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I was walking back to my car yesterday when I noticed the frost on the back windshield formed these long "straight" lines:

The temperature was about -10C and I was wondering what the mechanism behind these lines was (the horizontal lines I can guess have to do with the wires in the window but that doesn't really explain the other frost lines).

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  • $\begingroup$ Do you have a hydrophobic coating on your windshield? $\endgroup$
    – g s
    Commented Jan 10, 2022 at 20:58
  • $\begingroup$ Nope, I've had the car for ~8 years and have never applied any hydrophobic product. There was a previous owner; however, I don't think a hydrophobic product would last that long (8+ years?) $\endgroup$
    – Jspang
    Commented Jan 10, 2022 at 21:03
  • $\begingroup$ Probably not. Mostly straight lines and sharp angles are the norm in my experience, but relative "emptiness" of the pattern is unusual to me. I'd guess something present that water was able to stick to - either microscratches on the surface, or something on the surface like the web of an ambitious (and now frozen) spider. $\endgroup$
    – g s
    Commented Jan 10, 2022 at 21:09
  • $\begingroup$ @gs I almost wonder if there could be microscratches from ice scraping over the years that the frost has now filled, might explain the fact that the lines are so long if it's just filling the valleys in the glass $\endgroup$
    – Jspang
    Commented Jan 10, 2022 at 21:13
  • $\begingroup$ See also Geometric Condensation Patterns on Windshield. $\endgroup$ Commented Dec 23, 2022 at 17:57

3 Answers 3

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Patterns like this are most often caused by scratches in the glass surface that are too small to see with the naked eye, but which act as nucleation centers for the deposition of frost from supercooled (and supersaturated) air.

This happens because a glass surface tends to get a monolayer of water molecules adsorbed into its surface over time, and the sharp edge of a scratch then presents on a nanoscale what looks like a 3-dimensional piece of ice to the water vapor in the air- which is a preferred surface upon which more ice will form.

Try the following test: when the windshield is dry and free of ice or liquid water, and above freezing, bring over a steaming tea kettle and blow steam at a portion of the windshield where you have previously photographed the linear patterns. Those same scratches will then act as nucleation sites for the condensation process, and the pattern of tiny water droplets on the glass surface will reveal the scratches.

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  • $\begingroup$ oooh, please @Jspang do make that test, I'd love to see whether this is indeed as true as it is compelling :) $\endgroup$
    – rfl
    Commented Jan 11, 2022 at 15:19
  • $\begingroup$ Very interesting, I will test this out and see if it is indeed the case $\endgroup$
    – Jspang
    Commented Jan 11, 2022 at 17:51
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    $\begingroup$ What could have caused so many rectilinear scratches? $\endgroup$
    – akhmeteli
    Commented Jan 13, 2022 at 9:49
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Additional information: Just got into my car Dec 23 in Michigan and saw this frost pattern on my windshield. It is 6°F Wind chill -14 and the frost pattern is on the inside of the glass.

My hypothesis is that the extremely cold temperatures freeze the water vapor at random points on the windshield along the major stress lines that are intentionally designed into tempered glass.

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The following is pure speculation, but who knows... Maybe the car glass was at the ambient freezing temperature, but there was also rain falling, and drops continued moving in straight lines after hitting the glass and leaving a trace of water, which would freeze fast on the glass.

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  • $\begingroup$ Could be, although it was too cold for rain $\endgroup$
    – Jspang
    Commented Jan 11, 2022 at 4:04

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