What causes this frost pattern on my windshield?

Question

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).

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?) — Jspang, Jan 10 '22 at 21:03
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. — g s, Jan 10 '22 at 21:09
@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 — Jspang, Jan 10 '22 at 21:13

score 9 · Answer 1 · answered Jan 11 '22 at 06:41

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.

oooh, please @Jspang do make that test, I'd love to see whether this is indeed as true as it is compelling :) — rfl, Jan 11 '22 at 15:19
Very interesting, I will test this out and see if it is indeed the case — Jspang, Jan 11 '22 at 17:51

score 2 · Answer 2 · answered Dec 23 '22 at 16:27

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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.

answered Dec 23 '22 at 16:27

St. Christopher

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Welcome St. Christopher to Physics Stack Exchange. Thanks for the interesting additional information, but I didn't think the stress in tempered glass has such sharp and arbitrarily angled stress lines. See, for example Grid pattern on a car window when viewed through polarized sunglasses or "Influence of Cooling Jets on Stress Pattern and Anisotropy in Tempered Glass". – David Bailey Dec 23 '22 at 17:10
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It would be great if you (or others) could take photos of the patterns on different days (after the windshield has warmed up in between), to see if the frost follows the same lines. – David Bailey Dec 23 '22 at 17:58

score 0 · Answer 3 · answered Jan 11 '22 at 00:00

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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.

answered Jan 11 '22 at 00:00

akhmeteli

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Could be, although it was too cold for rain – Jspang Jan 11 '22 at 04:04

What causes this frost pattern on my windshield?

3 Answers3