Summertime Thermodynamics: Car Windows Open or Closed?

This may be a job for the MythBusters, but I’ll throw this out as a puzzle for interested blog readers. I don’t know the answer to this (though it wouldn’t be all that hard to determine experimentally), I just think it’s sort of interesting. There’s a poll at the bottom of this post, but it requires some set-up first.

So, it’s coming up on summer now, and I’ve been doing a bunch of errand-running this week, which means a lot of getting in and out of the car in sunny parking lots. Which raises the question: If you have an air-conditioned car, is it better to leave the car windows open a crack or closed while you run errands?

I think all of us, with the possible exception of oil company executives and some Republican members of Congress, would agree that if you are going to be gone a long time, it’s better to crack the windows. The glass windows of a closed car produce a greenhouse effect that heats the air inside the car to a temperature higher than that of the outside air. If you crack the windows, some of the heated air from inside the car can escape, to be replaced by cooler air from outside, leading to a lower equilibrium temperature than you would otherwise find.

So, if you’re parking your car in the morning on your way to work, the internal temperature will be a little more bearable if you crack the windows open slightly (assuming, of course, that you know it’s not going to rain, and that theft is not a major concern). But what if you’re just running a quick errand? The answer there isn’t as obvious (to me, anyway).

Assuming your car is air-conditioned, the air inside the car when you stop is considerably cooler than the air outside the car– depending on the weather and the setpoint of your A/C unit, probably something like 10 C cooler. The sun beating down on the car will eventually heat that cool air up, but that takes some time.

If you have the windows closed, the primary means of heating the car is by radiation– the sun beating down on the outside of the car, and through the windows, and all that. The car is essentially a sealed, partly transparent box, being heated from outside.

If you open the windows a crack, you add an additional heating channel: the cool air inside the car can mix with hot air from outside the car coming in through the windows. While this will ultimately limit the equilibrium temperature of the hot car to something less than that of the sealed car, in the short term (5-15 minutes, say), it might very well lead to faster heating. In which case, you would actually be better off leaving the windows closed than opening them a crack.

This is obviously something that can easily be tested experimentally using a temperature logger (or just a thermometer visible from outside the car), and I may do the experiment next week (weather permitting). For the moment, though, let me throw this out there to see what people think: If you’re going into a store for no more than 10 minutes, are you better off leaving your car windows closed or open?

If you would like to explain your reasoning, or question my reasoning, you know where the comments are.

39 thoughts on “Summertime Thermodynamics: Car Windows Open or Closed?

  1. if it’s no more than a 10 minute in-and-out store run, and your car’s already cool, and presuming the kind of weather and solar heating i’m getting where i live (45th parallel northern latitude, if we’re being ridiculously anal), then you’re better off leaving the windows closed. reasoning: empirical testing — i’ve tried that particular scenario both ways with my car. not as a deliberate test, mind, just because i happened to forget the windows cracked open or fully closed and then noted the outcome as i returned to the vehicle.

    you’re better off still finding a parking spot in the shade for those ten minutes, of course. but good luck with that, those few spots always get taken first.

  2. None of the choices really apply to me because I still drive a car with manual windows/locks–if I crack a window open, it’s only the front driver’s side window. Also, you didn’t offer the best solution (granted, it may not be available at the store you are visiting), which is to park in the shade. That said, if I have to park the car in the sun, the temperature is warmer than the mid 60s or so, and I’m not worried about car theft, I always crack the window (thus I voted for the “crack the windows” option). It doesn’t take very long for temperatures inside the car to become unbearably hot.

  3. Is it obvious the air temperature is the only relevant quantity and not also the intensity of the sunlight? Like, it could be warm outside but actually cloudy or even raining, I’d guess it makes a difference.

  4. What’s this shade y’all are talking about? In parking lots? I thought that was illegal or something, since most large lots, if they have trees, place them to provide minimum useful shade.

    For a short run I’d leave the windows closed, but around here (Tuscaloosa, Alabama) and put the sun visor up. It probably would be better to go ahead and crack them. Unfortunately, I often forget to crack my windows at work (long time, no shade unless I’m REALLY lucky); of course right now I can’t crack the windows because afternoon thunderstorms will be in the forecast for the next 3 months.

