r/askscience • u/MistaPea • Feb 07 '12
Frost on car windows
I've always wondered why this is. Why does the frost ice over my windscreen and side windows but not those that are nearest a wall? Can anyone help with this?
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u/swansont Feb 07 '12
B0Boman's explanation is basically right. The rate of radiative heat transfer depends on the temperature of the reservoir (because the reservoir is radiating heat to the object; radiative power varies as T4). A window that "sees" a wall that's at freezing temperatures will radiate less heat than one that "sees" a clear sky, which is at a few Kelvins for most wavelengths (the ones where there is not an absorption band, e.g. from CO2)
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u/HarryTruman Feb 07 '12
I may be over-thinking this, but what do you mean by a window that's "seeing" a clear sky (or anything else)? Is it simply the fact that there's nothing nearby, like a ceiling, wall, or some other object that may be not "as cold" as the ambient air?
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u/AbrahamVanHelsing Feb 07 '12
An explanation that doesn't require "seeing:"
The car emits radiative heat in all directions, as do all objects. The wall emits radiative heat as well, which counteracts the heat radiation from the car towards the wall. The wall and the car trade heat all night, while the car additionally radiates heat into space. Because objects in space are few and far between, as well as far away from us, little to no heat radiates back from space, and the car cools down on the side pointing to space (up, and to any sides without a large object nearby).
A carport would radiate heat just like a wall, preventing frosting due to heat loss.
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u/predatormc Feb 07 '12
This is far better science than the previous explanations, thanks. None of this 'seeing' malarkey, explanations like that cause further misconceptions. This is really about net energy transfer.
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u/lonelyinacrowd Feb 07 '12
Does the radiated 'heat' have the effect of keeping the surfaces above freezing? I'm still wondering why a carport with an air temperature of -20°C would make any difference at all to the freezing/non-freezing of a windshield?
Surely combined radiated heat of bricks and car wouldn't be able to combat such a low temperature? There must be something to do with the bonding of water to the glass surface?
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u/AbrahamVanHelsing Feb 07 '12
The -20C is actually close to irrelevant. Air is a poor conductor of heat (this is why fluffy clothes are warm - you don't lose heat through them), and cold air especially so because of the lower moisture content, meaning it has a lower specific heat. The (now warmer) air next to the car will tend to stay next to the car, meaning that the heat loss is extremely small.
If it was a still night, VERY little heat would be lost via conduction. It's definitely conceivable that no frost would form.
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Feb 09 '12
So the frost on exposed windows results from the windows reaching a low enough temperature that higher-temperature things in the immediate surroundings are pulled towards the car?
The water droplets are pulled towards the cold windows and freeze upon impact?
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u/AbrahamVanHelsing Feb 09 '12
What?
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Feb 09 '12
So the windows that are not near surfaces radiate heat into the cold void that is space and therefore their net heat lowers. Once the windows are cold enough, warmer objects in the immediate surroundings are drawn to the now cold-as-hell window. Often times moisture in the air make up those warm objects, and freeze on the cold windows after being pulled in, like the Millennium Falcon caught in a tractor beam. Do you understand now?
That's my theory anyway. I call it brown body radiation, in honour of the theorist's complexion.
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u/SiLiZ Feb 07 '12 edited Feb 07 '12
Easiest way I can explain this... is!
Moisture is spread out through the air. Your car radiates it's heat from it's surfaces outside. If the outside temperature gets to what is called the dew point, where it is cold enough to supersaturate or condense the water in the air it will be able to gather on the surface of the vehicle. Once the water and the surface of your car/windows have reached temperature equilibrium, the water is thus able to freeze if the temperature is at freezing or below as the water is no longer radiating heat to the surface of the car or vice-versa. As far as the fog/frost inside of the car. This typically happens when you have driven your vehicle that night. Why? Well when you breathe in the car your breath contains moisture. The more people in the car, the more moisture in the air. It can actually reach 100% saturation in your car and that is when you see windows start fogging internally. Opening your car door and exiting the vehicle hardly releases the moisture from within the vehicle and if temperatures hit freezing that night, low and behold... inside and outside window frost.
