Greenhouse Physics and Car Shades

I got a new comment on an old post asking an interesting question about thermodynamics:

I have a question that bears somewhat on this issue of keeping cars parked in the sun, cooler. You all know those accordion folded/aluminized shades you can put up inside the windshield and back window.

Seems to me putting them INSIDE is the wrong approach. They should be on the OUTside of the window acting as real shades and reflecting away the sun before it gets into the inside of the car.

This involves some of the same physics involved in the ever-popular issue of climate change, so it’s worth talking about the basic ideas involved. The key factor involved in heating the interior of your car to a temperature much higher than the outside air (or heating the Earth’s atmosphere to temperatures above those of interplanetary space, but let’s stick with the less controversial heating of cars) is that the light that comes into the car is not the same as the light that tries to leave.

Sunlight contains a very broad spectrum of different wavelengths of light, spanning all the way from the infrared into the ultraviolet. Not all of this light makes it through the glass into the car, though– the visible light mostly passes right through the glass, but most of the ultraviolet and some of the infrared get blocked by the glass. The visible light that comes down then gets partially absorbed and partially reflected by the contents of the car.

The visible light that is reflected stays visible light, and does very little to heat up the car– it goes right back out the way it came in. The visible light that gets absorbed, though, changes its character. When it gets absorbed, it increases the energy of the atoms and molecules making up the car interior, which raises the temperature.

That’s not the end of the story, though, because all of that energy doesn’t stay completely within the material. One of the most important ideas in physics is that any object with a temperature emits radiation with a characteristic spectrum of light. Describing this spectrum was an extremely difficult and important historical problem (here’s a couple of old posts on the topic), which led to the development of quantum mechanics, but the important thing here is that the radiation that goes out has a spectrum that depends only on the temperature. And for objects at everyday temperatures, the peak of that spectrum is in the infrared region of the spectrum (in the neighborhood of 10 microns wavelength).

In terms of the radiation involved, then, the interior of your car is essentially a machine for taking the visible light that comes into the car through the windows and turning it into infrared light. But that infrared light is in a wavelength range that gets blocked by the glass, which means that rather than leaving the car through the windows, it gets stuck inside, and bounces around heating things up. The total amount of energy inside the car increases, which makes everything hotter.

(In the Earth’s atmosphere, the role of the windows here is played by the various greenhouse gases (CO_2, methane, water. etc.), which strongly absorb radiation in the infrared, and re-radiate much of it back toward the Earth, where it heats things up. The basic process is the same, though: visible light comes in, infrared light tries to go out, and gets stuck inside, heating things up.)

The idea of those shiny car shades is that they reflect a greater fraction of the light that hits them than the fabric and so on in the car itself. Which means that visible light coming through the glass hits the shiny material and bounces back out as visible light (mostly), which means it doesn’t get trapped by the glass. So it doesn’t matter that the shiny stuff is inside the car– it bounces the light right back out, and keeps the interior cooler.

(This is the reason why some environmentalists suggest projects like painting all the roofs in New York City white. If you increase the amount of light reflected from the roof of a building, that means that more of the light headed out toward space remains in the visible region of the spectrum, and passes back through the atmosphere without contributing to heating things up. It’s unfortunately all too easy to make this sound silly, but it’s actually pretty solid, scientifically. And if we paid thousands of unemployed people to paint roofs white, we’d stimulate the economy at the same time, but that’s just crazy talk…)

It might be a tiny bit more effective to put the shiny stuff outside the glass than inside, in terms of the final equilibrium temperature of the car, but I doubt it’s a big effect. And putting the shades outside the car creates a different set of logistical problems, involving the shades blowing away or getting stolen. Keeping them inside the car insures that they’ll be around when you come back, which is probably worth a slight reduction in effectiveness.

If I were less busy, I might see if Chris still has the car that’s almost identical to mine, and put this to the test. It’d be kind of fun– just get a whole bunch of aluminum foil, and cover one car’s windows with foil on the inside, and the other with foil on the outside. Then, monitor the interior temperature of the two cars for an hour or so, and see if there’s any significant difference.

Maybe next summer. Unless any high school or college physics students are looking for a quick and easy environmental-ish experiment to try, in which case, feel free to take the idea, and drop me an email to let me know how it turns out.

Comments

  1. #1 Timberwoof
    September 19, 2011

    Chad, you wrote, “But that infrared light is in a wavelength range that gets blocked by the glass.” Does it? Does car window glass block IR?

