Oddball Question Re Frost on Windshield

[erugs]

This is an oddball question that doesn't really relate to hiking, except I'm sure a lot of us who hike experience it.

Why does the outside of the front windshield on my car frost up on cold nights when none of the other windows in the car do? I've noticed this when parked next to my friend's cars at his home in Maine; all three cars do the same thing. The three cars are facing east. It doesn't matter whether one or all of the cars have been used recently. It doesn't happen at my home in Manchester, but I'm facing north in an open-air style garage then.

It's not a problem, except that it makes getting going in the early hours take a little longer, require a little more effort and thought. Not just the get-up-and-go-right-away that I'm used to.

I did a little research online and found that I might not have the problem if I "coat them with a solution of 3 parts white distilled vinegar to 1 part water."

 

[Billy]

...Why does the outside of the front windshield on my car frost up on cold nights when none of the other windows in the car do? ...

Because the windshield is the most horizontal window on your car, so the condensation rolls off more slowly, thus has more opportunity to freeze.

Also, because that's the window you need to look out in order to drive, so it frosts over just to tick you off.

 

[TCD]

The aspect of the window (horizontal vs. vertical) might also have an impact on the extent of radiational cooling.

 

[w7xman]

Why does the outside of the front windshield on my car frost up on cold nights when none of the other windows in the car do? I've noticed this when parked next to my friend's cars at his home in Maine; all three cars do the same thing. The three cars are facing east. It doesn't matter whether one or all of the cars have been used recently. It doesn't happen at my home in Manchester, but I'm facing north in an open-air style garage then.
[/I]

Frost is a deposition of H2O molecules that undergo a phase change directly from gas (high energy) to solid (low energy). This process occurs a single molecule at a time, and is extremely exothermic (heat releasing). As a parcel of air cools, it will reach a saturation vapor pressure within the parcel, 'forcing' a deposition of a molecule. That deposition releases heat, temporarily warming the parcel and stalling the process temporarily in the parcel until the heat radiates.

Two thoughts on why it might only happen on the front windshield:

1) The amount of surface area held at the angle of the windshield allows for radiating heat (which generally ascends vertically) to leave other parts of the windshield unaffected by adjacent exothermic processes, whereas vertical windows are warmed by latent heat release from below...

and/or

2) The pitting on the windshield from the constant barrage of debris hitting it allows for more initial deposition sites, much the way gas bubble only form of pitted areas in a glass...

 

[walker]

Which Way is The Windshield Pointing

I'll put dollars to donuts the windshield in question in pointing toward a slight downhill slope. The warm(er) air rising from the lower elevation carries more moisture, condensing on the glass in question.

And yes, the pits in the windshield make a big difference. These little pits cause little pools of frozen water.

 

[DougPaul]

What is the sky exposure of the windows? For instance, a car parked with the back end under a tree will have less radiation cooling on the back windows than on the front windows because the underside of a tree is much warmer than a clear sky. (A clear winter sky is very cold. Clouds, foliage, and buildings are much warmer.)

If parked in the clear, a more horizontal window (generally the front) is more exposed to the sky than the side and back windows and thus more subject to frost.

Wind could presumably also affect the frost distribution.

FWIW, my car sits in a reasonably open spot and I sometimes get frost on all windows.

Doug

 

[Ed'n Lauky]

I live in Maine and generally speaking I have to scrape all the windows from the front to the back.

Actually, as I think about it, it is the windshield that generally frosts up first, followed by the others. So you may have been in one of those marginal situations where it wasn't quite cold enough to do the entire car.

I have noted on certain occasions that when the car was parked next to the house the windows on the side of the car next to the house weren't frosted up. That, one would assume, was because of heat radiated from the house.

 

[Little Rickie]

The windshield is flatter than the other windows so frost settles on it easier and first before the verticle windows.

 

[Kevin Rooney]

In my experience, if conditions are such that frost will occur, it will be the windshield first, then the back window unless it's my truck as they have a vertical window, like the sides. Vertical windows almost always frost last.

As to why - while many of the responses from our more technically-trained members are plausible, I'm not particularly persuaded that any of them are necessarily the correct one.

Good question, Ellen - thanks for posing it.