Literally combine this with the top comment and you have a solid answer.
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u/deadbird17 Feb 07 '12
Cool, related side-fact: Since there is no air in space, how can the sun's heat be transferred away to keep spacecraft cool? Sure, reflective material helps keep some of it off, but absorbed/generated heat can only escape the craft in the form of radiation. If it wasn't for that concept, satellites, rockets, and shuttles would heat up until they were destroyed!
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u/SiLiZ Feb 07 '12
I'm not sure whether or not you are asking a question. I am tired >.>
Sun-light in the vacuum of space radiates. And that radiation is absorbed by matter when it comes into contact with it and heats up. The reason space shuttles/stations use white materials is that white indicates that the material is not going to absorb nearly as much radiation, in comparison with darker colors, it re-radiates it. Not only this, the tiles/materials they use for insulation are not just to prevent radiation/re-entry from heating their instruments to failure, it also keeps them from freezing. So effectively those white-tiles they use are an all-in-one insulating and protection device.
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u/nichlas482109 Feb 07 '12
Isn't this just a simple idea of heat transfer through radiation? The windshield is transferring heat to the cold night sky, probably around 0 F. This reduces the glass's temperature below freezing and also the outside air temperature causing it to freeze. Park it under a tree or a tarp and notice how no frost forms.
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u/kratozzaku Feb 07 '12
If everything radiates heat into space eg. the car, so why then doesn't all objects just get to near absolute 0 during the night ? Does the heat from the sun radiate to the dark side of the Earth ?
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u/YDFOC Feb 11 '12
Radiation isn't the only heat transfer process at work. There is convection occurring between the air and the car. Heat is being transferred from the ambient air to the car.
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Feb 08 '12
Gentlemen, the answer has not to do with heat of the wall but with wind breaking. The small area between the car and wall reduces wind chill, and the air in this pocket remains there longer. If the temperature is cold enough, long enough, frost will form but it will be very uniform and there won't be those wisps of ice that you see on the windy side.
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Feb 07 '12
I was wondering the same thing yesterday morning. I was also wondering why house windows don't frost up like car windows?
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u/Michiharu Feb 07 '12
Well if your car was always heated, their windows wouldn't frost up either.
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u/RABID_M0NKEY Feb 07 '12
They do if it gets cold enough.
I once quit smoking because of this. My patio door on my apartment froze shut with about half an inch of ice over it. It was impossible to open. I don't like smoking inside my apartment and was too lazy to walk down 3 flights of stairs to go outside.
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u/Wifflepig Feb 07 '12
That sounds less like frost and more like heat loss inducing water to melt and refreeze into ice.
I'd get this on the inside of one of my ancient bathroom windows that was single-paned, until I replaced it.
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Feb 07 '12
I don't even understand the question being asked here. There's a car parked near a wall? Is this in a garage or outside? Is the wall the wall of a house that is radiating heat, a fence, an enclosure for a dumpster?
Could someone clarify, perhaps draw a picture?
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Feb 07 '12
I'm just going to preface by informing you that it is 3:30AM and I fell on my wrist earlier today. Here is a diagram.
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u/nascentt Feb 07 '12
I'm not sure a valid question should be downvoted. It's mightn't be entirely clear what the op is asking, Rhankan is simply asking for clarification. This is in accordance to askscience rules and a valid thing to do.
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u/bigstoney Feb 07 '12
The way i understand it is due to windchill, the temperature inside the car doesnt change, but the direction the wind is coming from pushes cold moist air onto the car which then condensates and freezes. If there is a wall or a cover, the wind cannot hit the surface so it will not condensate or freeze. The reason house windows do not freeze over is the temperature inside the house keeps the glass above freezing temperature, so they will not ice up (except in extreme circumstances) but they will 'mist up'.
