Stupid Physics Question

[LordoftheLeftHand]

Possibly the orientation. The windows being more horizontal, perhaps do not give as good a surface for moisture to adhere to as the more angled windshields.

This would be my guess as well. Probably the same reason you generally don't see frost on the side of objects, only on their top surface.

Most windshields (front windows) sit at a much better angle to collect moisture. Is your rear window also set at a similar angle to the front window?

LLH

 

[epepke]

If so, my first thought would be air flows. I presume that the front and rear of the car are designed to present less resistance to air flows and so, given any arbitrary air flow (i.e. winds, allowing for turbulence etc) air tends to flow more freely and in greater net volume (and possibly at a higher rate) when it's passing in a direction parallel to the normal driving direction of the car.

Also, dew drips off vertical surfaces faster.

 

[jj]

I have a very difficult time believing this theory. Clarsct, is the car parked less than 24 inches from other large objects?

Uh, why would the proximity matter? The angular portion of terrestrial vs. clear-sky-at-night that each is exposed to is the whole issue, really, for radiation.

A bit of work for you.

Suppose we have square foot of 45 degree stuff 1' away from the car, and 10x10 (100 sq ft) of the same material 10' away.

Which do you suppose, radiationwise, will have more effect. (cough, cough)

 

[Ladewig]

Uh, why would the proximity matter?

Because the original claim was

but the side windows, being vertical, are getting heat radiated back at them from trees, walls, other cars, etc., and thus aren't cooling as much.

surely distance matters if the trees, walls, and other cars are radiating heat AT the car in question; or am I not understanding your point?

 

[rudar]

Distance only matters if it's a point source (so the ``concentration'' of radiation drops off with distance). In this case, we're looking at how many angular degrees of the area in front of the window has ``stuff'' in it, versus open sky, assuming that most ``stuff'' will radiate a fair bit more than open sky will. If your ``stuff'' is a single object, then the further away it is, the fewer angular degrees it will cover. But here, we're assuming that a fair number of angular degrees contain ``stuff''; whether it's a mail box two feet away or a house 20 yards off doesn't really matter.

Not sure that's the best explanation. Perhaps someone else can do better...

 

[jj]

Because the original claim was



surely distance matters if the trees, walls, and other cars are radiating heat AT the car in question; or am I not understanding your point?

You're not understanding my point.

A big tree farther away provides the same radiation input (assuming it's the same temperature, colour, etc) as a small one closer. That's my point.

Side windows have the biggest projection from trees, etc. Slanted ones have more exposure to the sky, well, unless one is parked on the top of quite a hill.

 

[Ladewig]

Distance only matters if it's a point source (so the ``concentration'' of radiation drops off with distance). In this case, we're looking at how many angular degrees of the area in front of the window has ``stuff'' in it, versus open sky, assuming that most ``stuff'' will radiate a fair bit more than open sky will. If your ``stuff'' is a single object, then the further away it is, the fewer angular degrees it will cover. But here, we're assuming that a fair number of angular degrees contain ``stuff''; whether it's a mail box two feet away or a house 20 yards off doesn't really matter.
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It doesn't? I understand the concept you describe but surely there is a limit to how far radiant heat will affect things outside at night when the temperature is cold enough for frost.

I don't think your example is completely accurate because a house (on a night with frost) is a much different heat source from a mailbox, but even so, isn't twenty yards of air a pretty good insulator?

Are saying that a thermometer attached to the side window will have a significantly different reading from one left on the windsheild?

 

[TjW]

It doesn't? I understand the concept you describe but surely there is a limit to how far radiant heat will affect things outside at night when the temperature is cold enough for frost. (snip)

Nope. It's line-of-sight. Because it's radiated. It doesn't stop until it is absorbed by something.

 

[Ladewig]

Nope. It's line-of-sight. Because it's radiated. It doesn't stop until it is absorbed by something.

and it doesn't get absorbed by 20 yards of air?

 

[rudar]

It's EM radiation. Same as visible light. Certainly, it would get scattered by air, but only over distances on the order of how much air would make things look hazy. I say on the order, because different things in the air (small particles, water vapour, smog, etc.) probably will affect different wavelengths (i.e. the IR ``heat'' radiation we're talking about here versus visible light) somewhat differently, but not so much that 20 yards of (clear) air will have any significant absorption or scattering.

 

[clarsct]

Hmmmmmmmmm.

So when you walk out in the cold, you're still being 'heated' by the building behind you?

 

[rudar]

Oh, and as for the former post, again, yes, a house (especially one as poorly insulated as the sieve I'm in right now ) will radiate more than a mailbox, because it's warmer. But we're comparing your 375Kelvin mailbox and your 380Kelvin house (numbers completely made up) to your 3-4Kelvin open space (plus some backscattering by the atmosphere, a.k.a. the greenhouse effect), so the difference is not that huge.

And your idea of sticking (accurate, high-precision) thermometers on the different window is another great idea for a test that would distinguish between the radiation hypothesis and the vertical-surfaces-don't-collect-moisture hypothesis that was floated as one alternate. Of course, patnray essentially reports similar results when observing frost on the windshield, and unfrozen moisture on the sidewindows. This suggests that both surfaces collect moisture, but the windshield gets colder than the side windows. Which doesn't *prove* the radiation hypothesis, but does tend to argue against the moisture-collection or moisture-retention hypotheses.