  5. Assume a spherical car… (Couldn’t resist!)

    To be honest, I doubt that you get a lot of air exchange via cracked windows, at least until it gets really hot inside the car. So it probably doesn’t matter a whole lot unless you open the windows by >1″ or so.

    I’d guess that the biggest variable you control is how much solar flux you’re accepting via your windows. Depending on the car, you’ll usually get the most sun through the front windshield. So if you can park so that it’s not collecting a lot of sunlight — facing the car north, in most situations — that should have a major positive effect. Facing the car south on a sunny day at noon is probably the worst.

  6. Is it obvious the air temperature is the only relevant quantity and not also the intensity of the sunlight? Like, it could be warm outside but actually cloudy or even raining, I’d guess it makes a difference.

    Sunlight is the biggest factor, I think. Most of the heating of the car on a sunny day is going to be from direct absorption of sunlight, but on a hot-but-cloudy day, it will be much less, and conduction might become more important.

    I had in mind a sunny day and a treeless parking lot (so “park in the shade” is not an option), for the record. Not that anyone will read this comment before clicking poll buttons.

  7. From the title of the post, I had assumed you were talking about maximum speed. My brother did that with a 1970’s VW Bug. 3mph faster with the windows closed.

    For heat gain in Maryland, 5 mins is the max sitting time on a sunny day with windows closed, in my experience. I only open the windows 0.5″, since I figure I am letting expanding hot air out; I expect almost no cool air to enter. There is not much cool air in Baltimore in August.

  8. The other interesting question is: and while you [i]are[/i] driving, is it more fuel-efficient to open a window to make a slight breeze, or to leave everything closed and rely on air-conditioning?

  9. Half an inch is way too much; your car won’t be there when you get back. Crack the windows a quarter inch, and use an interior window shade!

  10. Hello, I am something of an expert on the topic of car windows. I am a technical researcher with for Bekaert Specialty Films, LLC. (we manufacture many coatings, including optical coatings for solar control window films- car tint). I worked on a research project at an auto R&D facility in Belgium to determine how fast cars will heat up with (and without) various types of solar control film on them, with the window cracked, etc.

    The result: under normal conditions (horizontal sea level radiation profile according to ISO 9845), sedans heat up extremely fast (it took sixteen minutes to get to 135F). Summer radiation flux tends to measure higher than this standard also. Essentially you’re looking at five to ten minutes or so in most places for interior temperatures to exceed exterior temperatures. You are almost always better off cracking your windows.

    Incidentally, adding solar control film, particularly reflective metalized film extends this period, but not significantly. It increases the time to equilibrium by 50% to 100% depending on coating performance. (We also learned it does not save on fuel costs, but it does make the cab more comfortable when a vehicle is in motion or the AC is on, and the AC needs to work less).

    Our company also sumbitted data and recomendations for the California Cool Car Standards bill AB32 (which ended up getting shelved anyway).

  11. You clearly don’t live in Phoenix. The correct answer is: leave the car running with the AC on.

  12. crack the windows, cold air sinks, hot air rises. the crack is near the top of the window. however, chances are pretty good that it’s the seat that is the hot spot when getting back in the car. if you really want to stay cooler, put a white towel on the seat.

    generally I leave the windows cracked for the first minute or two of running the AC anyway just to let hot air escape and be replaced by the cold air coming in. then turn the AC to recirculate…

  13. I agree with –bill. Temperatures will be over 100 °F at my house this weekend and will stay there until late August. I crack the windows for any stop over 30 seconds.

  14. Wouldn’t it depend on wind too? A stiff enough wind, and you actually get good air exchange through the window crack, decreasing the amount of time it would be better to keep it closed.
    Also, some of the Priuses have solar-powered AC, right? Is there any intrinsic reason you couldn’t design such an AC to run without the car running? So you could use –bill’s solution and still get to be a tiresome scold?

  15. MythBusters actually _have_ tested this.

    Result: it depends. There’s a cutoff point after which aerodynamic losses from open windows are more than from the running AC.

