Frost preferentially forms on non-vertical car windows?

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SUMMARY

Frost preferentially forms on non-vertical car windows due to several factors, primarily radiative cooling. Non-vertical surfaces, such as horizontal windows, have a greater exposure to the night sky, allowing for more effective heat loss through radiation. This phenomenon is influenced by the cleanliness of the surface, which provides nucleation points for frost formation, and the boundary layer effects associated with different orientations. The discussion confirms that frost formation is consistent across all vertical glass surfaces and car body components, with surrounding structures impacting radiative cooling efficiency.

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Frabjous
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Going out to my car this morning, I noticed that frost preferentially formed on non-vertical windows. I can come up with three explanations

1) non-vertical windows are dirtier, providing more nucleation points
2) horizontal vs vertical boundary layer effect
3) gravity driven vertical diffusion of water vapor effect

Of course these all could be wrong.

Does anyone know the correct answer?
 
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Does writing "4) other" justify saying you can come up with four explanations?
 
mjc123 said:
Does writing "4) other" justify saying you can come up with four explanations?

Corrected, although I think recognizing my own ignorance is worth something.
 
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Was that true of ALL vertical glass or just of that glass facing a tree or a man-made structure?
 
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phinds said:
Was that true of ALL vertical glass or just of that glass facing a tree or a man-made structure?

It holds for all ALL vertical glass.

It also holds for the car body. Hood, roof, trunk had frost. The rest did not.

The surrounding ground/concrete was frost free.
 
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Frost forms from radiative loss of heat cooling a surface. Plus radiative loss occurs more rapidly on surfaces with an unobstructed "view" of the night time sky and no other potentially radiating objects nearby, e.g., houses, walls. Does that help somewhat?
 
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It does help. The front windshield faced a brick wall (5 ft) of an unheated garage. The right side was 5 ft from a wood fence. The left side was 20 ft from a brick house. Is radiative cooling that view sensitive?
 
I agree that it probably is mainly radiative cooling. Frost occurs because an object is colder than the surrounding air and therefore the water vapor in the air condenses on that surface (air has to be humid enough). The main (only?) way to get a surface colder than the surrounding air is via radiation.

Non-vertical windows have a normal component towards the sky, to which a lot of heat can be radiated away.
 
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