I see people doing this a lot in the summertime around here. On a hot day, they’ll crack their car windows by 1-2", I guess on the assumption that it will allow air to circulate so that their car will only be as hot as the surface of the Sahara Desert when they get back, instead of being as hot as the surface of the sun.
Does this actually work?
If you mean “is the logic valid,” yes. Usually a surface exposed to direct sunlight will get hotter than the surrounding air. If that surface is the roof of a sealed car, the interior gets hotter than the outside air as well. So opening the window will lower the interior temperature.
If you mean “does a 1-inch gap really make a noticeable difference” - that depends on wind speed, outside temperature, color of the car, etc.
My unproven assumption is that the air is circulating inside the car. As the sun heats the car, the hot air inside is rising to the top of the car. So with a gap in the car windows, some of the circulating air inside is going to move out of the car. And being as the gap is near the top of the car, this air is the warmest in the car and the air from outside that replaces it will be cooler.
I suspect outside wind is more important than any natural convection current inside the car, even on a seemingly calm day. Especially if you crack open windows on both sides of the car, so wind can push air into the car from one side and out the other.
According to researchers at Stanford University School of Medicine:
This part is a little ambiguous:
The rate of rise is the same, but it’s unclear whether or not the same final temperature will be reached on hot days.
My own personal experience strongly disagrees with your Stanfrod study. When I am at work, leaving the windows cracked just 1 inch makes a huge difference in the temperature of my car at the end of the day.
What kind of day? A “relatively cool 72 degrees” day or a “hot” day?
I think the problem here is that the air coming in to the cabin has to go via the car’s ventilation system, so (a) there’s a small volume of air coming in and (b) the air gets heated along the way before it enters the cabin.
I wonder what the effect of leaving the windows wide open would be - other than your car getting stolen, of course.
Then I guess it depends on whether the thief turns on the A/C or not. 
I don’t crack my windows when it’s in the 70’s. I do when it’s over 100. It definitely makes a difference.
This is kind of an interesting drift of information. Your cite quotes the following paper:
“Heat Stress From Enclosed Vehicles: Moderate Ambient Temperatures Cause Significant Temperature Rise in Enclosed Vehicles,” Catherine McLaren, MD, Jan Null, CCM and James Quinn, MD, *PEDIATRICS *Vol. 116 No. 1 July 2005, pp. e109-e112. (This might be restricted content, so I don’t know if the paper is generally available.)
The paper says this:
Note that this confirms that there is no difference in the final temperature, but the rate of rise is different.
Now, this paper, in turn, cites a previous paper:
So let’s look at that paper:
“Heat Stress in Motor Vehicles: A Problem in Infancy,” K. King, K. Negus and J. C. Vance, Pediatrics1981;68;579-582. (a pdf; again I don’t know if it’s generally available.)
Note that the actual data from the King paper directly contradicts what the later McLaren paper says–not just contradictory data between the two experiments, but McLaren actually mischaracterized the data obtained by King.
In addition, the McLaren paper shows temperature rise for “cracked” windows that has an unexpected, unusual shape over time. I suspect their experimental procedure (“[R]ecordings were made first with the windows closed. Then the doors were opened to return the vehicle to ambient temperature, and a second hour of measurements was made with the windows cracked 1.5 inches.”) is flawed, although admittedly that’s hard to say with certainty. However, the mischaracterization of the earlier results does not give me confidence in the McLaren paper, and thus the Dr Greene site–although admirably cautionary–does not strike me as conclusive evidence.
I’m curious as to whether they also controlled for the 30° movement of the sun over the course of these two hours, and if so, how.
It all depends on how long the doors were left open when returning the vehicle to ambient. If they opened the doors, flapped them a few times and waited a couple of minutes, there’s no way this procedure is sound. The seats, floor, roof and dash would all still be significantly hotter than ambient, and will foul up the results. Use 2 identical cars, and alternate cracked vs. closed instead of always doing cracked second.
Exactly my point. The irregular shape of the data curve and the non-intuitive result (as well as the mischaracterization of earlier data) makes me think something is awry. If nothing else, consistently running the “cracked” trial after the “closed” trial, rather than alternating or randomizing the trials, is poor experimental design and the results should be called into question just because of this.
That definitely makes a huge difference. If I’m at home and know I’m going to be using the car in, say, half an hour, sometimes in summer I’ll go out and open all the windows. Over the course of that half hour the temperature in the car will drop from oven-like to pretty much whatever the outside temperature is.
Nice work, zut. 
And *after *the nice work?
Pardon?
Bad, wicked zut. 
X, it’s a Monty Python reference.
Good catch.
Do we have any reason to suspect an intention to eschew the results for a particular purpose? Is any of these studies funded by a company selling cooling fans, a group against animal cruelty, child abuse, etc? Or is it just sloppy science?