    It seems to me that part of the “greenhouse effect” is that the air inside the greenhouse gets warmer because it can’t leave. If you open a car window open (or an opaque sunroof, which would admit more light to begin with!) then, just as in a greenhouse with a window open, the air will circulate and the interior will be cooler.

    Is Chris’s car the same color as yours? Some German automobile journalists once got several cars of the same make and model, but different colors, and measured their interior temperatures throughout the day. As expected, the lighter-color cars were cooler than the dark ones—although all of them got “hot”.

    An additional variable to that experiment would be to use black construction paper instead of aluminum foil.

  2. #2 Clark
    September 19, 2011

    Timberwoof: It shouldn’t be too difficult to test IR transimssion through car window glass yourself. All you need is something to make IR (something hot), some glass and something that detects IR (a thermometer, or if you want something with pretty fast reaction time, your hand or your face.) Find something hot (but not too hot) and “shine” it through the glass. My first thoughts are to grab a toaster oven or small electric heater. I suppose lighting a fire would work, too. Turn it on and verify that from a distance of 1 foot or so you can feel the heat on your hand. Then insert the glass between the heat source and your hand. If the glass transmits the IR, your hand will still feel just as hot. If it doesn’t transmit, it’ll be cooler. There are a few things to keep in mind though. If you give the glass enough time to heat up it will start radiating itself, so then you won’t be measuring transmission. Also, a heat source like a blow drier depends heavily on the moving air from the fan, in addition to the heat. The glass will stop the air flow, so find a heat source that just radiates.

  3. #3 Chad Orzel
    September 19, 2011

    Chad, you wrote, “But that infrared light is in a wavelength range that gets blocked by the glass.” Does it? Does car window glass block IR?

    I will admit I don’t know the exact details of the reflectivity of car window glass, but I’m fairly certain it blocks IR in the multiple-micron wavelength range. If you want to do experiments at those wavelengths, you usually need to go to more exotic materials than ordinary glass. Mirrors for those wavelengths also tend to look like clear glass in visible wavelengths.

    The trapping of air is another big factor, which is why cracking the windows keeps the inside temperature a little lower. The trapping of radiation does come into play, though because it reduces the rate at which heat is radiated away.

    Clark: I like the idea of testing this with a piece of glass and a heater. You could probably do it with a toaster or an electric stove and a Pyrex baking dish (don’t put the dish directly on the burner, though, because they can explode that way).

  4. #4 marciepooh
    September 19, 2011

    “and a Pyrex baking dish (don’t put the dish directly on the burner, though, because they can explode that way)” And then you’ll have spaghetti all over the floor, burn part of the linoleum, and ruin supper. Just ask my mom. (She didn’t know the eye was still hot.)

  5. #5 Neil Bates
    September 19, 2011

    The commenter indulged a common intuitive misunderstanding. He would have been more to the point (there would be more effect) if it was a lousy sunshade inside, like black instead of white. However it is still true that some near UV and IR is absorbed by the window glass, so it’s not just a simple “visible” (or nearly so) light comes in, and some “visible” and some IR try to go out. Let’s say the window absorbs 50% of 360-400 nm. That absorption heats the glass some, and furthermore even a perfectly white shade sends it back for total absorption of 75% in the idea. It seems that sort of thing should make a noticeable difference, and it would be better to put the shade over the window – the main reason not to, is theft, wind, rain etc. Still, the OP is basically right in the abstract and per relevance to AGW.

    Yet it was pitiful when Energy Sec. Steven Chu (very impressed with his talk at “Jefferson Lab” where I sometimes work) proposed more white roofs to combat global warming: lots of trolls, often “skeptic”/right-wing types, derided him with comments around like ~ “it’s so dumb, the worst thing is to reflect the heat back into the air, so black is best” etc. The most pitiful thing is not their confusion about the principles, but their indulgent certainty that their intuitive, top-of-the-head take (or as instructed by Rush etc.) should niftily overrule what a Physics Nobel winner said.

    As for what type of roofs are best for cooling: of course they should reflect as much of incoming range as possible, but wouldn’t being dark in the emitting IR make it easier for them to shed what heat they did pick up?

