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Okay, I've been wondering this one all summer. Whenever I open my car door, I'm greeted by a wall of very hot air, which takes a while to cool down, of course, with the air conditioning. So I was wondering, if when I parked my car, I could flip a switch and create a perfect vacuum inside, would it be as hot when I got in hours later? Assume that my car is parked in direct sunlight without shade throughout the day.

Without air, would surfaces such as seats (indirect sunlight) get as hot or hotter? Ordinarily, does the air usually provide a bit of insulation to absorb some of the incoming heat? Or, does the air serve to trap the heat inside the car?

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The same amount of energy would get through the car windows whether or not there was air inside the car. This heat would be absorbed by the interior surfaces, which would radiate in the infrared part of the spectrum. Unfortunately, the car windows are just as opaque to that IR radiation whether or not air is in the car, so they would radiate that IR energy back into the car at the same rate as before. In a vacuum, there is no air for convection currents to carry heat away from the interior surfaces, so they would get somewhat hotter than they currently get. Naturally, air would be introduced into the car before you opened the door to get in, but it is doubtful that the air would have time to remove substantial heat from the interior surfaces before you sat down. This means that the seat would be somewhat warmer than it is now.

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    $\begingroup$ I concur, and the car would have the roof caved in a bit from the vacuum. You could use a inert gas like Argon to fill the car and have a similar effect without having to hold a vacuum. You could test it by placing a Argon welding gas hose into a parked car and comparing with identical car with air inside. Using SF6 gas would do the opposite and let a bit more of the heat convect to the car skin and lower the temperature by a couple of degrees. Neither will have much effect. Having a small fan attached to a side window circulating outside air is known to be effective. $\endgroup$ – KalleMP Nov 11 '16 at 8:49

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