticing the same things. My best guess for the patterns on the door panels is that they align with small scratches in the paint/topcoat, which I did find examples of when searching yesterday. It probably did cool faster, as you suggested. I noticed yesterday after work that there is indeed an arrangement of small dirt particles that matches the original frosting pattern.. I'm stumped on the lack of frost under the handle. This is different, and leaves one to wonder. $\endgroup$ Commented Jan 23 at 13:39

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