    That’s a different argument, about the efficiency of leaving the windows open vs. running the A/C while you’re driving. I’m talking about the heating of a parked car, with the engine off.

  16. I generally leave my moon roof cracked; I figure that way not too much hot air will seep in (the whole hot-air-rises thing) and as the internal temperature rises as a whole there will remain a vent for the heat.

  17. I drive a Jeep. No roof, no doors, no windows to crack open. The inside temperature is always the same as the outside temperature.

    And what’s this new fangled air conditioner thingy I keep hearing about?

  18. In the Los Angeles area it is common for closed (all windows shut) parked cars to exceed 140 F. I have a Raytek MiniTemp, which is great for this.

    To find out quickly how long it takes for your car to reach ambient temperature, park the car, turn the engine (and accessories) off, and watch the clock (and a thermometer).

    The point of cracking windows (two or more) is not to let heat escape, but to get convection currents going. If the air below your roof is 130 F and the outside air is 90 F, the outside is cooler and denser, and convection will be driven by denser air pouring (slowly) in, making hotter air ‘chimney’ out.

  19. Opening the door to get out of the car will lose a significant amount of the volume of cool air (especially as the cool air follows you in the low pressure created behind you as you exit the car). In a small car, I would imagine this amount to be a fair percentage of total volume. Perhaps in a mini-van or large SUV you might be able to retain enough cool air to make a difference, but in a small car I don’t think you could retain enough cool air to make keeping the windows closed a better option.

  20. Here in Austin, TX, a fair number of people leave the engine running and the AC on while parked to go to the grocery store. There are trees in the parking lot (research shows that trees in the parking lot increase profits), but one does not want to park under one inhabited by grackles.

  21. Crack the windows. Unless the entire interior of your car is cooled to the AC temperature, the car will heat up very quickly (ever felt the dash? It takes a long time to cool that off). If your drive to the store is short, the interior elements will still be hot. Another related test: When driving, after you feel comfortable, shut off the AC – how long can you bear it without turning it back on? For me, not very long. If no AC is running, cracked windows win whether the car is still or not.

  22. Crack the windows, but more than half an inch.

    Arlanthe’s comment doesn’t surprise me. My experience indicates that in the South, it doesn’t take very long for a car interior to get hot once the AC is off. One big reason is that the interior surfaces are probably already hot because the AC has cooled mainly the air but not the surfaces.

  23. Aren’t we forgetting a rather significant variable: That in order to exit the vehicle you have to open the door and in doing create an opportunity for warm/cool air exchange?

  24. The color of your car’s interior is going to affect the time it takes to heat up by varying the efficiency of the absorption. And then there’s location changing the insolation. A light interior in upstate NY is probably going to yield noticeably different results than a dark interior in Florida, under otherwise identical conditions (temperature, time of day, etc.).

  25. I’m in the part of the Bay Area which gets essentially central valley weather, so summer is (except this very wierd year) always sunny/dry, and maybe 10F cooler than Phoenix. Crack passenger and driver windows about an inch. Don’t bother with rear windows. The heatup of the car is amazingly quick. Unless your car is white, the roof will probably be 150F or more, and this heat will radiate down and heat it up pretty quickly regardless of window tints or windshield reflectors. At least our’s is a dry heat, I usually use the fan instead of AC below 90-95, so the interior is never much cooler than ambient. And I live and work in very windy climates so you get a lot of air circulation. In a humid climate the answer might be to close them for stops of under 10 to 20 minutes, as the AC has dried out the air a lot, it’s not just the heat (temperature), but the humidity.

    The Prius parked AC is a gimmick, essentially a PV powered ventillation fan, so you can get airflow without worrying about opening your windows or running down the battery. I once had seen some solar powered fan you can put on your car window. I wanted to try one, but an internet search came up empty.

    It is most efficient when starting on a hot day, to use open windows plus fans for the first few minutes of your drive, then close um and switch to AC.