    BTW, not a widely known fact about thermal radiation: the emission being proportional to absorption is (ideally) per each separate wavelength. Not all bodies are black or even gray/white bodies. So, if you could keep a body purple at high temperature, it would glow emerald green. For some reason we don’t hear much about that, or see many attempts to get some weird colors. It does come up in metallurgy, some of the subtle color variations. Mainly, most hot things just don’t have much absorption variation over the visible range?

  6. #6 clew
    September 19, 2011

    It seems to me that part of the “greenhouse effect” is that the air inside the greenhouse gets warmer because it can’t leave.

    Hang on, why do you think those are directly related? Holding air still doesn’t heat it up.

    How about, ‘if the windows were open, some of the trapped energy would convert to kinetic energy and move air through the windows.’

  7. #7 Omega Centauri
    September 19, 2011

    Neil, correct on all fronts. The absorption (and reflection) of solar radiation by typical window class is significant, probably at least twenty percent of overall. And of course, a lot of the heat transport, from the car is not radiative, but conducted/advected to the air. Also the heating of the nontransparent shell of the car, is quite important, which is why car color matters as well. Part of the problem is that what we regard as tolerable temperatures are not too far from what you’d get from the average radiation level on the earths surface (a quarter of sun sunshine if we didn’t have an atmosphere), so any dark object in full sunshine will have a radiation equilibrium temperature that is pretty unconfortable, and the time for thermal equlibrium if maybe an hour or two, which is short compared to the time of day. But solar radiation absorbed in the cars interior only adds yet more thermal impedance, which makes it hotter still.

  8. #8 Neil Bates
    September 19, 2011

    Clew, it’s not that the air is still per se (versus say, circulating around but still contained.) The issue is, it can’t interchange with other, cooler air – it is the same air kept in a small space. The heating of the insides of the car etc. just keeps heating it up more and more.

  9. #9 William Hyde
    September 20, 2011

    The Earth’s Surface is principally cooled by the upwards motion of air (loss of sensible and particularly latent heat). Yes, the upwards IR flux is bigger than either of these, but the net flux after accounting for downwards IR from the atmosphere is less. So a greenhouse functions principaly by suppressing this vertical motion, and only secondarily by trapping IR. The “Greenhouse Effect” is something of a misnomer, but we’re stuck with it.

  10. #10 Tom
    September 20, 2011

    No, the IR does not go through Pyrex. At least, not what gets picked up by an IR thermometer. http://blogs.scienceforums.net/swansont/archives/9837

  11. #11 Peter Erwin
    September 20, 2011

    There is a classic experiment from 1909 which demonstrated that real greenhouses don’t work by blocking IR light; R.J. Wood constructed two identical greenhouse-like enclosures, but gave one a roof of rock salt (transparent to optical and IR) and the other a roof of glass; the difference in internal temperatures was minimal. The real effect for physical greenhouses is, as others have suggested, due to the inability of air inside the greenhouse to convectively exchange heat with the rest of the atmosphere.

    Sadly, references to this experiment are sometimes trotted out by global-warming denialists to suggest that it somehow “disproves” the (atmospheric) greenhouse effect, as opposed to just showing that the latter is misnamed. (It’s as if by pointing out that “seahorses” aren’t actually “horses”, you had thereby disproved the existence of all fish in the Hippocampus genus….)

  12. #12 Peter Erwin
    September 20, 2011

    William Hyde @9:
    The Earth’s Surface is principally cooled by the upwards motion of air (loss of sensible and particularly latent heat). Yes, the upwards IR flux is bigger than either of these, but the net flux after accounting for downwards IR from the atmosphere is less. So a greenhouse functions principaly by suppressing this vertical motion, and only secondarily by trapping IR. The “Greenhouse Effect” is something of a misnomer, but we’re stuck with it.

    Yes, the Earth’s surface is cooled largely by convection, but that just transfers the energy higher into the atmosphere, where it is ultimately radiated away as IR flux. (And if the lower atmosphere were not mostly opaque to IR due to water vapor and CO2, you probably wouldn’t have the conditions for convection anyway.)

    The atmospheric greenhouse effect — that is, trapping of outgoing radiation by atomic or molecular absorption, leading to an increased surface temperature over what you would have without the absorption — happens regardless of whether there is convection. (Mars has a greenhouse effect due to the CO2 in its atmosphere, but very little convective heat flux.)

    Of course, the details of how the temperature varies with altitude (the lapse rate) below where the bulk of the IR flux is radiated to space do depend on whether there is convection, and how much, so it’s certainly important for working out the actual greenhouse effect in atmospheres. But it’s misleading to say that trapping IR is a “secondary” process.

