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Stupid Physics Question

clarsct

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Mar 14, 2005
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Something has been bothering me.

I live in the Northern Hemisphere, and here lately it has been frosting my car windows when I leave work(at about 0530), but it only frosts the front and rear windows, not the side windows. THe direction the car faces doesn't seem to have impact on this.

Why would this be? I get to work at 1730,(530 PM) so the heater and whatnot shouldn't be a problem for morning frost, I wouldn't think.

Any thoughts?
 
As in it's frosting up while you're moving with the heater on? I suspect you need work done on the blowers for your heater.
 
clarsct said:
Something has been bothering me.

I live in the Northern Hemisphere, and here lately it has been frosting my car windows when I leave work(at about 0530), but it only frosts the front and rear windows, not the side windows. THe direction the car faces doesn't seem to have impact on this.

Why would this be? I get to work at 1730,(530 PM) so the heater and whatnot shouldn't be a problem for morning frost, I wouldn't think.

Any thoughts?
Click to expand...

I may have misunderstood the question, but my understanding is that your front and rear windows frost up in the morning when the car is parked, but the side windows don't and it doesn't matter what direction the car is facing when parked, the same usually happens?

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. And if that's the case, then it's possible that a greater air flow along the front/back axis tends to cause a greater cooling effect along that axis than normal to it (i.e side to side). In other words what is usually called "wind chill factor" is greater parallel to the car than normal to the car.

I may of course be completely wrong, but that's the first guess that sprang to mind! :)
 
Is the frost forming on the inside surface or the outside?

Are the front/rear different in any noticible way from the sides (as in: are the side windows "tinted" with a plastic film)?

Interesting question.:)

Dave

Edited spelling.
 
Last edited:
Sorry, I should have been more clear.

Pragmatist nailed it, all in one shot.

When I spoke of the heater, I meant in the sense that the car is completely cold when the frost forms. If the front window were warmer I could attribute it to that, perhaps.

CaveDave:
No tinting, other than the blue at the top of the front window that all cars have. Would aerodynamics affect the dew formation?

I am assuming there is a physical phenomenon happening here, and it isn't the frost fairies playing a trick on me. I could almost wish that were the case, I need the money.

So what gives?
 
I'll bet you've had clear, cool nights.
I'm guessing that more heat is radiating away into space on the front and rear windows because they're at a more horizontal angle.
If you get nights that are just as cool, but with a cloud cover, I'd expect the effect to go away.
 
Huh?

I thought heat radiated in all directions equally.

But you are right about the clear and cool. Could you elaborate some?
 
I could be wrong.
But stuff around the sides of the car, like fences, or another car, or trees, is also radiating or reflecting heat.
So the side windows lose heat less quickly, because some of it is replaced by other objects radiating heat.
But when the heat is radiated to a dark night sky, it doesn't come back. So the more-horizontal portions of the car lose more heat than the vertical portions.
If there's a cloud cover, then the clouds are reflecting back some of the heat, and you don't get the effect.
 
clarsct said:
Huh?

I thought heat radiated in all directions equally.

But you are right about the clear and cool. Could you elaborate some?
Click to expand...

Objects emit radiation according to Stefan's law
[latex]P = e\sigma A T^4 [/latex]

Where T is temperature in Kelvin, A is surface area of the object, e is a constant called emissivity (related to the material, and less than 1), and sigma (the o-thingy) is a universal constant called Stefan's Constant.

Your car does radiate more or less equally in all directions, but for thermal equilibrium, all directions have to radiate back. The actual objects are at a similar temperature to your car (around 260 K, I'd guess), but space is at ~3K (amazingly not 0, despite it being mostly empty). So there's a factor of about 2604 difference between the energy your car radiates up, and the energy it receives from up, which cools it down. This has a less dramatic effect than you might think, (2604 is a huge number), because Stefan's constant is really small, and the temperature difference is reduced due to conduction and convection.
 
Could it relate to the material the windows are made of?

