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Discussion Starter #1
Every morning in the summertime when I come outside to my Volt, where it is parked in the shade of the house, the windshield is clear and dry, and the rear hatchback glass (what do you call that? Rear windshield?) is covered in condensation. Why is that? Why is the front glass dry and the rear glass wet? This is the case every single morning, even before the sun is up. I'm mystified. It's bothering me! Can anybody explain this?
 

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I think it's more meteorologists than physicists, but who knows. :)

I have found the same over the years - parked under my carport = window always clear. Vehicles on the street have condensation.
Also seems to relate to the moon cycles - full moon = pretty much guaranteed condensation. Or perhaps that's a red herring and it's because I could actually see the full moon = clear cloudless night and probably colder.

The mystery continues...
 

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That's a great observation, and I think there is an answer to be had from the world of physics! This is all about radiational cooling from the cold sky, and it's the same reason that frost can occur on the landscape even when the air temperature is above freezing (32-deg F). The temperature of the night sky is very, very cold, approaching absolute zero on a clear night, but easily way below 0-deg F. I'm not talking about the temperature of the air in the sky (which can also be cold), but the actual temperature of space. Radiational cooling happens any time that two objects have a different temperature, and heat always moves from the warm object to the cold object. Now the reason the back glass seems to be more susceptible to this phenomena than the windshield probably has to do with the angle of the glass with respect to the sky. The rear glass is much more horizontal and its surface therefore has a greater component of view toward the zenith. The coldness of the sky (space) is greater at the zenith that at the horizon.
 

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Also possible that heat from the engine compartment (charger) is having an impact. When it snows lightly, I notice melted snow on the hood from the low (but constant) heat coming from underhood.
 

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Try reversing the direction in which the car is parked and see if the condensation remains on the same end or not. Just thinking that perhaps it has more to do with the direction of nighttime breezes.
 

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Try reversing the direction in which the car is parked and see if the condensation remains on the same end or not. Just thinking that perhaps it has more to do with the direction of nighttime breezes.
Good thinking :) this will eliminate influence of external factor
 

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Neat thread. Sure beats stuff like, "Poll: What's your favorite breakfast cereal?"
 

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Condensation forms when the moisture in the air can no longer stay as vapour, usually due to the air cooling and lowering its ability to hold moisture. The rear window must be cooler in the morning than the front window or the air is warmer at the front of the car vs the rear of the car. The air between the house and the car could be slightly warmer I suppose but I think it has more to do with the car itself.

The windshield I believe is laminated glass and may have more insulation value than the rear window. Also, there are air vents along the front window and the air inside the car would slowly rise along the windshield from the vents, travel along the ceiling in the car, cool slightly and the fall at the back of the car to the floor level and that would be a natural, slow air current in the car. The natural convection could cause the slight difference in the temps of the two windows. That would be my guess.

Other factors could be driveway grade where tha back of the car is lower in the atmosphere than the front. Also, if the rear of the car is more to the west than the front then the morning eastern sun would start to heat the front of the car first. That would start to happen at sunrise.

Even oil residue on the windshield could cause a surface temperature difference.

All of the above could play a roll.
 

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All auto windshields are laminated, while all other windows are tempered glass. So there is a glass difference. However, I've always figured it was just proximity to the engine heat, keeping the front a bit warmer, a bit above the dew point, or delaying dewing due to radiative cooling. Since Volt has a lot less engine heat, it does seem that Volt dews over more than the ICE cars I've had.
 

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My guess is radiant heat from the building that you are parked next to. Your house is sitting in sunlight soaking in the heat. At night it radiates some of that heat back towards the car.
I have noticed in the parking lot at work, that the cars parked against the building, will almost always have condensation/frost on the window that is away from the building while the glass nearest the building is usually clear. That is why I always park with my windshield facing the building.
 