  26. Amega Centauri, an Atlanta TV station tested one of those solar-powered car ventilation devices and found that it did essentially nothing. It was no surprise, because only one window was opened and it was almost entirely sealed by a piece of rubber weatherstripping. As I mentioned above, the windows need to be opened more than a half an inch. I think there is very little air flow with an opening that size in the absence of wind, regardless of the inside-outside temperature difference.

  27. I remember seeing various solar-powered fan units mounted on enclosed aircraft* trailers. You drill a three-inch hole in the top of the trailer, and the fan fits directly into it.

    Something similar should we workable for a car, although it would take careful engineering to get the aesthetics right. Few people want a barnacle on their car roof, no matter how useful.

    [*] Gliders, to be precise.

  28. I have a black car with air conditioning that doesn’t work that well (and who but a global warming denier would cool their car’s interior to 70 when it’s 90 outside, anyway?), so I voted for cracking the windows.

  29. Yuppie swine that I am, driving an air conditioned car as I do (albeit a Honda Insight Hybrid) I have a spanner to throw into the works: I have tinted windows. This was not my idea; my dealer does this pro forma (but not pro bono). Nonetheless, it does affect my vote (close all the windows). Were I to park the car in the sun for longer than a few minutes,I would probably crack the windows. But I would also probably look for a tree to park under. I live in North Texas, where climate change does not exist (via the denial route), but where the sun always shines, even when there are tornadoes running through.

  30. If you leave the windows cracked you won’t have to worry about the temperature of the interior when you get back as the car will have been stolen.
    The new car delivered as a replacement will have been driven there, and will thus have a nicely cool interior.

    Ergo, it’s better to crack the windows as it will give you a cool car (albeit with a delay of hours or days) on your return to it.

  31. The answer depends on variables. I suspect in most summer cases ten minutes is about break even. So far, I think peter has the best answer in that not much cool air escapes and if it does get hot, you’ve got relief.

    As for helping prevent so much heat buildup, cars and houses etc. should have an extra cover set apart from the main body, at least in part. Like, a think shell set about 2-3 cm above the hood/roof/trunk, or 15 cm above roofs of houses. It would be a sort of self-contained shade, and not very much could conduct to the main body. I can understand aerodynamic objections etc. re cars, but there’s little excuse for not having done this more often for houses

  32. I know this isn’t the point, but if you want a practical suggestion, google for ‘solar car window fan’. It lets you keep the car locked but still have a little air circulation. For about ten bucks.

  33. Living in Las Vegas where the sunlight always seems extremely intense (rare to have clouds and we are 36° N). I find that parking facing the sun, installing a reflective and insulated sunshade, and cracking the windows about 1-2 inches helps keep the car bearable. However, it gets darn hot really quick anytime you park. Heck, I think people run red lights in this town to avoid sitting at intersections because the AC works worse while stopped at a light (due to getting more hot air from the cowl and reduced air flow through the condenser). It always sucks. No matter what you do.

  34. If you are parking in an area predominantly and highly democratic you should definitively close your windows! Otherwise they are going to break into your car! Simple.

  35. Heat exchange from one place to another happens b/c of a temperature difference(hot to cold). That heat energy is used for two things…causing a temperature increase by increasing the thermal motion of the air molecules or work (work in this case is gas expansion) I’ve always reasoned that by cracking the windows, you’re allowing for volume change and some of that heat energy goes into expanding the air (gas) inside of the car..and it expands into the surroundings. Therefore there is less heat energy left over to cause as much temperature increase had you left the windows closed. I have no idea if I’m correct, it just seems from a thermodynamic view this is what happens.

  36. 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.

    Has anyone ever studied this ?

  37. Following up on the comment posted by Lee Sept. 18, 2011, has anyone thought of making sure the reflected sunshades sit flat against the window as opposed to accordian style? Does this make any difference at all? Is there any tinting available that reflects the sun’s rays as opposed to simply filtering it? And if so, how expensive is it? Since an enclosed, small space heats up more quickly than a large, unenclosed space, most of this doesn’t matter when discussing cars. The best answers I’ve seen so far are don’t park in direct sunlight, let rising heat vent, and for comfort, AC is just about the only option in heat.

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