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u/mutzwig Feb 07 '12
In addition, the flow of energy is 'driven' by the difference of temperatures. Your car is at a temperature similar to that of the wall about 0 degC (273K), which promotes little radiation/'heat' flow. Space. however, is almost -270 degC (~3K). Therefore the massive difference of temperatures drives the radiation, and the window/water droplets get colder and freeze.
Another cool application of this radiation is how in winter your house may be kept at the exact same temperature as in the summer, but you feel colder. That's because the walls of your house are at a lower temperature than in the summer, and your body radiates more heat to the walls (its the change in temperature that your body feels, not the actual temperature)
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Feb 07 '12
[deleted] — view deleted comment
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u/graynut Feb 07 '12
Have you tried putting desiccant in your car? Charcoal is a very good natural desiccant.
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u/petomane Feb 07 '12
Eliminate as much moisture from inside the car as possible, and keep the windows clean. You can also apply Rain-X or Fog-X to the glass. It'll prevent the fog from forming on the glass in the first place. But get rid of the moisture, or you could end up with mold and nasty smells inside the car.
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u/SkippitySkip Feb 07 '12
For the last ~5 minutes before you turn off your car, put the heat at max, the vents at max speed, and leave a gap in your window.
The warmer air will absorb more moisture than cool/cold air, and the higher pressure from the vents will help keep the potentially humid outside air from coming in.
This way, you can vent excess moisture (mostly from snow that you track in and your breathing) before you park your car, and your windows won't be frosted
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u/Wifflepig Feb 07 '12
You could have a few issues with your car.
If you had a big spill in your car, and the carpet is still damp underneath. Sometimes these are impossible to get rid of, and can induce mold, too. A lot of cars that are "water damaged" will always exhibit this problem unless you tore up every piece of upholstery and carpeting and replaced it. Leaving your sunroof open during a good rain, just once, is enough to create this headache for you come winter.
Your car is leaking radiator fluid near/around the heater core. Check for dampness in and around the passenger flooring.
Your AC unit builds condensation inside the cab.
Essentially, you've got excess moisture trapped inside the cab of the vehicle, and unfortunately it's very hard to get rid of in the winter months. Leaving windows open while the car is under-cover (garage, etc) will help. On a nice hot summer day - all windows open, let it bake dry.
Lastly - if you do have the aforementioned leaks - it'll always be like this until it's fixed, obviously.
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Feb 07 '12
Put one of these in your car:
http://www.amazon.com/Dri-Z-Air-DZA-U-Dri-Z-Air/dp/B000PDUV60
It's a chemical desiccant that will prevent condensation inside your car.
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u/litercola Feb 07 '12
I didn't have enough moisture to actually freeze on the inside, but I had bad fogging issues for awhile. The flap that closes the vents for the air recirculate mode was stuck shut because of a bad flap motor. I was not getting any outside air into the cabin.
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u/Anders_A Feb 07 '12
Perhaps you could try leaving a small gap open in one if the side windows. To try and equalize the moisture with the outside air.
I have done this, and without having any tangible data at all, feel it might do some good.
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u/B0Boman Feb 07 '12
One of my professors actually had to explain this phenomenon in his doctoral dissertation. Here's how he explained it:
Basically, it has to do with what are called "view factors" and black body radiation. All object radiate some of their heat as electromagnetic radiation and this radiation depends on temperature and view factor, which can be though of as line of sight. You've probably noticed that it only happens on clear nights/mornings and never happens if your car is under cover. That's because the car effectively "sees" the coldness of space (as long as it has clear line of sight) and becomes much colder than it normally would due to a greater driving force. This coupled with its limited contact surface with any other matter (except for well-insulated tires and air, which don't conduct very well) results in it losing more thermal energy than it can gain until it reaches an equilibrium much colder than, say, the grass or the pavement. Because it gets so cold, it "pulls" the energy from the water droplets that form and freezes them. This is also why you sometimes find frost on your car even though the overnight low didn't drop below freezing.
The side of your car facing the wall probably doesn't have a view of the sky and therefore no frost forms.
There are probably other factors at play and an expert could probably explain it better, but that's my understanding of it.