 

[Nessmuk]

It really is a matter of radiation, a balance from both incoming and outgoing. Glass is opague to long wave infrared (radiative heat energy) so it is both an emitter and an absorber of energy, not a transmitter. Your side windows are exposed mainly to the surrounding terrain, which may have a temperature only slightly below freezing. Almost as much infrared radiation, little as it is at that temperature, is absorbed as is lost, so cooling and frost formation is relatively slow. But the angle of your windshield exposes it to the night sky, which if the sky is clear is a clean shot to infinity and extremely cold temps of deep space. So it doesn't receive much incoming radiation from local terrain and therefore cools off much more easily than the side windows. Only our atmosphere keeps unsunlit surface temperature from plunging to that of the dark face of the moon.

 

[Stan]

I wasn't gonna buy any of these theories until doug paul weighed in ... and I'm still not buying them.

The fact is that cars in different neighborhoods are subject to different Jack Frost window frosting specifications ... something to do with their union contracts and overtime for the rest of the windows. And, you'd think the rest of the car would be frosted, too, since the specific heat of metal is not that much different than glass for the temperature ranges involved, but nooooo, Frost doesn't even touch that ... and how about the headlights? Why don't they frost up? They'd be the easiest thing to defrost, that's why.

Ellen, there are some things you just have to accept and I guess this is one of them and it doesn't matter if they are facing Mecca or not.

 

[DougPaul]

And, you'd think the rest of the car would be frosted, too, since the specific heat of metal is not that much different than glass for the temperature ranges involved, but nooooo, Frost doesn't even touch that ... and how about the headlights? Why don't they frost up? They'd be the easiest thing to defrost, that's why.

The rest of the car (including the headlights) can also frost up. It's just that you don't pay enough attention...

The heat of the headlamp (once you turn it on) will defrost it fairly quickly.

Doug

 

[John H Swanson]

It really is a matter of radiation, a balance from both incoming and outgoing. Glass is opague to long wave infrared (radiative heat energy) so it is both an emitter and an absorber of energy, not a transmitter. Your side windows are exposed mainly to the surrounding terrain, which may have a temperature only slightly below freezing. Almost as much infrared radiation, little as it is at that temperature, is absorbed as is lost, so cooling and frost formation is relatively slow. But the angle of your windshield exposes it to the night sky, which if the sky is clear is a clean shot to infinity and extremely cold temps of deep space. So it doesn't receive much incoming radiation from local terrain and therefore cools off much more easily than the side windows. Only our atmosphere keeps unsunlit surface temperature from plunging to that of the dark face of the moon.

And IIRC, radiative heat transfer is to the 4th power of the temperatures of the two bodies, so when the glass (the first body) more directly faces outer space (the second body and where the temp is really cold), and there are no barriers, the radiative cooling is huge. Same for temp drops on clear nights. A similar expereince is heat from a fire. Hold a single sheet of newspaper between you and a fire and notice the effect (drop) in radiative heating. Or, perhaps a more relavant example - face a fire directly vs. turn at an agle to reduce the surface of your body that "sees" the fire.

 

[peakbagger]

Putting some mositure resistant material over the windshield held in by the wiper blades usually solves the problem. Cardboard works but if conditions are right, it can get damp and freeze to the windshield.

 

[daxs]

Supposedly spraying a mixture of 3 parts vinegar to 1 part water on the windshield at night will prevent frost from forming. The acetic acid in the vinegar raises the melting point of water....

 

[Tom Rankin]

You can buy a windshield cover too. Less fuss and mess...

 

[Kevin Rooney]

You can buy a windshield cover too. Less fuss and mess...

Yup. Or starting the vehicle 5 minutes before departure goes a long way, too!

No need to over-think this one, people.

 

[Ed'n Lauky]

The rest of the car (including the headlights) can also frost up. It's just that you don't pay enough attention...

The heat of the headlamp (once you turn it on) will defrost it fairly quickly.

Doug

Maybe in MA you don't notice it, but trust me, here in ME you notice it. The whole car very often looks like Frosty the Man of Snow.

 

[skiguy]

You can buy a windshield cover too. Less fuss and mess...

............or build a heated garage. So anyone want to estrapulate these theories to the goggle fogging and frosting phenomenon. So then this might actually be considered a hiking thread and therefore not get locked or deleted.

 

[Little Rickie]

Yup. Or starting the vehicle 5 minutes before departure goes a long way, too!

No need to over-think this one, people.

or use remote start.

How about that frost build up inside a tent, don't you hate that?