Frost preferentially forms on non-vertical car windows?

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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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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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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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