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That's a great observation, and I think there is an answer to be had from the world of physics! This is all about radiational cooling from the cold sky, and it's the same reason that frost can occur on the landscape even when the air temperature is above freezing (32-deg F). The temperature of the night sky is very, very cold, approaching absolute zero on a clear night, but easily way below 0-deg F. I'm not talking about the temperature of the air in the sky (which can also be cold), but the actual temperature of space. Radiational cooling happens any time that two objects have a different temperature, and heat always moves from the warm object to the cold object. Now the reason the back glass seems to be more susceptible to this phenomena than the windshield probably has to do with the angle of the glass with respect to the sky. The rear glass is much more horizontal and its surface therefore has a greater component of view toward the zenith. The coldness of the sky (space) is greater at the zenith that at the horizon.
Reference view factor and black body surfaces (Google search link).

Many articles are to be found. I hope you like complicated math. :D
 

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Neat thread. Sure beats stuff like, "Poll: What's your favorite breakfast cereal?"
Life cereal... hey Mikey!!!
 

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I have noticed a similar effect where my side windows will be clear or mostly clear of dew or frost while the front and rear glass is covered (along with the roof and hood). Also that objects, potted plants, and surfaces on my front porch, which has a roof over it, but is open to the weather on two sides, are dry when a heavy dew covers bushes, grass and trees, etc. nearby and seemingly everywhere else.

I don't understand it well enough to even begin to explain it, but I think it is due to the black body radiation effect mentioned above in a couple of other answers. I suspect your car is parked nose in toward your house and is getting some sheltering from the house from this effect due to the proximity of the building to the front of the car. Parking backwards might be an interesting experiment to prove whether it has anything to do with the properties of the car such as heat from the charging process, etc.
 

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Condensate will form once the water vapour drops to its dew point and then it needs to seed to something. Assuming no difference on the last factor, then as mentioned it has to mean the conditions next to the window have not met the dew point.

The other thought is that the effect may be a dynamic one, in which the air circulating against your house is turbulent and promoting rapid evaporation, preventing the seeding process, while the other way is not.

Also, the front and rear windows might differ in their construction. The front window usually has more plastic layers in it, due to safety requirements, and this thermally insulates the outer glass skin so it can be at a substantially different temperature to solid/less laminated windows. As any good physics experiment would go, try creating observations for new data points, like parking the other way around and see what happens then.
 

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Discussion Starter #15 (Edited)
More information: the Volt and our other car, a Prius, are always parked at an angle to the side of the western side of the house as shown in the two attached photographs, taken about 8 am, before any sunshine. NOTE: as seen, both the front and the rear glass surfaces of the Prius are always covered in condensation (contrasted with the Volt, where the windshield is always dry). This fact puts paid to the idea that the angle of the rear window is relevant, I think, because that angle looks the same or similar on both cars.


Okay, I tried to post these photos about five times and have failed, for reasons beyond my competence. So use your imagination.
https://www.dropbox.com/s/eecf3e4qocvepqj/IMG_0708.jpg?dl=0
https://www.dropbox.com/s/5h360uqpevytwfi/IMG_0709.jpg?dl=0
 

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The article above mentions the possibility that the windshield might have rain-X or some other treatment on it that may delay dew formation. This might even just be in the washer reservoir. Is there a difference between the two cars in that way?

How about what is over the cars? Is the front of only one covered by a tree, for instance?
 

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Discussion Starter #18
Same liquid used in both washer reservoirs. No Rain-X. There is a tree that is slightly closer to the Volt, but not by much. It's a cedar and does not overhang the car.
 

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It looks like only the Volt is plugged in. That may be keeping some electronics awake in the dash and or center stack, plus the LED, which might be convecting a little heat onto the windshield, keeping it clear.
 

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The mice nesting in the air ducts of my Volt create heat that radiates onto the front windshield, while my wet tent rainfly thrown in the rear hatch storage area provides higher humidity that condenses on the rear window. Hmmm . . . "Rear Window" sounds like a mystery starring Jimmy Stewart and Princess Grace.
 
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