I know that windshields (both front and back) are made out of a thick tempered-laminated safety glass. What the windows are made of varies from car to car, but often it is considered as important to the safety of the car, so they are made of something different.

Another thought:

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.

That's just a thought.

But my guess would be that it might relate to the material they are made of and it's thermal properties.

-Steve
 
TjW, Dilb, and DRBUZZ0 each made good points.

The reason I had for asking about which _side_ of the glass the frost was on was that absent of other distinctions, the orientation to the horizontal seemed to be the controlling factor: if on the inside it could be water vapor (from interior materials which might hold moisture) rising and impinging on the glass, or if outside, maybe a fine mist or fog falling onto the glass.

Or TjW's hypothesis about re-absorption from nearby warm-bodies might be correct.

Or maybe fairies DO exist.:eye-poppi :jaw-dropp

Dave

ETA phrase I left out (spaced out).
 
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CaveDave said:
Or TjW's hypothesis about re-absorption from nearby warm-bodies might be correct.
Click to expand...

A test of this might be to place an aluminum-foil shield between the windows on _one_ side of the car and the surroundings, angled from the bottom of the windows upward and outward so that full air contact is allowed, but ground and adjacent structure radiation is blocked. Then see if only that side frosts.

Just remember, same as in tinfoil hats, Shiny Side Out!:D

Dave
 
I'd agree that the car is radiating out more-or-less evenly in all directions, 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. You can see the effect of radiation from any object whenever it gets cold enough for the ground to frost; you'll notice a ring of no frost around trees, fire hydrants, mailboxes, etc. And if you're into back-country skiing, you've probably fallen into a tree well or two in your time. I've heard of folks needing a rope-rescue from a particularly deep tree-well. Basically the heat radiated from the tree melts a big honkin' hole in the snow around the tree. Well, there is also some effect of the branches intercepting some of the snow. But even if wind fills in the tree wells, they'll reappear after a while.
 
Hmmmmmmmm.

Cool. Sounds kind of plausible, when explained that way. Well, another one down..Thanks All. You know how it is when you notice something....your mind just needs to find a good explaination.

So much for my fairies...and I needed the money, too.
 
rudar said:
I'd agree that the car is radiating out more-or-less evenly in all directions, 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.
Click to expand...

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

I find DRBUZZ0's theory much more plausable. Dew condenses on the glass that is not vertical and then freezes.
 
Actually, the thermal equilibrium stuff can result in very weird things. Apparently, (I've never tried it myself) if you take a doer, (highly insulated thing used with infrared telescopes) and put water in it, and leave it outside on a clear night, the water will freeze, regardless of the temperature of the surroundings.

Cold air near the water stays down there and insulates the open top. The sides are already insulated. Then the water tries to come into thermal equilibrium with the sky. :D

Edit: typo :(
 
If you had a carport, or an easy-up awning, you could park with that over the windshield, leaving the rear window exposed.
If the rear window froze, and the windshield didn't, you'd have evidence for radiational freezing.
 
even better, get one of the tin-foil caps from one of the local conspiracy nuts, and suspend it above part of the window, or prop it up like a little umbrella. If you get a little circle of not-frozen windshield, even better evidence. If you don't, well, then I've been talking non-sense all this time :) Which could well happen.
 
Ladewig said:
I have a very difficult time believing this theory. Clarsct, is the car parked less than 24 inches from other large objects?

I find DRBUZZ0's theory much more plausable. Dew condenses on the glass that is not vertical and then freezes.
Click to expand...
This condition is very common on winter mornings following clear nights here in northern CA. I often find frost on the front and rear windows and unfrozen condensation on the side windows.
 
I see the same thing with my car. If I park it with the front towards my house the rear window gets icy, If I park with the rear towards my house the front window gets icy. ( if the sky is clear that is ). It is all about radiation. If you point an IR-camera up on a clear sky it shows a very low temperature, usually "of scale". As an employ at FLIR ( flirthermography.com ) I have made that demonstration may times.
 
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