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.