  13. #13 MRW
    September 20, 2011

    I believe that Peter Erwin is correct. Heating in greenhouses has more to do with trapping air than “the greenhouse effect.”

    Outside, the sun warms the ground, the ground warms the air, and the hot air rises away, so the heat doesn’t build up.

    In a car, the sun warms the dashboard, the dashboard warms the air, and the air is unable to carry the heat away, so the heat builds up. The IR being trapped is a comparatively minor cause.

  14. #14 Mark
    September 20, 2011

    Neil, for the most part, all paints are “black” in the IR. Dark paints are dark in the visible and IR, and shiny, metallic surfaces are typically reflective in both visible and IR.

  15. #15 Eric Lund
    September 20, 2011

    Yet it was pitiful when Energy Sec. Steven Chu (very impressed with his talk at “Jefferson Lab” where I sometimes work) proposed more white roofs to combat global warming: lots of trolls, often “skeptic”/right-wing types, derided him with comments around like ~ “it’s so dumb, the worst thing is to reflect the heat back into the air, so black is best” etc.

    Chu’s reasoning here is not just about absorption/reflection of radiation (which is a factor; there is a reason you see white roofs and metal roofs more frequently in tropical climates). It’s generally good to keep your roof temperature close to ambient. During warm weather, this means that your air conditioner (if you have one) doesn’t have to work as hard–that means you are spending less on electricity, and the reduction in greenhouse gas emissions is a bonus. It’s also desirable in cooler weather (although the reasoning is a good deal more complicated–part of the story involves preventing ice dams on your roof, which is a major consideration where I live) to keep the roof cool and rely on insulation to trap the heat inside.

  16. #16 William Hyde
    September 20, 2011

    12) My comment was about greenhouses, not the greenhouse effect. “Secondary” here applies to actual greenhouses which are, as I argued, mostly warmed by suppressing the loss of sensible and latent heat.

    It would indeed be misleading, if not outright absurd, to claim that IR absorption is secondary in the greenhouse effect. It’s rather difficult to cool the earth via convection. At least not without losing some of the atmosphere.

    But it’s still an unfortunate name, as it gives deniers another talking point.

    William Hyde

  17. #17 Mark
    September 20, 2011

    Atmospheric scientists have long recognized that using the term “greenhouse effect” with respect to the atmosphere is not physically accurate. Some have suggested calling it the “atmosphere effect” but that sort of defeats the purpose of a descriptive term. “Greenhouse effect” calls to mind the idea of trapping heat. Not many among the general public understand how a greenhouse works, much less how the atmosphere works. Don’t even talk about global warming. But it is true that a sun-heated car interior is experiencing the true greenhouse effect rather than the atmosphere effect.

  18. #18 Joe
    September 21, 2011

    Geoengineering! Sunshade = sulfate aerosol. (However, putting a sunshade in your car won’t disrupt the world’s monsoon systems…)

  19. #19 Mu
    September 22, 2011

    Mark, shiny surfaces do NOT reflect IR well. Ever touched a sheet of shiny metal lying in the sun? Metals have lousy emissivity, so the white painted aluminum is at room temperature, the blank sheet next to it is at 70C.

  20. #20 Mark
    September 27, 2011

    Mu, I did not say that shiny surfaces reflect IR well, I said that shiny, metallic surfaces reflect IR well. A shiny aluminum surface in the sun is hot precisely because the thermal emittance is low (that is, the thermal reflectance is high). It does not absorb much solar radiation because it has a low solar absorptance (relative to other surfaces), but for what it does absorb, it must reach a relatively high temperature in order to achieve thermal equilibrium by emission. It is somewhat counter-intuitive. As you note, a white-painted aluminum surface will be cooler because it has a high thermal emittance as well as a low solar absorptance. Look at the ratio of solar absorptance to thermal (measured at, say 300K) emittance. That ratio gives a good indication of equilibrium temperature (at least in space). Shiny aluminum surfaces act as good sunshades, but their own temperatures are not cool.

  21. #21 bjolley
    November 15, 2011

    I am glad I found this post through a news feed. I live in Arizona and I have always wondered if the reflective shades actually work. It was great to read about the science behind it. I personally have found that it is much more effective to tint the windows in order to reduce the amount of light that penetrates the window. Thank you for the info.

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