Back at the start of the summer, I asked a question about automotive thermodynamics: On a hot day, is it better to open your car windows a crack when making a short stop, or leave them closed? For a long term– say, leaving your car parked outside all day– I hope everyone will agree that leaving the windows slightly open is the better call, but the answer isn’t as clear for a short stop. There might well be some time during which the open-window car heats up faster as warm air from outside gets in, while the closed-window car holds in the air-conditioned goodness longer.
It occurred to me not long after posting that, while walking through the parking lot, that it was possible to test this with SCIENCE! A colleague in computer science has a car that is very nearly identical to mine, as you can see in this picture:
Chris’s car is similar enough to mine that I have more than once stood next to it like an idiot pressing the “unlock” button on my car remote and wondering why the door wasn’t opening. This makes for a nearly ideal test of the question of how to arrange your windows: just park both cars in the sun, one with the windows open and one with the windows closed, and monitor their temperatures over half an hour or so.
It took a while to get together, but yesterday, we did just that.
Here are the cars in the experimental configuration:
Chris’s car is on the left, my car is on the right. As you can see, they’re not quite identical– the interior of his car is a light tan, while mine is a light grey. The extedror color is the same, though, and the two cars are the same model year.
The high-tech temperature monitor can be seen at right. This is an indoor-outdoor thermometer, purchased at Lowe’s. I bought two identical thermometers, and we set them up with the “outdoor” probe hanging halfway down the center back seat, so it was not in the direct sunlight, and the wire was run out through the door of the car, so we could read the temperature outside. These read to the nearest 0.1 degrees Celsius, and refresh the display every ten seconds.
The cars were parked facing north (more or less), and the test started at around 3:45 in the afternoon, so the sun was more or less due west. Both cars were in direct sunlight, and we took care not to cast shadows on the cars. We ran the air conditioners in both cars for almost half an hour, then I opened the windows of my car roughly 1cm each, and we left Chris’s car closed up. We turned off both cars, exited quickly, then recorded the temperature once a minute for half an hour. Here are the results:
As you can see, there’s an obvious flaw with this test, namely that the air conditioning in my car works better than Chris’s. His car started about two degrees hotter than mine; the gap was closing a bit, but there’s a limit to how long we were willing to stand around in a parking lot with our car engines running.
The trend is pretty clear, though: My car, with the windows open, started out two degrees colder than Chris’s, but rapidly caught up and passed his car– within two minutes, the temperature in my car was higher than his. It took another 10-15 minutes for his car to catch up and pass mine, and they do show the expected long-term trend, with the closed-window car eventually reaching a hotter final temperature.
If you’re looking for the extra dorkiness you expect from a physics blog, here’s the same data with a couple of curve fits added:
The solid blue line is a double-exponential fit to the data, which corresponds to two different heating rates– a fast one (time constant of 2.7 min) due to direct exchange of cool car air with warm outside air, and a slow one (14 min) due to solar heating of the air inside the car. The solid red line is a single-exponential fit to the closed-car data, giving a single heating rate (about 8 min), presumably due to solar heating. The single exponential fit isn’t great, but SigmaPlot chokes on a double exponential fit to the same data, so this is all I’ve got. I wouldn’t put too much stock in this one.
We did some control testing by putting both cars in deep shade (that’s the picture at the top), and confirmed that they heat at more or less the same rate, and both come to just about the same temperature as the outside air. An attempt in partial shade was abandoned, as it was too difficult to get identical amounts of sun on the two cars. The sensors themselves are pretty close to identical– they take a surprisingly long time to come to the same value when put in the same car (in three-ish minutes, the gap between them had only cut in half), but given enough time, they record the same temperature to within a tenth of a degree or so.
What’s the take-home message from this? Well, the clear implication of the data is that for short trips, you’re better off leaving the windows of your car closed than open, to hold in the cool air. If you’re just running into the store for a few minutes, closed windows will be cooler than open windows. Beyond 10-15 minutes, though, the closed-window catches back up, and the difference is never all that large.
Obviously, this would be greatly improved by starting the two cars at identical temperatures. If I could think of a way for this to end with one of the cars exploding, I’d send it to Mythbusters as a story suggestion (Chris suggested sending it to Car Talk, too). For the moment, though, this is the state of the SCIENCE! on what you should do when you park your car on a hot day.