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.